Eindhoven University of Technology MASTER Novel ... · discovery of this giant magneto resistance (GMR) eﬀect thus has had a large impact on the development of hard drives. It was

Eindhoven University of Technology

MASTER

Novel applications in organic spintronics : studying anisotropic effects in OLED devices

Veerhoek, J.M.

Award date:2013

Link to publication

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Novel applications in organicspintronics

Studying anisotropic effects in OLED devices

J.M. VeerhoekAugust, 2013

Supervisors:ir. M. Cox

prof. dr. B. Koopmans

Eindhoven University of TechnologyDepartment of Applied PhysicsGroup Physics of Nanostructures (FNA)

Abstract

This work focuses on the combination of two relatively new fields, namely organicelectronics and spintronics. A recently discovered effect called organic magnetore-sistance (OMAR) combines these two fields. This effect is generally observed inorganic light emitting diodes (OLEDs) for a multitude of organic semiconductingmaterials (OSC). In these devices, application of an external magnetic field of 10mT shows a change in current of up to 10 % at room temperature.

OMAR is typically explained by spin dependent interactions between quasiparticles present in the OSC. A recurring element in explaining OMAR is found inthe hyperfine fields caused by nuclear spins of atoms in the organic material. Thesefields are randomly oriented throughout the material and will cause a precessionalmotion of the spin of the aforementioned particles. As a result, spin mixing of, e.g.an electron hole pair, can occur due to the difference in magnitude and orientationof the local hyperfine fields. Application of an external field will cause an effectivefield with equal magnitude and orientation at local sites. Spin mixing is therebysuppressed, resulting in a change in interactions between the quasi particles.

In this work, we introduce a ferromagnetic layer to a regular OLED device.When magnetic domains are present in such a layer, fringe fields are created whichalso interact with the spin of charge carriers. These fringe fields are anisotropic,however, their orientation is linked to the domain structure. Consequently, theangle at two neighbouring sites is very small, however, a difference in magnitudeis present at these two sites. Additionally, these fields are typically larger than thehyperfine fields. As a result, spin mixing can now also occur due to these fringefields. The typical OMAR effect is thus influenced by the presence of these fringefields. We show that the distance between the ferromagnetic layer and the organicmaterial is of great importance for the magnitude of the influence of fringe fields.Additionally, the orientation of the applied field is found to be of importance dueto the anisotropy of the ferromagnetic layer.

To fully understand the anisotropy in OMAR in the stray-field devices, we alsostudy the intrinsic anisotropy by changing the angle between a regular OLEDdevice and the external field. In the literature, a basic understanding of thisangle dependence has been developed. We observe two distinct regimes of thisangle dependence, at low and high field strengths. The measurements at lowfield strengths enable us to determine that electron dipole-dipole coupling is thedominant interaction causing the angle dependence in regular OLED devices.

i

ii

Contents

1 Introduction 11.1 Organic electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Organic spintronics . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Organic magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . 31.4 This thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Theory 72.1 Organic electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.1 π-Conjugated molecules . . . . . . . . . . . . . . . . . . . . 72.1.2 Hopping transport and polarons . . . . . . . . . . . . . . . . 82.1.3 Device physics . . . . . . . . . . . . . . . . . . . . . . . . . . 92.1.4 Trap states . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.1.5 Electron-hole interactions . . . . . . . . . . . . . . . . . . . 12

2.2 Magnetic field effects . . . . . . . . . . . . . . . . . . . . . . . . . . 142.2.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2.2 Line shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.2.3 Mechanisms and spin mixing . . . . . . . . . . . . . . . . . . 172.2.4 Causes of spin mixing . . . . . . . . . . . . . . . . . . . . . 202.2.5 Fringe field MR . . . . . . . . . . . . . . . . . . . . . . . . . 24

3 Methods 273.1 Device layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.1.1 Standard device layout . . . . . . . . . . . . . . . . . . . . . 283.1.2 Devices with a ferromagnetic layer . . . . . . . . . . . . . . 29

3.2 Device characterization . . . . . . . . . . . . . . . . . . . . . . . . . 333.2.1 Electrical characterization . . . . . . . . . . . . . . . . . . . 333.2.2 Optical characterization . . . . . . . . . . . . . . . . . . . . 393.2.3 Other measurement techniques . . . . . . . . . . . . . . . . 40

4 Ferromagnetic structures for fringe field MR 414.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.2 Cobalt deposited on a glass substrate . . . . . . . . . . . . . . . . . 42

4.2.1 Atomic force microscopy . . . . . . . . . . . . . . . . . . . . 424.2.2 In-plane Magneto Optical Kerr Effect . . . . . . . . . . . . . 434.2.3 Superconductive Quantum Interference Device characteriza-

tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444.3 Cobalt deposited on top of an organic layer . . . . . . . . . . . . . . 45

iii

4.4 Cobalt-Platinum multilayer stack . . . . . . . . . . . . . . . . . . . 474.4.1 Out-of-plane Magneto Optical Kerr Effect . . . . . . . . . . 474.4.2 Kerr microscopy . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.5 Simulation of fringe fields from a cobalt layer . . . . . . . . . . . . . 494.5.1 Simulation considerations . . . . . . . . . . . . . . . . . . . 494.5.2 Domain structure in a cobalt layer . . . . . . . . . . . . . . 49

4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5 Fringe Field Magnetoresistance 555.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 555.2 Electrical behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . 575.3 OMAR measurements . . . . . . . . . . . . . . . . . . . . . . . . . 58

5.3.1 Reference devices . . . . . . . . . . . . . . . . . . . . . . . . 595.3.2 Devices with a PMA layer . . . . . . . . . . . . . . . . . . . 595.3.3 Devices with a cobalt layer at the bottom . . . . . . . . . . 615.3.4 Devices with two cobalt layers . . . . . . . . . . . . . . . . . 64

5.4 Varying the spacer layer thickness . . . . . . . . . . . . . . . . . . . 655.5 Angle dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . 685.6 Voltage dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

5.6.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715.6.2 Cobalt induced voltage dependence . . . . . . . . . . . . . . 725.6.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

6 Angle Dependence of intrinsic OMAR in SY-PPV 776.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776.2 Electrical measurements . . . . . . . . . . . . . . . . . . . . . . . . 78

6.2.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 786.2.2 Low field effects . . . . . . . . . . . . . . . . . . . . . . . . . 796.2.3 High field effects . . . . . . . . . . . . . . . . . . . . . . . . 83

6.3 Luminescence measurements . . . . . . . . . . . . . . . . . . . . . . 846.3.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 856.3.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

7 Conclusion and Outlook 897.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 897.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Bibliography 93

A Sample preparation 99

B Used materials 103

C Additional measurement techniques 109

iv

D Relation between AD and MC measurements 115

E Fringe field calculation 117

F Cobalt in-plane magnetization switching 123

G PEDOT:PSS Spincoat calibration 127

v

1 Introduction

Organic light emitting diodes (OLEDs) have been used to produce televisions, aswell as screens for mobile phones. Organic electronics have thus found their wayinto consumer technology. In addition to technological applications, research is stillbeing performed on organic devices. Due to this research, new features in organicdevices are still being discovered.

In this thesis we will focus on a recently discovered effect called organic magne-toresistance (OMAR). As the name implies, magnetoresistance effects are observedin organic semiconductors. Closer inspection of this research field shows that itis actually a combination of organic electronics and spintronics. These researchfields will briefly be introduced in this chapter, as well as the OMAR effect. In thesubsequent section, we will discuss the motivation for the research project of whichthis thesis is the result. Finally, we will give an overview of the content of eachchapter in this thesis.

1.1 Organic electronics

Modern day society is getting more and more dependent on electronics. Not onlyconsumer electronics are of importance, but also industrial electronics. Examplesare computerized production facilities or the systems governing electronic money.Without these applications of electronic devices, the world would literally be adifferent place.

Conventionally, electronic devices are created using silicon based technology.Examples are the parts making up modern day computers. Currently, an alterna-tive in the form of organic electronics is finding its way to (consumer) applications.The most common applications are television and mobile phone displays based onorganic light emitting diodes. The materials used in this technology mainly consistof carbon and hydrogen atoms, with the addition of few other atoms like nitrogen.As these are the building blocks of organisms, they are named organics. Tradition-ally, these materials were thought to be insulating, however that changed in 1977.It was shown that certain organic materials could be considered semiconductingor even show metallic conduction by doping the organic material [1]. This discov-ery was awarded the Nobel Prize in Chemistry in 2000. Since then, more organicmaterials have successfully been developed, all showing similar (semi)conductingproperties.

1

2 Chapter 1. Introduction

Organic semiconductors are considered to be an important class of materials,because the production and application of organic materials is relatively easy. Asa consequence, the costs of applications using organic materials can be kept low.Furthermore, the chemical properties of organic materials can easily be tuned toobtain the desired properties from the material. An example might be changing aside-group of an organic molecule to change the colour of an organic light emittingdiode based on this material. This makes organic materials desirable to use inapplications.

1.2 Organic spintronics

We just discussed the application of organic materials as (semi) conductors. De-vices based on these materials thus operate using an electrical current, whichbasically consists of electrons (and holes) flowing through the material. In addi-tion to the charge, these particles also have an intrinsic property called spin. Thisspin can be considered as the intrinsic magnetic field of the electron and can onlyhave two values, ”up” and ”down”. When the spin and charge of an electron areexploited one speaks of a new area of research, called spintronics.

A traditional application based on spintronics is found in the read head ofa hard drive. Such a read head basically consists of two ferromagnetic layersseparated by a non-magnetic layer. The magnetization of one of the magneticlayers is fixed, while the other is allowed to switch its orientation. The relativeorientation of the two magnetic layers was found to be of importance when acurrent passes through both layer in a direction perpendicular to the layers. Thisis attributed to the spin of the electrons, which is scattered at the ferromagneticlayer if the magnetization is aligned anti-parallel to the spin of the electron. Asa result, one can distinguish the parallel and anti-parallel alignment using theresistance of such a structure. This principle is demonstrated in figure 1.1. Thediscovery of this giant magneto resistance (GMR) effect thus has had a large impacton the development of hard drives. It was awarded with the Nobel Prize in Physicsin 2007.

Figure 1.1: The operating principle of a GMR device, consisting of two ferromagnetic layersseparated by a non-magnetic layer. The relative orientation of the magnetic layers determinesthe resistance.

We have now discussed two emerging research fields, organic electronics and

1.3. Organic magnetoresistance 3

spintronics. Combining these two fields results in the field of organic spintronics,in which new possible applications might be found. One main advantage of thisfield is the long lifetime of an electron spin state in an organic material due to thelimited spin-orbit coupling in these materials. This spin-orbit coupling is related tothe atomic number Z as Z4 and causes spin mixing. Because spintronics is basedon the spin state, spin mixing is undesirable. Organic materials mainly containrelatively light atoms, thus little spin mixing occurs and the spin state has a longlifetime.

Finally we note that spin interactions are fundamental in the operation of or-ganic electronic devices. By inspecting spin interactions in these organic materials,we can thus improve organic based electronics. Examples are improved efficiencyand new or improved functionality.

1.3 Organic magnetoresistance

In 2003 Kalinowski et al. observed a change in the current of an OLED whenapplying an external magnetic field [2]. This effect is called organic magnetoresis-tance (OMAR). It is noteworthy that the effect occurs at room temperature andreaches relatively large values (10%) for small magnetic fields (∼ 10 mT). As such,the effect is potentially interesting in future applications like magnetic sensors.

A more comprehensive review was performed by Mermer et al., showing thatthe effect occurs in various π-conjugated molecules and polymers [3]. Since theoriginal discovery of OMAR, it has been reported to reach values of over 25%.Additionally, a change in the luminosity of the devices occurs as well when applyingan external magnetic field. Effects in the luminescence of over 50% have beenreported [4].

The change of the current as a function of the applied field shows a character-istic line shape. This is shown in figure 1.2 for a number of materials, as presentedin [3]. Note that the current has been substituted by the resistance in this graph.Further studies of this line shape have shown that several contributions at differentfield scales can be discerned.

As stated before, the characteristic field dependence is observed for many π-conjugated materials. Because no ferromagnetic materials are used to create de-vices based on these materials, OMAR is thought to be an intrinsic effect presentin organic materials. Due to this observation, research on OMAR is importantin understanding the fundamental processes occurring in these organic materials.With this knowledge, further advancements in organic materials and devices mightbe achieved.

The exact origin of the OMAR effect is still under discussion. In the literature,several mechanisms have been proposed to explain the OMAR effect, howeverno mechanism is able to explain all observed features. The next chapter willexplore one of these mechanisms in more detail, as well as the terms used in


-40 -20

Magnetic �eld (mT)

Ma

gn

eto

resi

sta

nce

(a

.u.)

0 40200

PFOAlq3

PtPPEPPE

PentaceneRRa-P3HT

RR-P3HT

Figure 1.2: Normalized organic magnetoresistance versus magnetic field for different materials.Figure obtained from Wagemans et al. [5].

the remainder of this paragraph. Despite of the absence of an all-encompassingtheory, consensus has been reached on the fact that OMAR is attributed to spininteractions between different (quasi) particles present in the organic material.The spins of these particles are also assumed to be mixed due to the presence ofhyperfine fields. Furthermore, an external magnetic field can suppress the spinmixing between different spin configurations and thereby alter the conductivity,and thus the current, in a device.

1.4 This thesis

The work presented in this thesis is divided in two parts. In the first part we focuson adding a ferromagnetic layer to a pristine device. The addition of a ferromag-netic layer can lead to the introduction of fringe fields. When magnetic domainsare present in this ferromagnetic layer, fringe fields are present around this layer.This is schematically shown in figure 1.3a, where a ferromagnetic layer is placed atthe bottom of a standard device structure. In regular devices, OMAR is usuallyattributed to the random orientation of so-called hyperfine fields. Because thefringe fields caused by a ferromagnetic layer are not random but have a preferen-tial direction per domain, the OMAR effect is expected to be influenced, thereforemaking it interesting from a fundamental point of view. From a practical stand-point, addition of a ferromagnetic layer might yield memory devices. To createsuch a device, the fringe fields and thus the magnetization of the ferromagneticlayer should be switchable between two discrete states. These states can then beused as a logical bit and thus as memory, where the organic layer is used as asensor.

In the first part of the thesis we will therefore focus on including such ferro-

1.4. This thesis 5

Non-magnetic

Spacer

Organic

(a)B (mT)

MC

(%

)

3

2

1

0

-40 -20 0 20 40

θ = 90°

θ = 0°

θ

B

(b)

Figure 1.3: (a) A schematic representation of a ferromagnetic layer with domains causingfringe fields (dotted lines) in a typical device structure. (b) The MC measurements performed byWagemams et al., showing the influence of changing the angle between the device and externalmagnetic field (indicated in figure). Figure adapted from Wagemans et al. [6].

magnetic layers in organic light emitting diodes. An extensive study of the effectscaused by including such a ferromagnetic layer will be performed.

The second part of this thesis discusses the effect of changing the angle be-tween the device and the external field on the organic magnetoresistance. Initially,OMAR models did not include any angle dependence because spin-spin interac-tions between polarons were taken to be isotropic. Only when sufficiently stabledevices were created, the angle dependence in OMAR was discovered as it is rel-atively small. Changes in the current of up to 0.1 % are observed when rotatingthe device, whereas the normal magnetoconductance reaches values of up to 4.8%[6]. Figure 1.3b shows the measured effect of changing the angle from 0◦ to 90◦.The device and externally applied field are schematically shown as well, indicatingthe angle between them.

Due to the relatively small effect, this effect has not been used in practicaldevices. Yet, it has recently been used to locally determine the direction andmagnitude of the earth magnetic field. Additionally it is suggested that animals usethe same principle for magneto-reception and navigation [7]. From a fundamentalpoint it is thus interesting to find out what the cause of this angle dependence is.

In this second part of this thesis we present measurements on pristine devicesbased on SuperYellow-PPV, the main organic semiconductor used in this thesis.A basic theory in the literature is expanded to explain the results obtained in thissection.

We will now present an overview of the different chapters in this thesis. Addi-tionally, a brief summary of these chapters will be provided.

Chapter 2 : Theory. This chapter provides a theoretical background on thephysics of organic electronics. Additionally, the magnetic field effects observed


in these organic semiconductors are introduced. Special attention will be given tospin mixing and the mechanisms causing it.

Chapter 3 : Methods. An overview of the device structures used in this the-sis are outlined. Furthermore, the electrical and optical measurement techniquesused to characterize the measured devices are discussed.

Chapter 4 : Ferromagnetic structures for fringe field magnetoresistance.The magnetic and surface properties of the used ferromagnetic layers are investi-gated in this chapter. Additionally, simulations are performed on the fringe fieldscaused by domains in such a layer. The results presented in this chapter will beused in the next chapter.

Chapter 5 : Fringe Field Magnetoresistance. This chapter presents results ob-tained by measuring devices which have been modified to contain a ferromagneticlayer. Results presented in the literature are reproduced to show their validity.Furthermore, we will show that the switching behaviour of the magnetization ofthe ferromagnetic layer can be observed by measuring the magnetoconductance inthese devices. Several properties of these devices are examined to validate a modeldescribing the fringe field magnetoresistance.

Chapter 6 : Angle Dependence of intrinsic OMAR in SY-PPV. In thischapter, we perform angle-dependent measurements on pristine devices based onSuperYellow-PPV, the main organic semiconductor used in this thesis. We willpresent the results of electric measurements and use these results to explain themechanism causing the angle dependence. Results of angle-dependent luminositymeasurements are also presented.

Chapter 7 : Conclusion and Outlook. In this final chapter, the main con-clusions from the preceding chapters are summarized. Additionally, an outlook onadditional research opportunities will be provided.

2 Theory

This chapter provides a description of the physics governing the behaviour of theorganic devices used in this project. In the first section the basic concepts of organicsemiconductors are discussed. The second section will then introduce magneticfield effects observed in these organic semiconductors. This second section willfocus on the basic characteristics of these magnetic field effects and some proposedmechanisms playing a role in causing these effects.

2.1 Organic electronics

Despite the fact that conduction in organic materials is a complex and active areaof interest, a basic theoretical description of electronic transport in these systemshas been established. In order to understand the devices created in this project,this theory will be described in this section. First, the electronic structure a singlemolecule will be examined, after which hopping between molecules is discussed.Finally, a general impression of the physics governing the current through organicdevices is given, including the effect of trap states and charge carrier interactions.

2.1.1 π-Conjugated molecules

Usually, organic materials are not considered conductive materials. Most electronsare strongly bound in localized states, causing a strong bond between the atomsin a molecule. When π-conjugation occurs in an organic molecule, the chargecarriers are delocalized over the molecule. As a result, the molecules now showconductive behaviour. Traditionally, this effect is described by alternating singleand double bonds in an organic molecule. In such a structure, the electronicorbitals of three of the four valence electrons of a carbon atom hybridize to so-called sp2 orbitals. This specific hybridization only occurs between neighbouringatoms in which the electronic orbitals overlap. These sp2 orbitals now overlap tofrom so-called σ-bonds, found in every C-C and C-H bond. The remaining valenceelectron occupies the pz orbital, which can overlap with neighbouring pz orbitals.A bond caused by such an overlap is called a π-bond [8]. Such a π-bond is foundin a double bond between two carbon atoms, in addition to the σ-bond which isalso present. When many neighbouring carbon atoms have this hybridization, theelectrons in the π-bonds are delocalized along the chain of carbon atoms, giving

7

8 Chapter 2. Theory

rise to conduction. An example of the sp2 and pz orbitals is given in figure 2.1a,which depicts ethene (C2H4).

HH HH HHHHσ-bond

CC

π-bond

HHH

π-bond

(a)

HOMO

LUMO

energy gap

En

erg

y

Vacuum level

C

V

(b)

Figure 2.1: (a) Orbitals and bonds in ethene (C2H4). The σ-bonds are all in-plain, while theπ-bonds are perpendicular to the plane. (b) The valence band (V) and conduction band (C) areseparated due to the Peirels distortion. The highest occupied energy level (HOMO) and the lowestunoccupied energy level (LUMO) are also indicated.

In an infinitely long π-conjugated molecule, the delocalization of electronswould lead to metallic conduction, which implies a valence band and a conductionband. The highest occupied molecular orbital (HOMO), at the top of the valenceband, and the lowest unoccupied molecular orbital (LUMO), at the bottom of theconduction band, are only separated by an infinitesimally small amount of energy,leading to a continuum of allowed energy states. However, as single bonds arelonger than double bonds (known as the Peirels distortion), the π-electron systemis split in two bands. An energy gap is thus present between the HOMO andLUMO levels [9]. The HOMO and LUMO levels, as well as the energy gap, areschematically drawn in figure 2.1b. This energy gap Eg is typically in the orderof a few eV [10]. As a result of this gap, π-conjugated organic materials are semi-conducting instead of conducting. We can thus refer to these materials as organicsemiconductors (OSCs).

2.1.2 Hopping transport and polarons

In the previous section, the assumption was made that the OSC consists of aninfinitely long chain of pi-conjugated atoms. In actual organic materials this is notthe case, as polymers have a finite length and small molecules are generally evensmaller. Due to the fact that these finite-length molecules have a random threedimensional orientation and position when creating a layer of the material, the π-conjugated system is broken into smaller parts [11]. This leads to the introductionof a positional disorder. Each part can now provide a place where a charge can belocated, leading to the localisation of energy states. As not all parts are necessarilyequal in length, a disorder is introduced in the localised energy states. As a result,the HOMO and LUMO levels are broadened.

The introduction of a charge on a segment of the molecule leads to a changein conformation of this segment due to Coulombic interactions. As a result, the

2.1. Organic electronics 9

energy of the molecular segment is lowered. The combination of the deformationand the charge can be considered a quasi-particle and is called a polaron. As acharge hops from one site to another, it will cause a deformation on the new sitewhile the deformation on the old site is undone. Conduction in disordered organicmaterials is thus not only caused by the charge carrier, but also by the deformationaccompanying the charge carrier. For simplicity, negative and positive polaronswill be referred to as electrons and holes, respectively.

As stated earlier, polarons can hop between sites in the OSC. Generally, energydifferences are present between sites, which can be overcome by emission or ab-sorption of a phonon, which is called phonon-assisted tunnelling [12]. The hoppingrate is dependent on the energy difference between the initial and final site andis usually described by the variable range hopping mechanism. The rate is thengiven by the Miller-Abrahams hopping rate [13]:

ωhop =ω0 · exp (−2α∆x) · exp

(∆EkBT

)for ∆E > 0

ω0 · exp (−2α∆x) for ∆E ≤ 0, (2.1)

where ω0 is the attempt frequency, α the inverse localisation length, ∆x the dis-tance between sites, ∆E the energy difference between sites and kBT the thermalenergy. The term exp(−2α∆x) represents the electronic wave function overlap,while the term exp

(∆EkBT

)is a Boltzmann factor included to take jumps upward

in energy into account. For hops where ∆E ≤ 0, it is assumed that it is alwayspossible to emit a phonon and therefore it is always possible to make that hopenergy-wise.

As the sites in an OSC are disordered in both position and energy, a trade-offwill have to be made between a long distance jump with a small energy difference,and a small distance jump with a larger energy difference. The conduction is thusstrongly dependent on the structural and energetic disorder [14]. The energeticdisorder is usually described by a Gaussian density of states (DOS), however,sometimes an exponential DOS is also used. A schematic representation of aGaussian DOS and the energetic and positional disorder is shown in figure 2.2.

2.1.3 Device physics

Typical devices created with organic materials are based on the principle thatcharge is injected into the OSC and that charge is moved through the device.The devices used in this thesis are organic light emitting diodes (OLEDs). Uponrecombination of charge carriers in this type of devices, light can be emitted.

The easiest way to facilitate charge injection is by sandwiching the organicsemiconductor between two metal electrodes. When both electrons and holes areto be injected, the electrode metals are chosen such that the work functions ofthose electrodes are aligned with the HOMO and LUMO energy levels of theorganic material. When different work function electrodes are used, an electric

10 Chapter 2. Theory

ΔE

En

erg

y

Density of states Space, time

Δx

Figure 2.2: Hopping transport in organic materials using a Gaussian DOS (left). The sites aredisordered in energy and position, leading to a hopping rate dependent on both the difference inenergy ∆E and in distance ∆x. Figure adapted from [15].

HOMO

LUMO

Φh

Φe

φh

φe

Vacuum level

qVbi

EF

Eg

(a)

Vacuum level

qVbi

(b)

Figure 2.3: (a) Energy diagram of an organic semiconductor without an applied bias voltage(V = 0). The electron (hole) injection barrier φe(h) and electrode work function Φe(h) are shown,as well as the band gap and the built-in voltage. (b) Energy diagram of an organic semiconductorwith an applied forward bias equal to the built-in voltage (V = Vbi), showing electron and holeinjection and transport. Both figures are adapted from [9] and [15].

field will be formed and a built-in voltage (Vbi) is present in the device. Theenergy diagram for this situation is schematically shown in figure 2.3a.

In order to allow drift of charge carriers through the device, the built-in voltagemust be overcome and thus a bias voltage V larger than Vbi must be applied. Oncethe bias voltage has reached the built-in voltage, charge carriers can be injectedand will start to drift towards the opposite electrode. The energy diagram forV = Vbi is shown in figure 2.3b.

Despite the statement that drift is only allowed when the bias voltage is largerthan the built-in voltage, it is still possible to inject charge carriers into the organiclayer and induce a current for V < Vbi due to diffusion. In this case, the current


density is best described by the Shockley diode equation [16]:

J = Js

(exp

(qV

kBTη

)− 1

), (2.2)

in which Js is the saturation current at negative voltage, V the applied voltageand η the so-called ideality factor.

Because the work functions of the electrode metals usually don’t exactly matchthe HOMO or LUMO levels, an injection barrier ϕ is introduced. This barrierprevents the direct transfer of charge carriers from the electrodes to the organicsemiconductor, however, it can be overcome by thermal activation or tunnellingthrough the barrier. When the injection barrier is limiting the current throughthe device, one speaks of an injection limited current (ILC). This situation occurswhen the bias voltage is just slightly higher than the built-in voltage, as relativelyfew charge carriers are injected due to the injection barrier. The charge carriersthus have little influence on each other and the injection is slower than the hoppingtransport.

When a larger bias voltage is used, the injection rate increases due to thefact that the injection barrier is lowered by the large electric field, as well as anincreased tunnelling probability due to a smaller injection barrier. From a certainmoment onward, the current will be limited by the hopping rate through the bulkinstead of the injection. For a unipolar device, where only one type of chargecarrier is injected, a space charge is generated which limits the current. This iscalled a space charge limited current (SCLC). The Mott-Gurney equation givesthe current density for such a unipolar device under the assumption of Ohmiccontacts:

J = 98εµ

(V − Vbi)2

L3 , (2.3)

where ε is the electrical permittivity of the device, µ the charge carrier mobilityand L the thickness of the device.

When both types of charge carriers are injected, one speaks of a bipolar device.In the case that the injection rates are similar for both charge carrier types andboth move similarly through the device, no net charge will be present in the deviceand the current would theoretically not be limited by charge build-up. Note thatsimilar movement does not mean ”in the same direction”, as positive and negativecharge carriers move in opposite direction due to the presence of an electric field.However, both charge carrier types behave differently and the mobility and injec-tion rates generally differ, resulting in a net space charge in the device, which limitsthe current. Another way to limit the current is the recombination of electronsand holes, see section 2.1.5.

2.1.4 Trap states

Because the OSC generally contains some impurities, additional energy states canbe present in the device. These additional states can lie significantly lower (or


higher) in energy, which causes electrons (or holes) to have a very small chance ofhopping away from these states. The tail of density of states of both the HOMOand LUMO can also provide energy states which can be hard to escape from [17].Effectively, charges present on such states can thus be trapped and these states aretherefore called trap states. Recently, doping of the OSC has received an increasedinterest. The dopant material can lead to additional energetic states, which canalso act as trap states. As a result of these trap states, the macroscopic currentthrough such devices can be changed, as well as causing changes in the so-calledOrganic Magneto Resistance (OMAR) effect, which will be discussed in section 2.2[18, 19, 20].

These trap states generally play an important role in the current through thedevice. The traps in an OSC generally have a certain energy distribution, allowingshallow and deep traps. This distribution is usually modelled as a Gaussian [21],exponential [22] or even as a set of discrete energy levels [23]. When the current ina unipolar device is trap limited, the voltage dependence is expected to be largerthan power 2 as compared to equation 2.3 [24, 25]. This effect can be explained bythe re-participation of trapped charge carriers upon an increase of the bias voltage.The larger electric field increases the chance that trapped charge carriers will hopback into the conduction channel, which causes a current increase. This is shownfor electrons in figure 2.4a. As a result, the current will have a dependence on thevoltage that is larger than quadratic.

LUMO

Energy

(a)

hν

(b)

Figure 2.4: (a) Re-participation of trapped electrons by means of an applied electric field. (b)Electroluminescence. Both figures assume V > Vbi and are adapted from [18].

2.1.5 Electron-hole interactions

Assuming a bipolar device, electrons and holes will be attracted to each otherdue to the Coulombic forces caused by their opposite charge. When both chargeshave approached each other sufficiently, they can be Coulombicly bound, forminga bound electron-hole pair (e-h pair). When the thermal energy of the pair issmaller than the Coulomb energy, the electron and hole are bound:

kBT <q2

4πεrc

, (2.4)


where rc is the Coulomb capture radius. Generally in organic materials, the per-mittivity ε is rather low (ε ≈ 3ε0), leading to a relatively large capture radius of∼20 nm at room temperature.

An e-h pair can gain additional energy by reducing the distance between thecharge carriers even further. As a result, the charge carriers will be more stronglybound because their wave functions will start to overlap. When this happens,exchange interactions will start to become important. This situation is called anexciton, basically the first excited state of a molecule. These excitons are generallyformed on the same molecule or segment of a polymer. Electron-hole pairs can, incontrast, reside on neighbouring molecules or polymer segments.

To inspect the energy level of an exciton, we should also consider the intrinsicspin of the charge carriers. When two charge carriers interact with each other, fourcombinations of both spins are possible. Note that charge carriers are fermions andthus have a spin equal to 1/2. In a classical view, three of these combinations, orstates, have a total spin aligned parallel and have a total spin of 1. The remainingstate has the two spin aligned anti-parallel and has a total spin of 0. The formerstates are degenerate in energy and are referred to as triplet states (T ), while thelatter is known as a singlet state (S). The degeneracy of the triplet states is lifteddue to Zeeman splitting when a magnetic field is applied. One state is lowered inenergy (T−), one is raised in energy (T+) while the third state (T0) is unaffected. Asa result, only interactions between S and T0 are usually considered when applyinga significant external field [26]. This will be important in the next section, wherethe effects of applying a magnetic field are discussed in more detail. Due to thewave function overlap in an exciton, exchange interactions are important and thetriplet states will have a lower energy than the singlet state.

From this exciton state, it is easy for the charge carriers to recombine andtherefore allow the molecule to return to the ground state. Usually, recombina-tion occurs under the emission of a photon. The rate at which this happens, ismeasured by the recombination mobility µr. As mentioned in section 2.1.3, therecombination will limit the bipolar space charge limited current. As the tran-sition from a triplet state to the ground state is spin-forbidden, it has to occurnon-radiatively which generally increases the lifetime of the triplet states (tens ofµs compared to ns for singlet states [27]). The recombination rate is thus mainlydetermined by the recombination of singlet configurations. The full energy dia-gram of all electron-hole states between free charges and recombination are shownin figure 2.5.

The recombination process is generally viewed as a bimolecular process. Langevintheory describes strong recombination, and gives the recombination rate as:

R = γnenh, (2.5)

in which γ is the recombination pre-factor and ne(h) the electron (hole) concentra-tion. The recombination rate is limited by the rate at which the two charge carriersapproach each other due to Coulombic attraction. The recombination pre-factor


Free charges

Electron-hole pair

Singlet excitonTriplet exciton

Ground state

En

erg

y

Figure 2.5: Energy diagram of charge carrier interactions. Free charges can gain energy byforming a Coulombically bound electron-hole pair. Such an electron-hole pair can gain additionalenergy by moving closer until an exciton is formed. An exciton can finally recombine radiativelyto reach the ground state of the molecule. Figure adapted from [20].

is therefore dependent on the electron and hole mobilities:

γ = q

ε(µe + µh) , (2.6)

where µe(h) is the electron (hole) mobility.When the recombination is strong enough, recombination will occur at a certain

position in the device. The recombination area has a finite width and at eitherside a current of only one charge carrier type will now be present. This is shown infigure 2.4b. Due to the fact that only one type of charge carrier is present at anypoint in the device, a net space charge can build-up, in a similar way to a unipolardevice.

When the space charge is limiting the current in such a bipolar device withLangevin recombination, the current density is given by [28]:

J = 98ε√

2πµeµh (µe + µh)µr

(V − Vbi)2

L3 , (2.7)

where V the bias voltage, Vbi the built-in voltage, µr the recombination mobil-ity and L the device thickness. While deriving this equation, it was assumedthat Ohmic contacts are present. Furthermore, the recombination mobility µr =ηr (µe + µh) is related to the recombination efficiency ηr of the charge carriers.

2.2 Magnetic field effects

Devices based on organic materials have shown interesting effects when introducingan external magnetic field. In this section we will define these magnetic field effects(MFEs) and inspect the line shapes associated with them. One of the currentlyproposed mechanisms will be discussed, indicating how these MFEs can be caused

2.2. Magnetic field effects 15

by an external field. Finally a link will be made to the influence of fringe fields,caused by a ferromagnetic layer.

2.2.1 Definitions

When applying an external magnetic field to organic materials, the resistance ofdevices based on these materials changes. This organic magnetoresistance effectis observed for devices based on polymers and small molecules. The effect caneasily be observed by measuring the current through a device while sweeping themagnetic field. The magnetoconductance (MC) is now defined as the difference incurrent at a certain field with respect to the current at zero magnetic field:

MC(B) = ∆II(0) = I(B)− I(0)

I(0) . (2.8)

Here I(B) is the current at a certain applied field B and I(0) is the current withoutan externally applied field.

The electroluminescence if also affected by an external field, and as such themagneto-electroluminescence (MEL) is defined as:

MEL(B) = ∆ELEL(0) = EL(B)− EL(0)

EL(0) . (2.9)

Like the magnetoconductance, the electroluminescence at a certain fieldB (EL(B))is compared to the electroluminescence without an external field (EL(0)). As in-dicated in equations 2.8 and 2.9, the MC and MEL are functions of the appliedmagnetic field. As such, they are usually plotted as a function of the applied field.A typical MC measurement is shown in figure 2.6, where the MC is plotted as afunction of the applied field.

From this figure, we observe that relatively large effects are observable at lowfield strengths. Furthermore, we can distinguish several contributions to the shapeof the MC, all with their own characteristic field scales. The inset of the figureshows features even occurring at field strengths below 1 mT.

2.2.2 Line shapes

As mentioned before, the MC is usually shown as a function of the magneticfield. Two functions are commonly used to describe the observed line shapes, theLorentzian and the non-Lorentzian functions [3]. The Lorentzian line shape isderived from simple theories of classical magnetoresistance and is given by:

MCLor(B) = MC∞B2

B2 +B20. (2.10)

MC∞ represents the magnetoconductance at infinite external field and B0 the halfwidth at half maximum. This function saturates as B−2 towards infinite appliedmagnetic field, while experiments show this saturation can be much slower.


-400 -200 0 200 400

0

1

2

3

4

-2 -1 0 1 2

0.0

0.1

0.2

0.3

0.4

0.5

0.6

MC

(%)

Field (mT)

MC

(%)

Field (mT)

Figure 2.6: A typical MC measurement performed on a typical device structure, using a biasvoltage of 5 V. The inset shows features occurring at very small field scales.

An alternative function was therefore proposed to describe this behaviour,which is called non-Lorentzian. It is described by:

MCnon−Lor(B) = MC∞B2

(|B|+B0)2 , (2.11)

in which B0 now describes the half width at quarter-maximum. This functionsaturates as B−1 towards infinite applied magnetic field. The parameter B0, oc-curring in both functions, gives information about the field scales of the mechanismcausing the MC. Both functions are shown in figure 2.7, showing the difference insaturation behaviour. The inset of the figure shows the difference between bothfunctions on a logarithmic scale.

When taking a closer look at the line shapes obtained from measurements,several contributions can be distinguished. These contributions are categorized bythe width of the contribution in terms of the magnetic field strength. Moreover,multiple contributions of the MFEs can be present, resulting in line shapes thatcan differ from the (non-) Lorentzian line shapes. At very large scales, on the orderof 100 mT or more, the so-called high field effect (HFE) occurs. At a smaller scale,on the order of 1-10 mT, the low field effect (LFE) is present. Recently an effecton an even smaller scale has been discovered, the ultra small field effect (USFE)at a scale of < 1 mT [29].

The HFE and LFE can be either positive or negative (independently from eachother). This sign depends on several parameters like voltage, temperature, devicethickness and the transition from unipolar to bipolar operation [30, 31]. Examplesof the different field effects currently known are shown schematically in figure 2.8.


-15 -10 -5 0 5 10 150.0

0.2

0.4

0.6

0.8

1.0

MC

/ M

C

B / B0

Lorentzian Non-Lorentzian 10-2 10-1 100 101 102 103

0.0

0.5

1.0

Lorentzian Non-LorentzianM

C /

MC

B / B0

Figure 2.7: Line shapes commonly used to describe the observed MC curves, the Lorentzian andnon-Lorentzian functions. The inset shows their behaviour on a logarithmic scale, showing thatthe non-Lorentzian needs a much larger relative field to reach saturation. Figure adapted from[5].

2.2.3 Mechanisms and spin mixing

No full consensus has been reached about the exact origin of these magnetic fieldeffects. Several mechanisms describing them have been proposed, however, none ofthem are able to describe all observed MFEs. A general observation, independentof the investigated mechanism, is the importance of spin mixing and the suppres-sion of this mixing process by introducing a magnetic field. The most commonlyused models to describe the currently accepted mechanisms are the electron-holepair model [32], some models based on triplet exciton interactions with other par-ticles [33, 34] and the bipolaron model [35]. For a detailed description of thesemodels we refer to reference [20].

The mentioned models all describe the interactions between specific (quasi)particles. For the e-h pair and bipolaron models, these particles are both fermionsand thus have spin 1/2. The combination of any two particles results in theformation of either a singlet or three triplet states as described in section 2.1.5.The statistical ratio between these the singlet and triplet states is 1:3. We nowconsider the possibility that a pair of charge carriers can change its configuration,i.e. change between the singlet and triplet states. The statistical ratio betweensinglet and triplet states can then be changed due to the conversion between states.If such a spin mixing mechanism is now magnetic field dependent, the conversionrate between states can be changed by applying a magnetic field.

We now focus on the electron-hole pair mechanism proposed by Prigodin etal., where the interactions between an electron and a hole are considered. Themechanism relates the MFEs to a change in recombination rate, which in turn isproportional to the recombination mobility µr. An overview of the mechanism is


−500 0 500

−1

−0.5

0

0.5

1

Magnetic !eld (mT)

Ma

gn

eti

c e

"e

ct (

a.u

.)

positive MC

negative MC

(a)

−500 0 500

0

0.2

0.4

0.6

0.8

1

1.2

Magnetic !eld (mT)M

ag

ne

tic

e"

ect

(a

.u.)

positive HFE

zero HFE

negative HFE

(b)

−5 0 5−0.1

−0.05

0

0.05

0.1

0.15

0.2

0.25

Magnetic !eld (mT)

Ma

gn

eti

c e

"e

ct (

a.u

.)

(c)

Figure 2.8: Schematic MC(B) curves of magnetic field effects. (a) Positive and negative lowfield effects. (b) Positive LFE with positive, negative and absent high field effect. (c) Ultra smallfield effect. All figures adapted from [18].

schematically shown in figure 2.9.We first focus on the situation where no external field is applied as displayed

in figure 2.9a. A free electron and hole can form a Coulombically bound e-hpair in either a singlet or triplet configuration, with the statistical ratio of 1:3.The electron and hole are still located on separate sites in the material and cantherefore change their state by means of spin mixing (in this case caused by thelocal hyperfine fields, see section 2.2.4).

From the e-h pair, either an exciton is formed or the e-h pair is dissociatedto free charge carriers. The singlet or triplet state of the e-h pair is conserved inthe exciton formation step. It is now assumed that the formation rate of tripletexcitons is higher than that of singlet excitons. This is indicated by the thick andthin arrows for triplet and singlet formation rates in figure 2.9a. Furthermore, thedissociation rates are assumed to be similar. Note that this is not necessarily thecase. Due to the larger formation rate of triplet excitons, many singlet e-h pairsconvert to a triplet exciton. The statistical ratio of singlet:triplet excitons will thusbe lower than 1:3. The path of of these converted singlet e-h pairs is indicated bythe grey dotted line in figure 2.9a. The recombination mobility of the total systemwill now be increased due to the fact that excitons are not able to dissociate intofree charge carriers. Using equation 2.7, we see that a large recombination mobilityresults in a relatively low current.

We now consider the situation where the spin mixing is suppressed by applyingan external magnetic field, as indicated in figure 2.9b. The energy levels of thetriplet state are now separated due to Zeeman splitting, by an amount ∆E =gµBB. As a result, only one of the energy levels of the triplet state is available forspin mixing. Spin mixing is thus reduced by applying an external field, resultingin an increased current and thus a positive MC(B) with a typical line shape asindicated in figure 2.8a.

As mentioned in section 2.1.5, recombination of singlet excitons leads to theemission of a photon, while for triplet exciton it does not. The amount of emittedphotons thus depends on the ratio between singlet and triplet states and conse-


Free charges

1 3

1

3

B=0

En

erg

y

HF

(a)

Free charges

1 3

1

3

B≠0

En

erg

y(b)

Figure 2.9: The e-h pair mechanism. (a) Without an external field, spin mixing is possiblebetween singlet and triplet e-h pairs. As a result, the singlet e-h pair can follow the faster routeto recombination by converting to a triplet e-h pair. This route is illustrated using the grey dottedline. (b) Applying an external field results in the suppression of spin mixing, causing the singletand triplet e-h pairs to follow their own paths.

quently on the magnetic field. As a result, MFEs are also present in the elec-troluminescence when considering the mechanism described above. Using the e-hpair model, increasing the external magnetic field leads to an increase in singletexcitons and thus an increase in electroluminescence. The MEL(B) caused by achange in spin mixing is thus positive using the e-h pair mechanism. A positiveMEL(B) is indeed observed in measurements, indicating that the triplet excitonformation rate is higher than the singlet exciton formation rate.

Simulations

Besides the proposed empirical functions to describe the MFEs, the typical lineshape can also be retrieved by simulating a two site spin system. Particle interac-tions in such a two site system are mathematically described using the theory ofopen quantum systems. This theory allows the quantum system to interact withan environment. Using the density matrix formalism, which describes the proper-ties of mixed quantum states, an equation governing the system dynamics is found.For a brief description of this formalism and the necessity to use it, we refer toreference [36]. This so-called Stochastic Liouville equation is introduced with con-siderable success in a range of fields. The equation has first been introduced byScully et al. [37] and is given by:

∂ρ

∂t= − i

~[H(t), ρ(t)]− 1

2 {Λ, ρ(t)}+ Γ, (2.12)


in which the left hand term describes the evolution of the density matrix ρ intime. The first right hand term corresponds to the behaviour of a closed system,the second right hand term is a (spin-selective) sink term and the last term is anexternal source term dependent on the state of the system. We note that thisequation is usually solved for steady state systems as typical measurements arealso performed in steady state.

As we stated before, multiple mechanisms have been proposed to describe theOMAR effect. The Stochastic Liouville equation can be used to describe thesemechanisms as they are all based on the reduction of spin mixing by applying amagnetic field. In chapters 5 and 6 we will refer to simulations performed on thee-h pair mechanism. Figure 2.10 schematically shows the e-h pair mechanism andthe parameters relevant in forming and dissociating the singlet and triplet e-h pairand exciton states. Electron hole pairs are generated at a rate r, which is splitbetween the singlet and triplet states as 1:3. The singlet e-h pair can dissociateat a rate qS, while the triplet pair dissociates at a rate qT . From the e-h pair, anexciton can be formed at rates kS and kT for singlet and triplet states, respectively.For a more complete description of the simulated system we refer to reference [36].

Free carriers

Triplet

e-h pair

Singlet

e-h pair

¼r ¾r qT

qS

hf

Exciton states

kS

kT

Figure 2.10: Schematic diagram of the e-h pair mechanism and the relevant parameters informing the various states in the system.

2.2.4 Causes of spin mixing

In the previous section, we discussed the e-h pair mechanism, which uses spinmixing to explain the MFEs observed on small field scales. We mentioned that thelocal hyperfine fields were responsible for spin mixing, however we did not describethe mechanism by which this spin mixing occurs and what these hyperfine fieldsare. Additionally, several other mechanisms can also cause spin mixing, which canbe observed at different field scales.


Hyperfine fields

Many models describing the MFEs use so-called hyperfine fields to explain spinmixing. The hyperfine field is mainly caused by the nuclear spin of hydrogenatoms, which are abundant in organic materials. Because the spatial distributionof hydrogen atoms is random, the hyperfine field is randomly oriented as well. Thelocal hyperfine field now causes a precessional motion of the spin of the chargecarrier(s). The frequency of this precession is given by:

ω = gµBB

~, (2.13)

where g is de Lande factor, µB the Bohr magneton, ~ Planck’s constant and Bthe magnitude of the external field. The Lande factor for an electron spin is wellknown and approximately equal to 2.

Due to its random orientation, the hyperfine field can differ significantly for twonearby sites considered in the hopping process. As a result, the precession of thespin can change the mutual orientation of the spins and thereby allow a transitionbetween singlet and triplet states. Note that the local field at one site is requiredto be sufficiently different from the field at the next site to allow a transition. Thespin precessional motion for two sites with different hyperfine fields is shown infigure 2.11a. The spin of the charge carrier is indicated by the blue arrow, theprecessional motion by the dashed ellipse.

Bhf,1 B

hf,2

(a)

Bhf,1+B

ext Bhf,2+B

ext

Bext

(b)

Figure 2.11: Two sites considered in the hopping process are displayed. (a) The local hyperfinefields Bhf,1 and Bhf,2, as indicated in light grey arrows, cause a precessional motion of the chargecarrier spin as indicated in blue. (b) An external field is now applied, causing the local effectivefield to be oriented less randomly. The precessional motion is indicated by the dashed ellipses inboth figures.

When an external field is applied, the effective field at the two sites is given bythe sum of the external field and the local hyperfine fields. Increasing the externalfield in strength will change the orientation of the effective field to the directionof the external field, until the size of the hyperfine fields is negligible . As thespatial orientation will become less random, the spin mixing will be suppressed.


Additionally, a larger effective field will increase the precession speed and therebyreduce the angle between the spin and the effective field. This is indicated infigure 2.11b, in which the spin precession is shown for an external field in additionto a random hyperfine field.

Spin mixing by hyperfine fields is not only implemented in the previously dis-cussed e-h pair mechanism, but is found in other models as well. The way thecurrent is influenced depends on the exact model. It is important to realise thatlittle control can be exerted over the hyperfine fields and thus over the spin mixing.The hyperfine fields only vary little between different materials and once a devicehas been created, it cannot be changed.

∆g-effect

In addition to spin mixing by hyperfine fields, it can also be caused by the ∆gmechanism. This mechanism is based on a difference in Lande factors between anelectron and a hole. The main difference with the hyperfine interactions is thatthe effect occurs at large instead of small applied magnetic field strengths due tothe small difference in Lande factors.

This mechanism can be understood by considering an electron in the presenceof a magnetic field. The spin of the electron will now start to precess aroundthe magnetic field. Due to the presence of weak spin-orbit interactions in organicmaterials, the Lande factor of an electron polaron will be slightly different for thatof a free electron. Furthermore, the Lande factor of electron and hole polarons willbe different as well.

Due to this difference in Lande factor, the precession frequency of electron andhole polarons will be different. From equation 2.13 we can see that this differencein precession frequency (∆ω) can be expressed as:

∆ω = µBB

~∆g. (2.14)

We immediately see that this frequency difference scales with the applied magneticfield and thus becomes more important with increasing magnetic field strengths.The spin mixing lost by the suppression of the hyperfine interactions can thus beincreased again by this ∆g mechanism. In the e-h pair mechanism this resultsin a negative high field effect. It is negative because spin mixing is increased byapplying a magnetic field, instead of being suppressed by the hyperfine fields.

∆B-effect

Kersten et al. have proposed an alternative mechanism which causes spin mixing[38]. It is based on the difference in effective magnetic fields at two sites consideredin the hopping mechanism. This ∆B mechanism considers the precession frequencyof polarons, as with the ∆g mechanism. This time, the Lande factor is assumed to


be constant and as such we can now consider the difference in precession frequencyas:

∆ω = gµB

~∆B. (2.15)

This mechanism does have some requirements in order to explain magneticfield effects. When considering effects at very low fields strengths, the differencein magnitude of the fringe fields at neighbouring sites must be larger than thedifference in hyperfine field strengths. This must at least be valid for a significantfraction of the sites contributing to the current. A density matrix analysis of thismechanism, see section 2.2.3 for more information, allows the singlet fraction tobe determined as a function of the relative hopping rate, in this case for excitonformation in the e-h pair mechanism.

The relative hopping rate can be defined as the ratio between the hoppingfrequency (equation 2.1) and the precession frequency of a spin in the hyperfinefield (equation 2.13):

r = ωhop

ωhf

. (2.16)

Using this hopping rate we can distinguish a number of regimes. First there isthe slow hopping limit, in which the hopping frequency is much lower than theprecession frequency. In this regime, the maximum amount of spin mixing isachieved because the spins can precess many times before either of them hops tothe next site. On the other hand, in the regime where the hopping frequencyis much larger than the precession frequency no spin mixing can occur. This isexplained by the fact that the spins do not have time to precess before either ofthem hops to the next site. The transition between these two regimes is knownas the intermediate hopping regime. The ultra small field effect we mentioned insection 2.2.2 is attributed to this intermediate hopping regime.

Comparison between the situation where ∆B is caused only by the hyperfinefields and the situation where ∆B is taken statically equal to 10Bhf shows thatthe singlet fraction is different for intermediate hopping rates. The singlet fractionfor these two situations is shown as a function of the hopping rate in the left graphof figure 2.12. As a result, spin mixing is increased by the presence of fringe fieldsin this regime.

Note that the singlet fraction is calculated using a certain ration between singletand triplet exciton formation rates, kS and kT . This ratio is defined as:

γ = kS

kT

. (2.17)

The results in the left graph are calculated using γ = 1/2, indicating that tripletexcitons are formed twice as fast as singlet excitons.

From the singlet fraction as shown in the left graph of figure 2.12, the magneticfield effect can be calculated using:

MFE = χS,hf − χS,fringe

χS,fringe

, (2.18)


10-3 10-2 10-1 100 101 102 10316

18

20

22

24

26

10-3 10-2 10-1 100 101 102 103

-20

-10

0

10

20

30 Fringe fields Hyperfine fields

Sing

let f

ract

ion

(%)

Relative hopping rate r

= 1/2 = 2

Mag

netic

Fie

ld E

ffect

(%)

Relative hopping rate r

Figure 2.12: Left: The singlet fraction is displayed as a function of the relative hopping rate.These curves show the difference between hyperfine and fringe fields. Right: The magnetic fieldeffect is shown as a function of the hopping rate for larger triplet formation rate (γ = 1/2) andlarger singlet formation rate (γ = 2). Figure adapted from [38].

where χS indicates the singlet fraction, either caused by hyperfine fields (subscripthf) or by a static difference in the local field magnitude (subscript fringe). Theright part of figure 2.12 shows this magnetic field effect for γ = 1/2 and γ = 2.Note that a sign change occurs when the singlet exciton formation is larger thanthe triplet exciton formation.

2.2.5 Fringe field MR

We introduced the concept of a ∆B mechanism, however we have not shown howsuch a system might be realised. One way of creating such a system is by intro-ducing (ferro) magnetic nanoparticles, causing inhomogeneous fringe fields [39].Another way is by introducing a ferromagnetic layer close to the organic layer.Such a layer will also provide fringe fields when a domain structure is present inthis layer. The fringe fields caused by the domain structure of a ferromagnetic layerwill depend on the magnetization of the layer. As such, control can be exerted onthe fringe fields by applying an external field which changes the magnetization ofsuch a layer. A domain structure causing fringe fields in a typical device structureis illustrated in figure 2.13a.

The addition of an out-of-plane magnetized ferromagnetic layer to a standarddevice has already been shown to influence the MC when switching the magneti-zation of this layer [40]. In these devices, the distance between the ferromagneticlayer and the organic layer is varied, showing that the effect is strongly dependenton the relative location of the layers. Wang et al. proposed a model in which spinmixing is suppressed in a similar way as applying an external field to hyperfinefields. This model is unsuccessful in describing the observed line widths and lineshapes. Deviations from the regular MC curves are observed for the field strengthswhere the magnetization of the magnetic layer is switched from -MS to +MS. Thisindicates that fringe fields are the cause of the observed deviations, as fringe fieldsare only present when switching the magnetization of an out-of-plane magnetized


Non-magnetic

Spacer

Organic

(a)Magnetic field (T)

Ma

gn

eto

co

nd

uctivity (

%)

-1.0 -0.5 0.0 0.5 1.0

12

10

8

6

4

2

0

100 nm

50 nm

20 nm

15 nm

(b)

Figure 2.13: (a) A schematic representation of a ferromagnetic layer with domains causingfringe fields (dotted lines) in a typical device structure. Figure adapted from [38]. (b) TheMC measurements obtained by Wang et al. showing the influence of a switching out-of-planemagnetized layer.

layer. The MC results obtained by Wang et al. are shown in figure 2.13b. Thedistance between the organic and ferromagnetic layers is indicated in the figure.

We note that the gradients in the fringe fields are an important factor as thedifference in fringe fields (∆B) at neighbouring sites is related to this gradient. Thegradient in the fringe fields is dependent on the distance from the ferromagneticlayer, as is the absolute magnitude of the fringe fields. As a result, the distancebetween the organic layer and the ferromagnetic layer becomes a tool to tweak theeffect of the fringe fields on the MC.

In this project we will use ferromagnetic layers with an in-plane or an out-of-plane magnetization, depending on the device. The resulting fringe fields willdiffer between the two types of layers. The ∆B mechanism can qualitatively beexamined using these device types.


3 Methods

The first section of this chapter provides an overview of the device structures usedin this thesis, which is necessary to understand the device operation. The secondsection will discuss the main measurement techniques used in this thesis, focusingon the electrical and optical characterization.

3.1 Device layout

All devices created for this project have the same general layout. An organic layeris sandwiched between two electrodes, which inject charge carriers into the organiclayer, but also from the contacts of the device. In practice, several additionallayers are added to enhance the performance of the devices. Based on the desiredfunctionality of the devices, several options are available in the choice of substrate,organic material and electrode materials. An artist impression of a typical lightemitting device structure is shown in figure 3.1.

Hole injection layer

Organic layerCathode

Bu�er / electroninjeciton layer

Glass substrate

Anode

Figure 3.1: An artist impression of the structure of a typical organic light emitting diode device.

The standard device layout will be discussed in the following subsection. Modi-fications of this structure are considered in the subsequent subsections. The clean-ing procedure of the substrates is discussed in Appendix A, as are all relevantdeposition techniques. An overview of the materials used in the devices discussedin this thesis is provided in Appendix B, outlining their properties.

27

28 Chapter 3. Methods

3.1.1 Standard device layout

For standard non-magnetic devices, a glass substrate with a pre-patterned indiumtin oxide (ITO) layer is used. A general impression of the device fabrication processis given below, however the fabrication details for each layer are discussed in moredetail in Appendix A.2. Before the actual layers are deposited, the substrate iscleaned according to Appendix A.1.

A layer of poly(3,4-ethylenedioxythiophene) with poly(styrenesulfonate) dopant(PEDOT:PSS) is spin coated on top of the ITO layer in order to increase the holeinjection. Other hole transporting materials like TPD can be used instead of, orin combination with, the PEDOT:PSS layer.

The actual light emitting organic layer (SuperYellow PPV in this thesis) isdeposited on top of the hole transport layer. This is done by means of solutionspin coating.

The organic layer is covered with a thin layer (1 nm) of lithium fluoride (LiF)and a 100 nm layer of aluminum (Al) by thermal evaporation. These layers functionas the top electrode, where the LiF facilitates an easier electron injection. Bothlayers are deposited using a mask, ensuring that only a specific pattern is deposited.

The total layer composition of a standard non-magnetic device is shown infigure 3.2a. A top view of a typical device is shown in figure 3.2b. The colours inboth figures match, showing the ITO layer in light grey and the aluminum layerin dark grey. The active area of the device, where the current flows through theorganic layer, is defined by the crossing of the top and bottom electrodes. Thetypical size of this area is 10 mm2.

Glass substrate

ITOITOPEDOT:PSS

Active layer

LiFAl

200 nm

45 nm

100 nm

1 nm

100 nm

(a) (b)

Figure 3.2: (a) The layers of a standard non-magnetic organic light emitting diode. (b) Topview of a typical device showing two electrodes patterned with an overlap, which defines the activedevice.

The energy levels of a standard device structure is shown in figure 3.3a. LiFis an insulating salt, but a thin layer of the material between the aluminum con-tact and the organic layer is known to change the work function of the combinedLiF/Aluminum layer and improve charge carrier injection.

The important information we can find in these diagrams is that the differencein energy levels must be small enough to allow injection and hopping between the

3.1. Device layout 29

subsequent materials. Note that holes are injected at the ITO/PEDOT:PSS sideof the device while electrons are injected at the LiF/Aluminum side of the device.

3.1.2 Devices with a ferromagnetic layer

In order to investigate the effect of fringe fields on the change in current through adevice, a ferromagnetic layer is added to the standard device structure as describedin the previous section. Several options are available to add such a layer, whichwill be briefly discussed in this section.

In the first experimental investigation, a relatively thick cobalt layer is ther-mally evaporated either at the bottom or at the top of a device. Energy diagramsfor such devices are shown in figure 3.3. Energetically, the behaviour of thesedevices should not change because the additional cobalt layer is always separatedfrom the organic layer by a conductive material, the spacer layer. This is eitherPEDOT:PSS or aluminum, depending on the location of the cobalt layer. Thespacer layer ensures that no direct spin injection can occur, thereby excluding spinpolarization effects.

SY-PPV LiFPEDOTITO

Vacuum level

-4.7-5.1

-2.8

-5.2

Al

-4.1

(a)LiF

Vacuum level

PEDOT

-5.1

SY-PPV

-2.8

-5.2

Al

-4.1

Co

-5.0

Al

-4.1

(b)CoLiFITO

Vacuum level

-4.7-5.0

PEDOT

-5.1

SY-PPV

-2.8

-5.2

Al

-4.1

(c)

Figure 3.3: Band diagrams for multiple device structures. A standard SY-PPV based device(a) is modified by adding a cobalt layer at the bottom (b) or at the top (c).

The relatively thick cobalt layer normally has a total in-plane magnetization,however the thermal evaporation process is known to produce relatively inhomo-geneous layers. As such, magnetic domains are easily formed and these domainscan even have an out-of-plane component. Fringe fields are formed due to domainformation, making these layers suitable for further investigation.

Surface roughness considerations

When creating devices with cobalt positioned at the bottom of the device, we donot need the transparent ITO electrode because cobalt is not transparent. Wetherefore deposit cobalt directly on a glass substrate. Devices created this wayturned out to be shorted or have such high leakage currents that no measurementscould be performed on them. To resolve this issue, an aluminum seeding layer is


added to the device. This seeding layer is already indicated in the energy diagramdisplayed in figure 3.3b.

The addition of this seeding layer has some consequences. A scanning electronmicroscopy image of a 10 nm aluminum layer deposited on a pre-patterned ITOsubstrate is shown in figure 3.4a. It is clear that the aluminum layer shows islandgrowth and that the layer thereby has a significant roughness. The island growthis known to occur independently of the aluminum layer thickness.

(a) (b)

Figure 3.4: SEM images of a 10 nm aluminum layer deposited on (a) a pre-patterned ITOsubstrate and (b) a pre-patterned ITO substrate covered with 10 nm of LiF. Island growth of thealuminum can be discerned, as well as conglomerations of LiF causing additional roughness.

When depositing cobalt layers either at the top and bottom of the device, theyunderlying surface is always rough. Depositing cobalt at the top of a device resultsin an even less well-defined surface, because not only the aluminum contributesto the roughness but the organic layer as well. The morphology of the organiclayer is also influencing the aluminum growth pattern, as well as the presenceof LiF. Inspection of a 10 nm LiF deposition on top of an ITO substrate showsconglomeration of the LiF. A SEM image of such a deposition, covered with 10 nmof aluminum, is shown in figure 3.4b. The LiF layer in our devices typically hasa thickness of only 1 nm, however it might still lead to an additional contributionto the roughness.

Top positioning

The easiest way to create devices with a cobalt layer is to simply evaporate such alayer on top of a normal stack. This can be done in addition to the normal stack,or by replacing the top electrode. Because fringe fields rapidly decay in strengthwith disstance, the distance to the organic layer should be minimized. To thatend, it is preferable to replace the top aluminum layer with a cobalt layer.

Devices are created with a cobalt layer instead of the standard aluminum layer,using either SY-PPV or Alq3 as the organic layer. These devices were all shorted or

3.1. Device layout 31

had very high leakage currents without any injection current, which is attributedto cobalt diffusing into the organic layer. To prevent such damage, a normaldevice with an aluminum top electrode was created with a cobalt layer on top ofthat structure. The aluminum layer was reduced in thickness to limit the spacebetween the cobalt and organic layers. The composition of the standard andmodified devices are schematically shown in figures 3.5a and 3.5b.

Glass substrate

ITO

ITOPEDOT:PSS

Active layer

LiF

Al

200 nm

45 nm

100 nm

1 nm

100 nm

(a)

Cobalt 15 nm

Glass substrate

ITO

ITOPEDOT:PSS

Active layer

LiF

Al

200 nm

45 nm

100 nm

1 nm

x nm

Al 100 nm

(b)

Cobalt 30 nm

Aluminum ~100 nm

Glass substrate

ITOPEDOT:PSS

Active layer

LiF

Al

45 nm

100 nm

1 nm

100 nm

(c)

Figure 3.5: Schematical representations of the layers used in the devices discussed in thissection. (a) A default structure with non-magnetic electrodes. (b) Compared to the defaultdevice, the aluminum layer is reduced in thickness to x nm, a cobalt layer is added as well as anadditional aluminum capping layer. (c) Compared to the default device, the bottom ITO layerhas been changed to a cobalt layer on top of an aluminum wetting layer.

Long deposition timeIn first instance the cobalt layer was grown over a period of approximately one

hour. This long period was necessary because the cobalt could not be heated toomuch, as it would then react with the Tungsten boat holding it. Because largeamounts of power are required to significantly heat the cobalt in order to evaporateit, all elements in the vacuum chamber are exposed to large amounts of heat inthe evaporation process.

Measurements of the magnetic field dependence, as well as IV measurements,show that the modified devices are damaged. The IV curves show a much higherleakage current while the field dependent measurements show results which area factor 30 smaller. These results indicate that the organic layer is most likelyinfluenced by heat exposure during the cobalt deposition process.

Short deposition timeIn order to limit the heat exposure, the cobalt layer should be deposited at a

much higher rate. This is achieved by replacing the default disposable Tungstenboat by a much thicker, re-usable Tungsten boat. As a result, the used depositionpower and thus temperature of the cobalt could be greatly increased. Devicescreated this way do not show the large changes in electronic behaviour and MCmagnitude as observed before.


Because this device structure allows easy tuning of the aluminum spacer layerbetween the organic and cobalt layers, devices are created with varying aluminumthicknesses. Results concerning these devices will be discussed in section 5.4.

Bottom positioning

Alternatively, the ferromagnetic layer can be used at the bottom side of the de-vice. As mentioned before, an aluminum seeding layer is added to make sure thesedevices are stable and noise in the electrical signal is limited. Schematic represen-tations of the default device structure and the modified device structure with acobalt bottom electrode are shown in figures 3.5a and 3.5c.

Energetically, little to no difference should be noticeable when inspecting theIV curves. Injection should be achieved at roughly the same voltage, becausethe PEDOT:PSS layer can be considered metallic and the work function of theunderlying layers should therefore not be of major influence. The (metallic) layersadjacent to the organic layer are equal in all device types, an additional cobalt layeris thus not expected to have a major influence due to the relatively thick spacerlayers. Measurements performed on these devices are discussed in chapter 5.

Out-of-plane magnetized ferromagnetic layers

Besides these thick cobalt layers, it is also possible to add multilayer stacks with anout-of-plane magnetization. These layers are sputtered at the physics departmentas described in Appendix A.2.4. Typical material combinations to create thesemultilayer stacks are cobalt and platinum or cobalt and nickel. A capping layer istypically used to make sure the top layer(s) of the multilayer are not affected byoxidation.

The exact composition of the multilayer stack is shown in figure 3.6, whichdisplays the full device based on this multilayer. A 20 nm tantalum seeding layeris used, which is capped with a 4 nm platinum layer. Ten repetitions of cobalt/ platinum (0.4 / 1.2 nm) are then deposited on top of the platinum. These arethe layers that actually cause the magnetization to be perpendicular. A final layeris added to protect the multilayer from oxidation. Spin coating was found to beeasier on top of aluminum than on platinum, so the capping layer is made of analuminum layer of 2 nm. Note that the final platinum layer is twice as thick,functioning as an extra protection of the cobalt/platinum multilayer.

A layer of PEDOT:PSS is typically spin coated on top of these multilayers,making sure that hole injection is no problem. As an additional advantage, theenergetic behaviour of these devices should be similar to the standard devicesbecause the PEDOT:PSS is fully conductive. The work function of the multilayerstack is therefore of no importance, as injection into the organic material occursthrough the PEDOT:PSS.

3.2. Device characterization 33

Glass substrate

ITOPEDOT:PSS

SY-PPV

LiF

Al

45 nm

100 nm

1 nm

100 nm

Ta 20 nm

Co/Pt (0.4/1.2)10

nm

Pt 4 nm

Pt 1.2 nm

Al 2 nm

Figure 3.6: Device structure with a perpendicularly magnetized bottom electrode.

3.2 Device characterization

The devices created in this thesis are mainly characterized by electrical measure-ments with some additional optical measurements. Because other characteristicsare also important, the additional measurement techniques used in this thesis arebriefly explored in Appendix C.

3.2.1 Electrical characterization

It is important that the current through the devices used in this thesis is stable,because we are measuring the electrical properties as a function of the appliedmagnetic field. To measure the electronic behaviour of a device, the top andbottom electrodes (or the cathode and anode as shown in figure 3.1) are connectedto a source-meter (Keithley 2400). A voltage is then applied over the two electrodesand the current is measured as a function of either voltage or time. The setupused to do these measurements is schematically shown in figure 3.7a.

If the voltage is swept from 0 to a certain voltage V dependent on the devicetype, several contributions to the current can be distinguished, see figure 3.8a.Some leakage current will always be present due to pinholes and other parallelcharge transport paths. The magnitude of this leakage current is determinedby the amount of pinholes and generally increases as the organic layer becomesthinner. This leakage current can be modelled as being linear with the voltageand thus with a constant resistance. This is not entirely accurate, but will sufficeas the leakage current is of no interest for the device properties.

Once charge carriers are efficiently injected into the organic material, the in-jection current will become dominant. One can distinguish unipolar and bipolarcurrents as discussed in section 2.1. These current contributions are schematicallyshown in figure 3.8a. The sum of these current types is also shown. Note thata linear leakage current and a quadratic trap-free unipolar current is assumed,while traps are assumed to be present in the bipolar current. As discussed in sec-


Bottom electrode

Top electrode

Organic layer(s)

Source-meter

A

(a)

BSample

A

Source-meter

Photodiode

A

Currentmeter

(b)

Figure 3.7: (a) A schematic representation of the setup used to measure the device current.(b) Schematic representation of the setup used to measure magnetic field effects. Figure adaptedfrom [18].

tion 2.1.4, the voltage dependence in that case is larger than quadratic, here setto a third power for illustration.

A bipolar device can have both unipolar and bipolar contributions; howeverthese contributions are not necessarily both discernible as the leakage current canbe larger in magnitude than the unipolar current. The injection current in abipolar device can thus consist of one or two contributions on top of the leakagecurrent. The latter case is shown in figure 3.8a. The general shape of the IV-curvecan now be used to verify the correct operation of a device.

Drift correction

When measuring the current as a function of time, at a constant voltage, a drift inthe current will be visible. Several components with different time constants arepresent in this drift, ranging from short to long periods [20]. It is important tokeep this drift in mind when varying external parameters during a time-dependentmeasurement. In order to reduce the drift, the devices are ’conditioned’ by apply-ing a constant voltage or current for a certain time. Usually this conditioning isperformed for a period in the order of an hour. As a result the drift is reduced.Furthermore, it has been observed that conditioning generally (greatly) increasesthe magnetic field effects in these devices.

In a typical measurement, a constant voltage or current is applied for a certaincharge time before the actual measurement is performed. The fast components ofthe drift occur in this period, which leaves only slow components in the measure-ment. These components can then be compensated, either with an exponential orparabolic correction in the time domain. An example of such a typical measure-ment curve is shown in figure 3.8b.


0 1 2 3 4 5 6 7 8 9 10100

101

102

103

104

105

106

Cur

rent

den

sity

(a.u

.)

Voltage (V)

Total current Leakage current Unipolar current Bipolar current

(a)

Measurement

Exponential fit

MeasurementCharging

0 5 10 15 20 25 300.56

0.57

0.58

0.59

0.60

0.61

Cu

rre

nt (m

A)

Time (min)

(b)

Figure 3.8: (a) A schematic display of the various contributions to the measured device current.(b) A device is charged so only slow time components in the drift current remain.

Magnetic field effects

The magnetic field effects occurring in organic devices are measured by applyinga magnetic field to the devices. The setup used to measure these MFEs consistsof a cryostat (only used at room temperature in this thesis) in which the sam-ple is placed. A coil magnet is placed around the cryostat, which is able to beswept from -500 mT to +500 mT. The sample is connected to a source meter asdescribed in section 3.2, allowing the application of specific voltages or currents.The sample is placed between an electromagnet while it is connected to a source-meter. Additionally, a photodiode is present which can be used to measure theelectroluminescence. The photodiode generates a current which is measured usinga current meter. A schematic representation of the setup is shown in figure 3.7b.

Note that the sample can have several orientations with respect to the exter-nally applied magnetic field. In chapters 5 and 6 we will often refer to two distinctorientations, namely parallel and perpendicular. In the first case, the external mag-netic field is oriented along the surface of the device. This is schematically shown infigure 3.9a. In the latter case, the external magnetic field is oriented perpendicularto the surface of the device. This is schematically shown in figure 3.9b.

A comparison between magnetoconductance measurements in the perpendicu-lar and parallel orientations can be made. In order to make a good comparison,we first introduce the absolute difference between both orientations:

∆MC = MC‖ −MC⊥. (3.1)

We now divide this difference in MC by the absolute value of the MC in the parallelorientation at 500 mT. When comparing measurements performed at differentvoltages, the voltage dependence present in the MC at 500 mT as measured in theparallel orientation is now taken into account. Any remaining voltage dependencecan be attributed to either an anisotropic voltage dependence as a function ofthe magnetic field, or to an additional (anisotropic) contribution caused by an


B

n

B

(a)

B

n

B

(b)

Bθ

nx

y

z

(c)

Figure 3.9: (a) In the parallel orientation, the magnetic field is oriented along the surface ofthe device. (b) In the perpendicular orientation, the magnetic field is oriented perpendicular tothe surface. (c) The measurement setup as used while measuring the angle dependence.

additional ferromagnetic layer. We introduce this quantity as:

δMC = ∆MC

MC‖,500mT· 100% = MC‖ −MC⊥

MC‖,500mT· 100%. (3.2)

This equation will be used in chapter 5, where we analyze the influence of aferromagnetic layer on the magnetoconductance. Additionally, it will be used inchapter 6, where we analyze the angle dependence of a reference device.

In order to measure angle dependent magnetic field effects at a certain exter-nally applied magnetic field, a step motor is installed in the measurement setupdescribed above. This step motor allows the sample to be rotated 360 degreesin 400 discrete steps. A schematic drawing of this setup is shown in figure 3.9c,showing the sample which can be rotated around the horizontal axis as indicated.This horizontal axis corresponds to the z-direction in the measurement setup. Themagnetic field ~B and the angle with the normal ~n of the device are also shown inthis figure. When the sample rotates, the angle θ between the magnetic field andthe normal changes. The perpendicular orientation corresponds to θ = 0◦, whilethe parallel orientation corresponds to θ = 90◦.

Magnetic field dependence at constant angleMagnetoconductance measurements can easily be carried out by applying a

bias voltage and sweeping the magnetic field. This is done in a forward sweepdirectly followed by a backward sweep. An example of this process is shown infigure 3.10a. As mentioned before, a drift in the current occurs and should becompensated.

Because a magnetic field is applied, only data points with the same applied fieldare used to fit a function (exponential or parabolic) for drift correction. An exam-ple is shown in figure 3.10b, where three exponential fits are performed. The fit isthen subtracted from the measured current and finally, the forward an backwardsweep are averaged. This averaged current is then used to calculate the magneto-


-500 -400 -300 -200 -100 0 100 200 300 400 5001.14

1.16

1.18

1.20

1.22

1.24

1.26

1.28

1.30

1.32

Backward sweep

Cur

rent

(A

)

Field (mT)

Forward sweep

(a)

0 1 2 3 4 5 61.14

1.16

1.18

1.20

1.22

1.24

1.26

1.28

1.30

I=I3-A3e-t/t3

I=I2-A2e-t/t2

Cur

rent

(A

)

Time (min)

I=I1-A1e-t/t1

(b)

-500 -250 0 250 500-10123456789

1011

MC

(%)

Field (mT)

(c)

Figure 3.10: (a) Forward and backward sweeps of the magnetic field at a constant applied biasvoltage. (b) Three exponential decaying fits are made on the measurement data to determine thedrift. (c) The resulting magnetoconductance after the drift current has been eliminated. Imagesbased on [18] but created with own data.

conductance using equation 2.8. An example of such a final magnetoconductanceresult is shown in figure 3.10c.

It is also important to note that a number of point distributions are availablewhen sweeping the magnetic field. For small scales, a linear distribution maybe sufficient, however, when various field scales of the different magnetic fieldeffects are present, an exponential point distribution will generate far better results.Especially the ultra small field effect can simply be missed when doing a full 500mT sweep with a linear distribution, due to the small field size. Figure 3.10c showsa measurement using an exponential point distribution.

When the measurements are completely processed, we can introduce a normal-ization function based on the maximum MC in the parallel orientation:

Normalized MC⊥,‖ = MC⊥,‖

MC‖,500mT, (3.3)

where MC⊥,‖ represents the MC in either the perpendicular or the parallel orienta-tion and MC‖,500mT the MC at 500 mT in the parallel orientation. By normalizingthe MC this way, we can easily compare the shape of the MC between various


devices in either the parallel or perpendicular orientation.

Angle dependence at constant fieldAs mentioned before, a step motor is used in order to rotate the sample in

a static magnetic field. Due to the stepwise rotation, the sample endures smallbumps as it accelerates and stops during rotation steps. These bumps in turn canlead to movements in the electrical contacts with the device, causing noise in themeasured electrical signal. This noise can be observed in the measurements. Byaveraging the measurement over a number of turns, this noise is then reduced.

The current is directly measured while rotating the sample over several turns.Before starting the rotation, the current is turned on for a period of 400 seconds sothe fast component of the drift does not influence the measurement. During thisinitial period, the device is positioned perpendicularly to the external field, whichis designated as ’0◦’. An example of a raw measurement is shown in figure 3.11a,in which the rotation is started at 400 seconds. The measurement in the figure isperformed on a reference device at 5V with an external magnetic field of 500 mT.The drift-corrected data of this measurement is shown in figure 3.11b, in whichan exponential correction based on the period between 400 and 1000 seconds isapplied.

To further enhance the measurements, the drift-corrected current is expressedas a function of the measurement angle. This data is then averaged over a numberof 180◦ or 360◦ periods to reduce the noise level. Note that averaging over 180◦is only valid when the angle dependence and the actual rotation movement isassumed to be symmetric. Figure 3.11c shows the drift-corrected measurement ofthe example measurement as a function of the angle, averaged over a period of360◦.

To finally present the data, the current is expressed as a percentage of thecurrent at 0◦ using the following expression:

δI = I (θ)− I (0◦)I (0◦) · 100%. (3.4)

This equation is applied to the averaged current from the example measurementmentioned above, resulting in the right hand axis in figure 3.11c. Typical intrinsicchanges in the current due to rotation of the sample in the magnetic field are inthe order of 0.05 %. Note that this value applies to an external magnetic fieldof 500 mT and an applied voltage of 5 V on a reference SY-PPV device. Thetotal magnetoconductance in such a device at the same conditions is 4.5 % to 5%, roughly two orders of magnitude larger.

Equation 3.4 can be linked to equation 3.2 by inspecting the definitions usedfor the quantities used in their definitions. It is found that δMC at a specificexternal field B is proportional to δI at the same field as measured at 90◦. Anadditional factor is included to take the parallel MC at 500 mT into account, aswell as the change in current induced by applying a magnetic field. This can be


0 200 400 600 800 1000455

460

465

470

475

480

485

490

495

Cur

rent

(A)

Time (s)

(a)

400 500 600 700 800 900 1000

453.40

453.45

453.50

453.55

453.60

453.65

Cur

rent

(A)

Time (s)

(b)

0 60 120 180 240 300 360

453.40

453.45

453.50

453.55

453.60

453.65

Cur

rent

(A)

Angle (degrees)

0.00

0.01

0.02

0.03

0.04

0.05

I (%

)(c)

Figure 3.11: The current as measured while rotating a reference sample with respect to theexternal magnetic field. The measurement is performed at 5V and a field of 500 mT. (a) Theraw current as measured. (b) Drift corrected current, using an exponential correction. (c) Thecurrent averaged over a period of 360◦. The absolute current is shown on the left axis, while thechange in current with respect to the current at 0◦, δI, is shown on the right axis.

written as:δMCB = IB (0◦)

I0 (0◦) · δIB (90◦)MC500mT (90◦) . (3.5)

The full derivation is given in Appendix D.On a final note, these measurements can also be performed at constant current

instead of constant voltage. In that case, equation 3.4 is replaced with:

δV = V (θ)− V (0◦)V (0◦) · 100%. (3.6)

3.2.2 Optical characterization

Due to the fact that the cryostat is almost completely free of light, sensitive elec-troluminescence measurements can also be performed in this setup. The light


produced by a device is detected by a photodiode, translating the light into acurrent. This current is measured using a very sensitive current meter (Keithley6430), enabling the measurement of the luminescence L. The luminescence canthus be measured as a function of voltage, time, externally applied magnetic fieldand the angle between the field and the sample, using the same setups as describedin the previous section.

By sweeping the magnetic field while measuring the photocurrent at a constantvoltage, magneto electroluminescence (MEL) effects can be measured. Because theapplied current is also measured, both the MC and MEL are measured simulta-neously. The same drift correction procedure as used for magnetoconductancemeasurements is used, however, equation 2.9 is used instead of equation 2.8 toshow the magnetic field dependence.

Furthermore, angle dependence measurements can also be performed, using thesame procedure as described in the previous section. A constant voltage or currentis applied to the sample, while the emitted light is measured using a photodiode.By rotating the sample by means of the step motor, the angle dependence in theMEL is measured. Equation 3.4 is now replaced by:

δL = L (θ)− L (0◦)L (0◦) · 100%, (3.7)

in which the current is replaced with the luminescence.

3.2.3 Other measurement techniques

Besides the electrical and optical measurements we perform on the devices dis-cussed in this thesis, we also use a number of other techniques. The magneticproperties of the ferromagnetic layers used in our devices are characterized usingthe Magneto Optical Kerr Effect (MOKE) and the Superconductive Quantum In-terference Device (SQUID). Furthermore, the surface of certain non-organic layersis inspected using Atomic Force Microscopy (AFM) and Scanning Electron Mi-croscopy (SEM). The thickness of organic layers is finally measured using stylus-based profilometry. These techniques are all discussed in Appendix C, except forSEM. If the reader is interested in this technique, we refer to [41].

4 Ferromagnetic structures forfringe field MR

The measurements discussed in the following chapter have all been performed ondevices modified to contain a ferromagnetic layer. The influence of these ferromag-netic layers is directly coupled to their magnetic properties. In order to characterizethe ferromagnetic layers used in this chapter, several techniques are utilized to ex-amine the surface and the magnetic properties of the layers. To get an impressionof the fringe fields caused by a relatively thick cobalt layer, the magnetization of acobalt layer is simulated. From these simulations, the fringe fields are calculated asa function of the distance from the cobalt layer. The chapter is finally concludedby a summary of the main findings of this chapter.

4.1 Introduction

The influence of ferromagnetic layers on the magnetoresistance is thought to becaused by fringe fields. These fringe fields are caused by domains present in theferromagnetic layer. It is therefore important to characterize the ferromagneticlayers we will be using in the next chapter. Properties like the surface roughnessand the magnetization in the parallel and perpendicular orientations are thereforeinspected using atomic force microscopy (AFM), the magneto optical Kerr effect(MOKE) and a superconducting quantum interference device (SQUID). The re-sults of these characterizations are presented in sections 4.2 and 4.3 for certaincobalt layers and in section 4.4 for a ferromagnetic multilayer stack.

Furthermore, the behaviour of fringe fields is of great importance to understandtheir influence on the magnetoconductance. We therefore perform some basiccalculations on the magnetization of a cobalt layer using LLG simulation software.From these simulations we will then determine the fringe fields as a function of thedistance from the cobalt layer. The results from these simulations are discussedin section 4.5.

41

42 Chapter 4. Ferromagnetic structures for fringe field MR

4.2 Cobalt deposited on a glass substrate

In order to correctly characterize the cobalt layers, samples are created in the sameway as fully functional devices, however, no organic materials or top electrodesare deposited. The resulting layers of these samples consist of either only a 30 nmcobalt layer (indicated by Co) or a 135 nm aluminum layer with a 30 nm cobaltlayer on top (indicated by AlCo). The cobalt layers in both sample types aredeposited at the same time, and should thus be equal in thickness.

4.2.1 Atomic force microscopy

The surface roughness of the Co and AlCo layers is investigated using AtomicForce Microscopy (AFM). For reference purposes, a standard ITO substrate wasalso measured. The measurements on the cobalt layers are performed using thecontact mode, the ITO substrates are measured using the tapping mode.

The Co layer shows a relatively flat surface with a maximum height differenceof 4 nm on a surface area of 1 µm x 1 µm. No large patterns can be discerned on thesurface, only a small grain-like structure. The AFM scan is shown in figure 4.1a.

4 nm

0 nm

(a)

15 nm

0 nm

(b)

8 nm

0 nm

(c)

Figure 4.1: AFM scans of several layers. (a) 30 nm cobalt directly deposited on glass. (b) 30nm cobalt deposited on a 135 nm layer of aluminum. (c) Pre-patterned ITO layer.

The AlCo layer shows features with a maximum height difference of 15 nm,again on a 1 µm x 1 µm surface area. This surface does show larger, flake-likepatterns, on the scale of a few hundred nanometres. It is known that aluminumdepositions created using thermal evaporation show island growth in the order of10-100 nm. This has been confirmed by SEM imaging, where a 10 nm aluminumlayer was deposited on a substrate. See section 3.1.2 for more details. The AFMscan is shown in figure 4.1b.

The pre-patterned ITO layer used in default devices shows a maximum heightdifference of 8 nm on a 10 µm x 10 µm surface area. We should note that dueto the large area being scanned, a deformation in the scan occurs and the actualheight difference is smaller. This surface shows flake-like patterns, but smallerthan on the AlCo layer, as seen in figure 4.1c.

4.2. Cobalt deposited on a glass substrate 43

In conclusion, we found that cobalt layers directly deposited on a glass substrateare significantly less rough than a cobalt layer deposited on an aluminum wettinglayer or a standard ITO layer. Additionally, the aluminum wetting layer has aprofound influence on the surface structure of cobalt layer deposited on top of it.

4.2.2 In-plane Magneto Optical Kerr Effect

To investigate the in-plane switching behaviour of the thermally deposited cobaltlayer, in-plane MOKE measurements are performed. During the measurement,the external field is swept from -5 mT to +5 mT. The magnetization of the cobaltlayer is then measured as a function of this external field. The results for both theCo and AlCo layers are shown in figure 4.2.

-5 -4 -3 -2 -1 0 1 2 3 4 5

-1.0

-0.5

0.0

0.5

1.0 Aluminum (135 nm) + Cobalt (30 nm) Cobalt (30 nm)

MO

KE S

igna

l (a.

u.)

Field (mT)

backward sweep

forward sweep

Figure 4.2: MOKE measurements of a 30 nm cobalt layer thermally evaporated directly on glass(black circles) and on top of an aluminum spacer layer of 135 nm (red triangles).

The hysteresis loop of the Co layer is shown in figure 4.2 using black circles.The coercive field of this layer is found to be 1.4 mT. Before the magnetizationfully switches, a decrease in magnetization is visible, indicating that the externalfield is not applied exactly along the easy axis of the cobalt layer. This can becaused either by misaligned in the measurement setup, or a preferential direction(easy axis) of the magnetization which is not directed along the surface.

The hysteresis loop of the AlCo layer is shown with red triangles in the samefigure. The coercive field of this cobalt layer is found to be 1.9 mT. In comparisonto the Co layer this is higher, however the more direct switching indicates that thefield is now fully directed along the easy axis of the cobalt layer.


4.2.3 Superconductive Quantum Interference Device char-acterization

The in-plane magnetization of a AlCo layer is already determined using the MOKEtechnique, however the MOKE measurements have a significant amount of noisepresent in the signal. Additionally, out-of-plane magnetization measurements arenot possible using the MOKE setup because relatively large fields are required tofully pull an in-plane magnetized layer out-of-plane. Due to this fact, the MOKEsetup we use is unsuitable for these measurements as it is unable to reach fieldslarger than a few hundred militesla. The Superconductive Quantum InterferenceDevice (SQUID) on the other hand can reach fields of ± 7 T.

To improve accuracy and also determine the out-of-plane magnetization of theAlCo layer, squid measurements are performed in the next two subsections.

In-plane measurement

A SQUID measurement with a logarithmic point distribution in the magnetic fieldis performed on an AlCo layer. The result is shown in the left part of figure 4.3.The forward sweep is shown in the black circles while the backward sweep is shownin red circles.

-15 -10 -5 0 5 10 15-1.0

-0.5

0.0

0.5

1.0

SQU

ID s

igna

l (a.

u.)

Magnetic field (mT)-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

in-plane

Magnetic field (T)

out-of-plane

-6 -4 -2 0 2 4 6-1.0

-0.5

0.0

0.5

1.0

SQU

ID s

igna

l (a.

u.)

Magnetic field (T)

Figure 4.3: SQUID measurements of an AlCo sample. Left: in-plane measurement for fieldsranging from -20 mT to +20 mT. Right: out-of-plane measurement for fields from -2 T to +2T. The inset displays a fast sweep from -7 T tot +7 T, showing that saturation actually occursat 1.6 T.

We first note the differences with the MOKE measurements performed on asimilar layer, created on the same substrate. A less direct switching from negativeto positive saturation is observed, showing a gradual switch with a coercive fieldof 1.2 mT. This coercive field is in the same order of magnitude, however it issmaller then measured in the MOKE setup. This can be explained by the relativeinaccuracy of the magnetic field sensor in the MOKE setup. Furthermore, we notethat the remanence is approximately 50% where it was 100% while measured in

4.3. Cobalt deposited on top of an organic layer 45

the MOKE setup. This can be explained by the fact that a MOKE measurementis performed on short timescales (a minute for a full sweep) while a SQUID mea-surement is performed on longer time scales (4 hours for a full sweep). The cobaltlayer thus has time to equilibrate during a full sweep in the SQUID, allowing aless direct switching.

Out-of-plane measurement

Assuming an ideal in-plane magnetized layer and using the Stoner-Wohlfarth the-ory, the magnetization should depend linearly on the external perpendicular field.Once the magnetization of such an ideal system is fully out-of-plane, saturation isreached. As the thermally deposited cobalt is expected to be magnetized in-plane,such a linear relation is expected to be found using out-of-plane measurements.

A SQUID measurement with a field range of 2 T is performed on an AlColayer, with a quick measurement at a maximum field of 7 T to confirm that satu-ration is achieved. The results of both measurements are shown in the right partof figure 4.3. Saturation occurs at approximately 1.6 T, which is not unexpectedfor an in-plane magnetized cobalt layer. Before saturation is reached, however,two regimes can be distinguished where only one is expected. For small fields, themagnetization changes rapidly. For fields between 0.3 T and 1.6 T, the magneti-zation changes linearly, as expected from theory. Tangent lines are drawn in bothregimes, emphasizing the field ranges of the fast and slow change in magnetization.

A possible explanation for the rapid change for small fields can be found inthe fact that the aluminum seed layer is very rough as discussed in section 4.2.1.Because clusters of cobalt are formed with different orientations, an angle betweenthe surface of the device and the surface of the cluster is introduced. The orien-tation of the magnetization of such a cluster is thus also different, allowing themagnetization to locally have an out-of-plane component with respect to the de-vice surface. If this is assumed, clusters with an out-of-plane component can bepulled out-of-plane more easily than a fully in-plane magnetized layer. Clusterswhich have no out-of-plane magnetization component then require a higher exter-nal field to be pulled out-of-plane, resulting in the expected linear change of themagnetization.

4.3 Cobalt deposited on top of an organic layer

In some of the devices discussed in the next chapter, a cobalt layer will be depositedat the top of a full device structure. An extra surface roughness is thereby possiblyintroduced, due to the disordered organic layer which is now located beneath thecobalt layer. An aluminum spacer layer will be deposited between the organicand cobalt layers as discussed in section 3.1.2, however for thin aluminum filmsthe organic surface may still influence the cobalt layer. As a result, the domainformation in the cobalt layer may be influenced. It is therefore important to


inspect the magnetic behaviour of these devices. Note that an aluminum layer isalso deposited on top of the cobalt layer in order to reach a thickness of at least100 nm for the top electrode. Due to this capping layer, MOKE measurementscannot be performed because the laser used in MOKE measurements has a limitedpenetration depth of approximately 15 nm.

The aluminum spacer layer is varied in thickness, using values of 10, 25, 50 and100 nm. Because the spacer layers were grown in separate steps, they are exposedto the environment and thereby a parameter of influence is introduced. The cobaltlayers of these devices were all grown at the same time, after which they weredirectly covered with an aluminum capping layer. The cobalt is thus not directlyexposed to the environment because the vacuum was not broken between thedeposition steps. Out-of-plane SQUID measurements as discussed in the previoussection are repeated for devices with a cobalt layer on top. The results of theSQUID measurements are shown in figure 4.4.

-2000 -1000 0 1000 2000-1.0

-0.5

0.0

0.5

1.0

SQU

ID s

igna

l (a.

u.)

Field (mT)

100 nm 50 nm 25 nm 10 nm

Figure 4.4: SQUID measurements of devices with a cobalt layer on top of an organic layer. Analuminum layer with varying thickness is sandwiched between the organic and cobalt layers, seelegend.

From this figure, we see that the difference in spacer layer thickness has aneffect on the magnetization of the cobalt layer. However, no clear relation canbe found between the thickness and the change in shape. One of the features wecan distinguish is the fact that with a 10 nm spacer layer, the cobalt layer reachessaturation at a field of 1.3 T. For the other spacer layer thicknesses, saturation isreached at approximately the same value 1.7 T.

Inspection of the line shapes shows that the two regimes, as discussed in theprevious section, are present for all spacer layer thicknesses. The rate at whichthe magnetization changes at small fields differs significantly between the devices,without a clear relation to the spacer layer thickness. The fast regime does occurat the same range of field strengths, resulting in different slopes of the slow regimeas a different part of the magnetization remains to be pulled out-of-plane.

4.4. Cobalt-Platinum multilayer stack 47

4.4 Cobalt-Platinum multilayer stack

In addition to in-plane magnetized cobalt layers, we will also use a multilayerstack with perpendicular magnetic anisotropy (PMA). Because the magnetiza-tion is now perpendicular to that of the cobalt layers we discussed so far, wecan now use MOKE to measure the out-of-plane magnetization. Additionally, aKerr microscope can be used to visualize the domains present when switching themagnetization.

4.4.1 Out-of-plane Magneto Optical Kerr Effect

We use the polar MOKE technique as described in Appendix C.1.1 to check amultilayer structure for correct out-of-plane behaviour. The composition of themultilayer structure is discussed in section 3.1.2. Two measurements are per-formed, before and after a 5 minute cleaning operation using isopropanol in anultrasonic bath. The perpendicular magnetization as a function of the externallyapplied magnetic field is shown in figure 4.5 for both measurements.

-60 -40 -20 0 20 40 60

-1.0

-0.5

0.0

0.5

1.0 Before cleaning After cleaning

MO

KE s

igna

l (a.

u.)

Field (mT)

Figure 4.5: MOKE measurements of a Co-Pt multilayer stack, showing an out-of-plane magne-tization. Measurements from before and after a cleaning step are shown, only a minimal changein the magnetic behaviour is observed.

The magnetization is indeed shown to be perpendicular, validating the chosenmultilayer structure as a PMA electrode. The magnetization starts to switch ata few militesla, quickly reaching the point where no nett magnetization remains(M = 0) at 10 mT. After that point, the magnetization is directed in the otherdirection, reaching the saturation magnetization at 50 mT. Only a minimal changeis observed due to the cleaning step, indicating that the layers are not damaged.After cleaning, the magnetization starts changing slightly earlier, indicating thatextra nucleation points are created, allowing domains to be formed at lower fields.


The multilayer stacks we will use in the next chapter will have a slightly differ-ent composition due to differences in the growth process. As a result, the switchingbehaviour of the magnetization is changed to somewhat higher field strengths. NoMOKE measurements have been performed on these multilayer stacks.

4.4.2 Kerr microscopy

A multilayer structure with more repetitions (15) is inspected using a Kerr mi-croscope. In this device, the Kerr rotation used in the MOKE is applied to amicroscope. As a result, the magnetization of a surface can be inspected insteadof a single point. The purpose of these measurements is to visualize the domainstructure, the difference in the composition of the multilayer is therefore of minorimportance.

Figure 4.6 shows the gradual switching of domains in the multilayer device.A positive field of 80 mT is applied perpendicular to the device structure. As aresult, few to no domains will be present. The field is then slowly swept from +80mT to -80 mT. In the range of +2 to -20 mT, the domains switch from a positiveto a negative magnetization in the z-direction. At -20 mT, only few domainsremain and the magnetization is almost fully switched. From these images we canthus conclude that domains actually form when switching the magnetization ofthe multilayer structure.

Figure 4.6: Kerr microscope images of the out-of-plane switching magnetization of a multilayerstack. The applied magnetic field is swept from +80 mT to -80 mT, showing a switch occurringbetween approximately +2 and -20 mT.

4.5. Simulation of fringe fields from a cobalt layer 49

4.5 Simulation of fringe fields from a cobalt layer

Experimental characterization of thermally evaporated cobalt layers has given usinformation about the switching behaviour of the magnetization, however we havenot gained insight in the fringe fields caused by such a cobalt layer. Unfortunatelywe do not have the required equipment to experimentally obtain information onthe domain structure and the accompanying fringe fields, which can be calculatedusing the domain structure. An implementation of such a calculation is given inAppendix E. In collaboration with Kersten et al., this implementation was usedto calculate the fringe fields of the perpendicularly magnetized electrodes as usedby Wang et al. based on the XMCD data obtained from Wang et al. [40].

4.5.1 Simulation considerations

In order to get an impression of the domain structure and the accompanyingfringe fields of a relatively thick cobalt layer, we can use simulation software. Tothis end, we use the LLG Micromagnetics Simulator (2.63c). This software isbased on the Landau-Lifshitz-Gilbert equation to calculate the magnetization of amagnetic material. Parameters like an external field can be changed, for exampleto find the response of the magnetic material to this external field in terms of themagnetization.

We should note that the simulated material is an ideal homogeneous materialby default. In practice however, we know that the thermally grown cobalt layersare not homogeneous. Defects are presents for example, and the substrate is notperfectly flat. As a result, the anisotropy will vary as a function of the positionon the material. In order to get random domains in such a simulation, we need toinclude an anisotropy with a random orientation into the material. That way, themagnetization will not have a single preferential direction throughout the mate-rial. The energy landscape is thereby changed, such that it is energetically morefavourable to form multiple domains instead of one large domain.

4.5.2 Domain structure in a cobalt layer

We use the simulation software with a pre-defined cobalt material setting, whichis set-up to match experimentally obtained results for the material. The satu-ration magnetization equals 1.414 · 106 kA/m, the exchange stiffness A equals3.05 · 10−12 J/m and the magnitude of the anisotropy is 4 · 10 J/m3, which aretypical values for crystaline cobalt. We also introduce a random orientation (in x,y and z coordinates) to the anisotropy as discussed in the previous section. Simu-lations are run on a piece of cobalt with dimensions 2000x2000x15 nm, where thethickness corresponds to layers used in the devices discussed in chapter 5. The cellsize was set to 8x8x15 nm. The results of the simulations indeed show domainswith a size varying between 50-1000 nm after a certain stability criterion for the


simulations has been met.From the found magnetization structure, we now calculate the magnetic fringe

fields caused by the magnetization at several distances from the cobalt layer. Thisis also done using the simulation software mentioned in the previous section. Thefringe fields are separated in the x, y and z components. A set of results is shownin figure 4.7

0.5 1.0 1.50.0

0.5

1.0

1.5

2.0

HzHy

y-po

sitio

n (

m)

x-position ( m)

Hx

0.5 1.0 1.5

x-position ( m)

0.5 1.0 1.5

x-position ( m)

-200-150-100-50.00.0050.0100150200

Figure 4.7: The simulated x-, y- and z-components of the fringe fields caused by domains in acobalt layer at a distance of 10 nm from the surface. The indicated field strengths are in mT.

We note that the absolute value of the field strength in the z-direction is largerthan those in the x- and y-directions. The gradient in the z-direction, or perpen-dicular to the device surface speaking in terms of the orientations discussed insection 3.2.1, is thus largest. This is important, as this is the direction in whichthe current through a full organic based device (see section 3.1.2) flows.

In order to show fringe fields caused by the magnetization structure, the fringefields are calculated at regular distances from the surface of the magnetic material.To make a practical visualization, we select a line at position x = 76 nm, rangingfrom y = 716 to y = 1276 nm. The x, y and z components of the magnetizationalong this line are shown in figure 4.8.

At this line, the z-component is varying around 0, while the y-component variesbetween -70% and -100% of the saturation magnetization. The x-component alongthis line is varying from -80% to +60% of the saturation magnetization. In otherwords, the magnetization at this line is mainly changing in the x direction andless so in the other directions. Note that this also shows that the simulations donot correspond to the actual cobalt layers discussed in the previous sections. Weshowed that the magnetization of those layers has a finite magnetization in thez-direction for certain domains. This kind of magnetization is not present in thesesimulated layers. Application of an external field will thus change the simulateddomains in a different way compared to the actual cobalt layers. Without theapplication of an external field, we can still analyze the fringe fields caused by thissimulated layer.

The fringe fields in the z-direction are now calculated along this line as afunction of the distance to the surface. Figure 4.9 shows these fringe fields in a

4.5. Simulation of fringe fields from a cobalt layer 51

700 800 900 1000 1100 1200 1300-1.0

-0.5

0.0

0.5

1.0

M /

MS (

a.u.

)

y-position (nm)

Mx My Mz

Figure 4.8: The magnitude of the components of the magnetization as a function of the y-position. The x-position is fixed at 76 nm.

contour plot. The domains causing the fringe fields are shown below the graph,indicating the direction of the x-component of the magnetization. The size of thedomains is shown to be of importance when considering the distance at which thefields are still significant.

750 800 850 900 950 1000 1050 1100 1150 1200 1250

25

50

75

100

125

Position (nm)

Hei

ght (

nm)

-90.00-80.00-70.00-60.00-50.00-40.00-30.00-20.00-10.000.00010.0020.0030.0040.0050.0060.0070.0080.0090.00100.0

Figure 4.9: The simulated fringe fields in the z-direction as a function of the y-position atx = 76 nm. The domains and the sign of the x component of the magnetization are shown belowthe graph.

From the cross-section presented in this figure, we can now show the fieldstrength of the z-component of the fringe fields as a function of the distance to thesurface. Figure 4.10 shows that the fieldstrength stronly decays with increasingdistance to the surface. The gradient in the fringe fields also decays as a functionof the distance to the surface. Additionally, the features in the fringe fields aresmoothed with increasing distance, because the fringe fields are caused by anincreasing area of magnetization on the surface

Note that the data displayed in figure 4.10 only shows the trend for a cross-section of the full dataset. Larger gradients may thus be present and should be


700 800 900 1000 1100 1200 1300

-100

-50

0

50

100

Fiel

d (m

T)

y-position (nm)

128 nm 64 nm 32 nm 16 nm 8 nm

Figure 4.10: The simulated z component of the fringe fields as a function of the y-position. Thex-position is fixed at 76 nm.

taken into account when considering the fringe fields in further study.According to the ∆B model discussed in section 2.2.4, the changes in spin

mixing are caused by differences in the fringe fields at two sites. Such a differencecorresponds to the gradient of the fringe fields along the z-direction, because thehopping transport in organic semiconductors occurs mainly in the z-direction.From the simulations we therefore calculate the gradient in the z-direction of thez-component of the fringe fields, that is: dBz/dz. Because in the model it doesnot matter which site has the larger fringe field, we now take the absolute value ofthis gradient. Furthermore, we average this quantity over the full matrix of fringefields to get a statistically valid result. The resulting quantity, < |dBz/dz| >, isshown in figure 4.11. A r−2 function is plotted through the data, showing a perfectmatch.

In addition to information about the distance dependence of the fringe fields,the influence of the external field on the magnetization can also be investigated.Application of an external field can be simulated relatively easily, however wealready mentioned that the simulated domains do not have the out-of-plane com-ponent we showed to be present in the actual cobalt layers. As such, the simu-lated magnetization and thus the fringe fields will change in a different way. Theanisotropy of the simulated layer should be defined in a different way, such thatdomains with an out-of-plane component are present. It should then be confirmedthat the magnetization as a function of the applied field changes in a similar way tothe results showed in section 4.2.3. Ideally, the results of these simulations wouldbe used as input for simulations used to calculate the influence of the magnetic fieldon the singlet fraction present in the organic material. From this singlet fraction,the magnetic field effect can be calculated using various models, see section 2.2.4for more information.

4.6. Summary 53

0 50 100 150

0.0

0.5

1.0

1.5

<|dB

z/dz|

> (m

T/nm

)

Height (nm)

Figure 4.11: The average absolute value of the gradient in the z-component of the fringe fieldsalong the z-direction. The results are fitted with a r−2 function, showing a perfect match.

4.6 Summary

In this chapter we find, using AFM, that the surface roughness of the substrateis important when spin coating PEDOT:PSS. The addition of a rough aluminumseeding layer has improved device operation when using cobalt as a bottom elec-trode.

Characterization of the in-plane magnetized cobalt layer shows that only smallfields of approximately 1.5 mT are required to switch the magnetization of thelayer. The presence of an aluminum seeding layer is shown to have some influenceon the coercive field, however due to the small fields involved these differences willnot show up in actual organic light emitting devices.

Inspection of the out-of-plane magnetization shows the presence of domainswhich are not fully in-plane magnetized, i.e. they have an out-of-plane component.Fields of up to 1.6 T are required to fully pull the magnetization out-of-plane, butat fields up to 300 mT a fast and significant change in magnetization is alreadyobserved.

Cobalt layers deposited on top of an organic layer (covered with an aluminumspacer layer of varying thickness) show different magnetization behaviour. Themagnetic field required to pull the magnetization out-of-plane is not constant.Furthermore, the speed at which the magnetization changes differs for the variousspacer layer thicknesses. We finally note that the spacer layer thickness is notdirectly related to the change in magnetic behaviour.

A cobalt-platinum multilayer stack is shown to have a perpendicular magne-tization. The magnetization gradually switches from -MS to +MS between 10and 50 mT. Cleaning the electrode using isopropanol and an ultrasonic bath isshown to have a minute influence on the switching behaviour. Kerr microscopy


has visually shown that domains are gradually switched.Simulations of the magnetization of a thick cobalt layer have been performed

as well. The fringe fields calculated from this magnetization have shown thatthe magnitude and gradient in the field strength both get smaller for increasingdistance to the layer. The magnitude decreases with r−1 while the gradient in thefield decreases with r−2. The distance from the cobalt layer to the organic layeris therefore of great importance. Additionally, we note that as the organic layertypically has a thickness of 100 nm, the gradients and field strengths at one endof the device are significantly different compared to those at the other end. For afull comprehension of the effect of these fringe fields, the simulations will have tobe extended to show the influence of an external field on the magnetization of thecobalt layer and thus on its fringe fields.

5 Fringe Field Magnetoresistance

Introducing a ferromagnetic layer to an organic semiconductor device structure hasseveral potentially interesting applications. One of those applications is creatinga memory device by utilizing the randomly oriented fringe fields caused by thislayer. Such a device should have two clearly defined states which can be set andread, corresponding to a digital 0 and 1. The suggested fringe fields should thusbe able to significantly change the resistance of a device. Additionally, the fringefields should be controllable such that the states can be discerned and are stableover time. Before such a device can be fabricated, it is important to understandthe underlying physics.

We will first introduce the various devices used in this chapter. In the subse-quent sections we will investigate the electrical behaviour and the influence of anexternal magnetic field in combination with the ferromagnetic layer. The influenceof changing the distance between the ferromagnetic layer and the organic layer isthen inspected, as is the effect of changing the angle between the magnetization andthe external field. We will then see what the effect is of changing the bias voltageand finally conclude with a summary of this chapter.

5.1 Introduction

In standard devices, the application of an external magnetic field alters the currentthrough a device as discussed in section 2.2. A standard device is schematicallyshown in figure 5.1a. The addition of a ferromagnetic layer to such a device cancause the presence of fringe fields in the organic layer when domains are presentin this layer. As a result of these fringe fields, additional spin mixing can occur inthese modified devices.

The magnetization of the ferromagnetic layer is influenced by applying an ex-ternal magnetic field. The effect caused by this external field depends on the ori-entation of the layer’s magnetization with respect to the external magnetic field,as demonstrated in chapter 4. A sufficiently large field will remove any domainsas the magnetization will be homogeneous in the layer, while intermediate fieldsmay induce domains in the layer. In the first case, no fringe will be present whilein the latter case fringe fields will occur.

In our experiments the external magnetic field is swept, forcing the magneti-zation of a ferromagnetic layer to change. We assume that domains are formed

55

56 Chapter 5. Fringe Field Magnetoresistance

during this change. Consequently, the fringe fields caused by these domains changeduring this field sweep. The largest changes in the current through an organic de-vice are expected when the ferromagnetic layer switches gradually, because thegradient in the fringe fields then changes most.

The actual formation of domains is dependent on the used ferromagnetic layer.In this chapter we will use a ferromagnetic multilayer stack to confirm the resultsobtained by Wang et al. [40]. This multilayer stack, consisting of a number ofthin cobalt an platinum layers as described in section 3.1.2, has an out-of-planemagnetization due to its perpendicular magnetic anisotropy (PMA). The magneticbehaviour of the PMA layer we use is confirmed in section 4.4. The PMA layer isadded to the bottom of a standard device structure, which is schematically shownin figure 5.1b. These devices will be referred to as PMA devices. The resultsreported by Wang et al. were based on a device with an Alq3 organic layer, whichis a small molecule. They reported that similar results were obtained for a polymerbased device, however no proof was shown in this report. The organic layer wewill use in this thesis is a polymer, SuperYellow PPV.

In addition to this multilayer stack, we will also use devices in which a relativelythick cobalt layer of 15 nm is used. Such a layer has an in-plane magnetizationand therefore will behave differently compared to the PMA device. The magneticcharacterization of these cobalt layers in sections 4.2 and 4.3 has shown that themagnetization is not fully in-plane, the magnetization also has an out-of-planecomponent. This layer is positioned either at the bottom or at the top of a standarddevice structure. The structures of such devices are schematically shown in figures5.1c to 5.1d. When referencing these devices, we will specify the location of thecobalt layer. The devices with a cobalt layer at the top have a non-magnetic spacerlayer with a varying thickness. The used thicknesses are 10, 25, 50 and 100 nm.Finally, we show some preliminary results on devices with a cobalt layer at the topand bottom of the device, see figure 5.1e.

Organic layer

Non-magnetic

Non-magnetic

(a) (b) (c) (d) (e)

Figure 5.1: Schematic representation of the devices used in this chapter, showing the organiclayer sandwiched between two electrodes. Ferromagnetic layers are indicated in red. The arrowsindicate the preferential direction of the magnetization of the ferromagnetic layer. (a) Nonmagnetic reference structure. (b)-(e) Structures with a ferromagnetic electrode.

5.2. Electrical behavior 57

5.2 Electrical behavior

In this section, we characterize the devices discussed in the introduction withoutan externally applied field. The current as a function of the applied bias voltageis measured. From the current and the surface area of the device, we calculatethe current density to directly compare devices, which do not necessarily have anequal surface area.

The current density for a PMA device and a reference device made in the samebatch is shown in the left graph of figure 5.2. The leakage current is linear andsimilar for both devices. The built-in voltage is equal for both devices, showingthat the presence of the PMA layer does not influence the energy levels of theactual electrodes contacting the organic layer. For higher voltages, approximatelythe same voltage dependence is found, J ∝ V 3.6 and J ∝ V 3.5 for the referenceand PMA devices, respectively. In the intermediate regime, we observe that thecurrent density in the PMA device increases more slowly, indicating a more difficultinjection of holes into the device. Note that the current density of the referencedevice is limited, causing a plateau at 125 A · m−2.

The right graph of figure 5.2 shows the current densities measured for a deviceswith a cobalt layer at the bottom. A reference device created in the same batchis also shown. Again, both leakage currents are similar and the built-in voltagesare equal. The voltage dependence at higher voltages is reduced from J ∝ V 3.7

for the reference device to J ∝ V 3.0 for the cobalt device. This change in voltagedependence might be caused by a change in transport properties, caused by thepresence of the cobalt layer. In the intermediate regime, the current density in thecobalt device also increases more slowly than the reference device.

0 2 4 610-4

10-3

10-2

10-1

100

101

102

103

0 2 4 6

Cur

rent

den

sity

(Am

-2) J~V3.6

J~V3.5

Voltage (V)

Reference PMA

J~V1

J~V3.7

J~V3.0

Voltage (V)

Reference Cobalt bottom

J~V1

Figure 5.2: The current density is measured as a function of the applied bias voltage. Left: Theresults are shown for a device with a PMA layer at the bottom of the device structure. Right:The results are shown for a device with a cobalt layer at the bottom of the device structure. Bothgraphs show the results of the same measurement performed a reference device created in thesame batch.

The final set of devices we will inspect are devices with a cobalt layer on top.The current densities for these devices, as well as a reference device, are shown in


figure 5.3. We first note that for large bias voltages, the current densities are verysimilar, indicating that the energy levels in the device are not influenced by theadditional cobalt layer. The device with a spacer layer of 50 nm shows some stepsat 2.5 and 3.0 V as well as some noise, which are related to bad electrical contacts.Furthermore, we note that the devices with a spacer layer of 50 and 100 nm havea lower leakage current, as can be observed for V ≤ 1.5 V. Comparing the cobaltdevices with the reference device, we note that the onset of the injection currentappears to be lowered to some extent for all cobalt devices, however no clear trendcan be observed. Finally note that the current densities are a bit lower comparedto the reference devices in figure 5.2. On the other hand, no distinction can bemade between the cobalt and the reference devices, unlike the devices shown inthe same figure.

0 1 2 3 4 5 610-4

10-3

10-2

10-1

100

101

102

Cur

rent

den

sity

(Am

-2)

Voltage (V)

Reference 100 nm 50 nm 25 nm 10 nm

Figure 5.3: Current density for device with a cobalt layer at the top of the device structure.The measurement on a reference device is also shown.

From these measurements, we conclude that the devices all operate correctly.Some differences occur between the voltage dependence, however the leakage cur-rent is low in all devices. This is important when measuring field dependently,because magnetic field effects are measured in the injection current.

5.3 OMAR measurements

In this section we will discuss the effect of applying an external field on the currentthrough a device. The measurement procedure is discussed in section 3.2.1. Notethat we will refer to several orientations of the magnetic field with respect to thedevice as described in figure 3.9. We will first consider the magnetoconductance(MC) for a reference device. These measurements will function as a reference whenintroducing a ferromagnetic layer to the standard device structure in the followingsubsections.

5.3. OMAR measurements 59

5.3.1 Reference devices

The current is measured while an external magnetic field is applied perpendicularto the device surface. The magnetic field is swept from -500 mT to +500 mT. TheMC is then calculated using equation 2.8. The result of a measurement performedat a bias voltage of 5 V is shown in figure 5.4.

-400 -200 0 200 4000

1

2

3

4

5

MC

(%)

Field (mT)

Figure 5.4: The magnetoconductance is measured for a reference device at a bias voltage of 5V. A typical non-Lorentzian line shape is observed, as well as a positive high field effect.

A maximum MC of 4.9 % is found at a magnetic field of 500 mT. At a magneticfield of 100 mT, the MC has already reached a value of 4.4 %. The MC(B) curveshows the non-Lorentzian line shape in combination with a positive high fieldeffect.

The MC is now measured in both the perpendicular and the parallel orien-tations. These measurements are performed at a bias voltage of 3 V, where theMC reaches values of up to 11.3 % at an external field of 500 mT. We now useequation 3.2 to calculate the difference between both orientations to inspect thedifference in line shape. The resulting δMC is shown in figure 5.5.

From the figure we see that a small difference occurs between the measurementsin both orientations. The MC in the parallel orientation is slightly smaller than theMC in the perpendicular orientation, resulting in a negative δMC. The differencebetween both measurements is largest at an external field of 10-20 mT, showing adeviation of 1.5 %. Note that the units of δMC are a percentage of the full MCcurve, so for example a δMC of -1.5 % at a field of 12 mT means that the MC isreduced from 3.153 % to 3.106 %.

5.3.2 Devices with a PMA layer

We now introduce a layer with perpendicular magnetic anisotropy (PMA) to thestandard device structure. As mentioned in section 5.1, the resulting device is


10-2 10-1 100 101 102

-1.5

-1.0

-0.5

0.0

MC

(%)

Field (mT)

Figure 5.5: δMC is calculated as a function of the field for a reference device measured at 3 V.

similar to the devices used by Wang et al. [40]. As shown in section 4.4, themagnetization of this layer is shown to be pointing in the perpendicular direction.The device structure is schematically shown in figure 5.1b.

The current is measured while sweeping the external field from -150 to +150mT (forward) and backwards to -150 mT. The field is oriented perpendicular tothe device surface during this measurement. The magnetization and the externalfield are thus oriented (anti) parallel with respect to each other. The MC of sucha measurement performed at a bias voltage of 5 V is shown in figure 5.6 for theforward and backward sweeps. The direction of the field sweep is indicated usingarrows. Note that the field shown here is limited to -100 mT to +100 mT. Thesefield strengths are indicated in figure 5.4, showing that further increasing the fieldstrength has little effect on the MC.

The first halves of the forward and backward sweeps of the magnetic field showthe basic line shape observed in the reference device. When inspecting the secondhalf of the field sweeps, deviations occur between 5 and 70 mT. These results aresimilar to the results obtained by Wang et al., see the results presented for a 50nm spacer layer in figure 2.13b. These deviations are explained by the presence offringe fields, caused by switching the magnetization of the PMA layer.

Moke measurements in section 4.4 have shown that the switching of the magne-tization of the PMA layer is dependent on the sweep direction of the external field.The magnetization was shown to switch for 5 < |B| < 50 mT, which is close to theobserved deviations in the MC occuring between 5 < |B| < 70 mT. We note thatthe PMA layer composition is slightly different in this section, causing slightly dif-ferent magnetic properties of the PMA layer. As a result, the observed deviationsin the MC are attributed to fringe fields caused by switching the magnetization ofthe PMA layer.

In conclusion, we have been able to reproduce the results reported by Wang etal. We have used devices based on a polymer instead of small molecules as used


-100 -50 0 50 1000.0

0.5

1.0

1.5

2.0

2.5

Backward sweep Forward sweep

MC

(%)

Field (mT)

Figure 5.6: The magnetoconductance is measured at a bias voltage of 5 V for a device with aPMA layer. In addition to the shape of the reference device, a difference between the forwardand backward sweep directions of the magnetic field is observed.

by Wang et al. The observation they made that the deviations in the MC occurdue to fringe fields are consistent with our observations.

5.3.3 Devices with a cobalt layer at the bottom

Instead of adding a PMA layer, we now add a cobalt layer at the bottom of astandard device structure. The magnetization of such a cobalt layer is directedalong the surface of the device, as indicated in figure 5.1c.

The current is measured while sweeping the external field from -500 to +500mT. Measurements are performed for parallel and perpendicular orientations of theexternal field with respect to the device surface. The results of the measurementsperformed at 3.6 V are shown in the left part of figure 5.7. The MC is now usedto calculate δMC using equation 3.2. The calculated δMC is shown in the rightpart of figure 5.7 for measurements performed at a bias voltage of 3.6 V. Forcomparison, the δMC calculated for a reference device at 3.5 V is also shown.

Several features can be distinguished when inspecting this figure. The lineshapes of both devices show similar behaviour at low field strengths (∼ 10 mT),however at low and high field scales deviations occur. We first focus on the differ-ence between the measurements on the cobalt and reference devices at small fieldscales. We will then discuss the deviating behaviour at large field scales.

Ultra small field effects

We compare the results of the cobalt and reference devices as shown in figure 5.7for small field scales. At a field of 1.5 mT, a step can be observed in the δMCdata of the cobalt device. This step can be attributed to the in-plane switching


10-1 100 101 102

-2

0

2

4

6

8

-400 0 400

0

2

4

6

8

Reference 3.5 V Cobalt 3.6 V

MC

(%)

Field (mT)

MC

(%)

Field (mT)

B B

Figure 5.7: Left: MC as measured at 3.6 V for a cobalt device in the perpendicular and parallelorientations. Right: δMC displayed for reference and cobalt devices, measured at 3.5 V and 3.6V respectively.

of the cobalt layer. A more thorough analysis is given in Appendix F, where thesame effect is also discussed for devices with a cobalt layer on top of the devicestructure. A more detailed view of the normalized MC curves is shown in in theleft part of figure 5.8. We use the normalized curves defined by equation 3.3 toeasily compare the line shape of the various measurements.

-1 0 1-0.02

0.00

0.02

0.04

0.06

0.08

0.10

-1 0 1Simulations

Field (mT)

Nor

mal

ized

MC

(a.u

.)

Reference (perpendicular) Parallel Perpendicular

Measurements

Field (mT)

0.0 mT 0.5 mT 1.0 mT

Figure 5.8: The normalized MC shown for small fields. Left: Actual measurements performedon a cobalt and a reference device. Right: Simulations performed on the MC with the additionof a static magnetic field in the z-direction.

At very small field scales (∼0.5 mT), the reference device shows an ultra smallfield effect. This USFE is not present in the measurements performed on a devicewith a cobalt layer. Additionally, there is little difference between the paralleland perpendicular measurements of the cobalt device, causing δMC to remainapproximately zero at low field scales, as observed in figure 5.7. In referencedevices, the change in orientation does influence the MC at these field scales,thereby causing δMC to be non-zero.


In section 4.2 it was shown that the magnetization of the cobalt layer is pointingin-plane (or parallel in terms of the definitions of section 3.2.1). The in-planemagnetization was found to switch at a field of 1.5 - 2.0 mT when sweeping theexternal magnetic field. When the external field is zero, a remanent magnetizationis present in the cobalt layer. If domains are now present in this state, fringe fieldsare present even at zero applied field.

The right part of figure 5.8 shows some very basic simulations of the USFEusing the electron hole pair mechanism as described in section 2.2.3. Simulationswere carried out for a standard device in which 50.000 random hyperfine fieldswere averaged in calculating the magnetoconductance. Inclusion of an additionalsmall static z-component in the magnetic field is shown to remove the ultra smallfield effect. This additional static magnetic field represents the fringe fields causedby the ferromagnetic layer. The graph shows simulations where additional fieldsof 0.5 and 1.0 mT were used. In the case of 0.5 mT, a small USFE can still beobserved, however the field scales at which it occurs is drastically reduced. Whenthe additional field has a magnitude of 1.0 mT, no USFE can be observed.

Because we add a static value to the z-component of the magnetic field, thesesimulations are very crude. From section 4.5 we know that the fringe fields causedby a cobalt layer are not static at each site, they are changing in magnitude as wellas in orientation. Besides averaging over the random hyperfine field, the additionof a single static component to the simulations should therefore be replaced byaveraging over a random additional component simulating the fringe fields causedby the ferromagnetic layer. Because the current through the device experiencesa different gradient during its hopping process, averaging over random additionalcomponents is a good way to take the gradient in the fringe fields throughout thedevice into account.

High field effects

We now return to the MC measurements shown in figure 5.7. The small differenceobserved between both orientations in the reference device at intermediate fieldstrengths is also observed in the cobalt device. At high fields, the line shapemeasured in the perpendicular orientation differs from the parallel orientation. Atfields larger than 150 mT the MC decreases, while at fields larger than 400 mT itstarts to increase again. These features are clearly visible in the difference profileshown in the figure.

If we consider a sufficiently large positive or negative magnetic field perpen-dicular to the device, we know that the magnetization of an in-plane magnetizedcobalt layer can be pulled out-of-plane, as demonstrated in section 4.2.3. As aresult, fringe fields occurring from domains in the cobalt layer will disappear whensaturation is reached, affecting charge transport in the nearby organic layer. Be-cause the magnetization orientation of the domains is directly influenced by themagnitude of the external magnetic field, the fringe field distribution will also bedependent on the magnitude of the external magnetic field. The change in the MC


line shape as measured in the perpendicular orientation can thus be attributed tothe presence of the cobalt layer.

The large change in magnetization of the cobalt layer at relatively low fields (0- 100 mT) does not generate a measureable change in the MC. For fields greaterthan 150 mT, a difference in the magnetoconductance is observed. This thresholdcan be caused by either the switching behaviour of the ferromagnetic layer, or bythe penetration depth of the fringe fields caused by domains in the ferromagneticlayer. In the first case, a certain field may be required to induce fringe fields whichhave a gradient in the appropriate spatial order of magnitude needed to influencethe hopping process. The fast switching at low fields might even switch domainsin such a way that the fringe fields do not significantly differ while sweeping themagnetic field in this regime. In the second case, a certain amount of magnetizationshould be pulled out-of-plane to cause sufficient penetration of the fringe fields inthe organic material. However, we have already demonstrated that it is likely thatfringe fields penetrate the organic material without the presence of an externalfield. As a result, it is likely that the switching behaviour of the magnetizationcauses the MC to change at large field strengths.

5.3.4 Devices with two cobalt layers

In the previous subsection, we added a cobalt layer at the bottom of a standarddevice structure. It is now interesting to see the effect of adding two cobalt layers,at both sides of a standard device. Because the fringe fields have strongly decayinggradients into the device, their presence at both sides might enhance the observedeffects.

We try to minimize the distance between the cobalt and organic layers in anattempt to achieve the largest fringe field gradients in the organic layer. To thatend, we reduced the PEDOT:PSS layer thickness from 45 to 35 nm. Appendix Gshows the calibration of the PEDOT:PSS thickness with respect to the spin coatspeed. Furthermore, we use a 10 nm aluminum spacer layer at the top of thedevice. In an attempt to increase the fringe field gradients present in the device,we furthermore decreased the thickness of the organic layer from 100 to 60 nm.The full device structure is shown in figure 5.9a. Fabrication of a device with twocobalt layers proved difficult as only one device from a whole batch was functional.Nevertheless, we want to present the measurements we performed on this device.

The current was measured at a bias voltage of 5 V in both the perpendicularand parallel orientations. The MC measured in the perpendicular orientation isshown in figure 5.9b. We now separately show the MC in the forward and backwardsweeps, as a hysteresis effect occurs. Additionally, we show the difference betweenthe two sweeps.

From the figure, we immediately see that the maximum MC achieved is rela-tively small. In the parallel orientation (not shown), a value of 2 % is reached atan applied field of 500 mT. We should keep in mind that the reduced thickness of

5.4. Varying the spacer layer thickness 65

Cobalt 15 nm

Glass substrate

ITOPEDOT:PSS

Active layer

LiF

Al

35 nm

60 nm

1 nm

10 nm

Al 100 nm

Al 50 nm

Cobalt 15 nm

(a)

-400 -200 0 200 400-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

MC

(%)

Field (mT)

Backward Forward Difference

(b)

Figure 5.9: (a) The schematic structure of a device with two cobalt layers. (b) MC as a functionof the applied field. The forward and backward sweeps are shown, as well as the difference betweenthe two. The MC is shown for a measurement performed at 5 V in the perpendicular orientation.

the organic layer causes a larger field at the same applied bias voltage. The biasvoltage of 5 V thereby corresponds to a higher voltage in devices with a regularthickness of the organic layer. Because an increase of the bias voltage is shownto decrease the magnitude of the MC, the found values for the MC are easilyexplained [20].

Figure 5.9b shows a difference in the MC when comparing the forward andbackward sweeps. A decrease in the MC occurs between 100 < |B| < 300 mT.Note that this decrease is in addition to the change in line shape we attributeto fringe fields from the cobalt layer. If we now inspect the device structure asshown in figure 5.9a, we note that we only have in-plane magnetized cobalt layerswhich are separated approximately 100 nm. With such separation, the fringe fieldsproduced by one layer on the other is estimated to be in the order of 10-20 mT,see section 4.5.2. Because the externally applied field is approximately one orderof magnitude larger than this field strength when the decrease occurs, the fringefields are small in comparison to the external field. An explanation for this effectis not found.

5.4 Varying the spacer layer thickness

In this section we will inspect the effect of changing the thickness of the spacerlayer between the organic layer and the cobalt layer. By changing the distancebetween the organic and cobalt layers, we can investigate the effect of the fringefield gradients on the charge transport in the organic layer. The cobalt layer is nowdeposited at the top of a standard device structure, as the aluminum spacer layercan easily be varied in thickness. Furthermore, the layer is deposited in a period


of approximately one minute to prevent damage to the organic layer, as discussedin section 3.1.2. The general structure of these devices is shown in figure 5.1d.

We measured the current of the various devices as a function of the appliedmagnetic field at a bias voltage of 5 V in the parallel and perpendicular orientations.From these measurements the MC was calculated. The results, normalized usingequation 3.3, are shown in figure 5.10. The results for the parallel orientation areshown in the left part, while the results for the perpendicular orientation are shownin the right part.

-400 -200 0 200 4000.0

0.2

0.4

0.6

0.8

1.0

Perpendicular

Nor

mal

ized

MC

(a.u

.)

Field (mT)

Reference 100 nm 50 nm 25 nm 10 nmParallel

-400 -200 0 200 400

Field (mT)

Figure 5.10: The normalized MC for devices with spacer layers of 10, 25, 50 and 100 nmas measured at 5 V. Left: The data for the parallel orientation shows a broadening, which isindicated by an arrow. Right: The data for the perpendicular orientation shows an increasedeffect of the cobalt on the high field effect, also indicated by an arrow.

Focussing on the results in the parallel orientation, we see that the introductionof a cobalt layer on top of the standard device layout (thus using a spacer layerof 100 nm) causes a minute broadening of the line shape. If we decrease thethickness of the spacer layer, we see that the line shape broadens a little withevery decrease. When the spacer layer is reduced to 10 nm, a large broadeningoccurs. The same broadening can be observed in the perpendicular orientation.However, this broadening is only clearly visible in the device with a 10 nm spacerlayer in this orientation. In the perpendicular orientation we see the same deviationin shape we observed in device where the cobalt layer was situated at the bottomof the device, see section 5.3.3. With a decreasing spacer layer thickness, the effectat high fields is clearly increasing.

We now use equation 3.2 to analyze the difference between the parallel andperpendicular orientations. The results are presented in figure 5.11. In this figure,δMC as measured at 5 V is shown as a function of the applied magnetic field forspacer layer thicknesses of 10, 25, 50 and 100 nm. A reference device was alsoanalysed for comparison.

From this figure we can clearly see the increase of the effect as a function of thespacer layer thickness. When a cobalt layer is introduced on top of the referencedevice, an effect of up to 5 % is observed at high fields. Reducing the thickness ofthe aluminum layer increases this effect up to 58 % at 5 V. This is much larger than

5.4. Varying the spacer layer thickness 67

10-1 100 101 102

0

10

20

30

40

50

60

MC

(%)

Field (mT)


Figure 5.11: δMC as a function of the applied magnetic field for devices with a cobalt layer ontop, as well as for a reference device. The spacer layer in the cobalt devices is varied between 10,25, 50 and 100 nm. The measurements are performed at 5 V.

the devices where the cobalt layer was added at the bottom, where the maximumeffect measured at 5 V was approximately 6 %. We note that the effect of thecobalt layer can reach up to 71 % for the device with a 10 nm spacer layer whenmeasuring at 2.5 V.

Note that the line shape of δMC is not equal for each sample. This can clearlybe observed when inspecting the local maximum found for each of the cobaltdevices at high fields. These maxima can be clearly observed in figure 5.11. For a100 nm spacer layer the maximum is located at roughly 300 mT, while for a 50 nmspacer this is reduced to 400 mT. At 25 nm the maximum is located at the edge ofthe sweep range, around 480 mT, and for a 10 nm spacer layer the maximum is noteven reached at 480 mT. The shift of the local maximum is emphasized using anarrow in the graph. Additionally, this 10 nm device shows a different behaviour atlow field strengths (10 - 100 mT). All other devices show a small, gradual increasein δMC while the 10 nm device shows a much larger increase.

The maximum value found in δMC can be plotted as a function of the spacerlayer thickness. The measurements performed at 2.5 V are analyzed, the maxima inδMC are plotted in figure 5.12. Note that the maximum in δMC for the device witha 10 nm spacer layer is located at fields larger than 500 mT, which we cannot probe.The values used for the 10 nm spacer layer might therefore be underestimated. Ar−2 function is fitted through the data, showing a match.

Now consider the ∆B mechanism discussed in section 2.2.4. The ∆B modelassumes that the difference in the local field can be approached by a differencein the z-component of this field. As mentioned in section 4.5, the difference inz-component corresponds to the gradient of the field in the z-direction. The modelfurthermore assumes that only two sites (or 1 hop between the two sites) areinvolved in the spin mixing process. In that case, the spin mixing and thus the


0 25 50 75 1000

20

40

60

80

100

MC

max

(%)

Spacer layer thickness (nm)

Figure 5.12: The maximum value of δMC as a function of the spacer layer thickness. Theapplied bias voltage is 2.5 V. A r−2 function is fitted through the measurement data.

MC are expected to change with the gradient in the fringe fields. If multiple sites(and hops) are participating in the spin mixing process, not the gradient but thefield itself is expected to scale with the MC.

From the simulations performed in section 4.5, we found that the gradient ofthe fringe fields has a r−2 relation with respect to the distance from the magneticlayer. Comparing this observation to the r−2 fit performed on our measurements,it is shown that the gradient in the fringe fields indeed changes the spin mixingand thereby the MC. If we assume that only two sites have to be considered forspin mixing to occur, the ∆B mechanism correctly explains the observed thicknessdependence.

5.5 Angle dependence

As we will see in the next chapter, the angle dependence in reference devices isclearly measureable. Measurements of the angle dependence in devices with acobalt layer are therefore also interesting, because contribution of the cobalt layerto the the angle dependence can be determined. To this end, the angle dependenceof the reference devices can be subtracted from these measurements. In this sectionwe will inspect the angle dependence of devices with a cobalt layer at the bottomof the device.

The current is measured as a function of the angle between the normal of thedevice and the externally applied magnetic field, while keeping the voltage andthe external field constant. The exact measurement and processing procedure isdescribed in section 3.2.1 in the section Angle dependence at constant field. Theangle dependence in cobalt devices is measured at 6V, while the reference devicesare measured at 5V. These values are chosen such that the current through bothdevices is approximately the same. Measurements are performed for magnetic field

5.5. Angle dependence 69

strengths of 20, 100, 200, 300, 400 and 500 mT. The drift-corrected measurementsperformed at 400 mT are shown on the left hand side in figure 5.13 for both devices.Note that we have used equation 3.4 to present the change in current as a functionof the angle.

0 60 120 180 240 300 3600.0

0.2

0.4

0.6

0.8

1.0

0 60 120 1800.00

0.05

0.10

0.15

0.20

FWHM

Normalized reference Cobalt Difference

I /

I max

(a.u

.)

Angle (Degrees)

Current Shift

Reference Cobalt

I ()

Angle (Degrees)

Figure 5.13: The relative change in current is shown as a function of the angle between thedevice normal and the externally applied magnetic field of 400 mT. The cobalt and referencedevices were measured at 6 V and 5 V respectively. Left: δI shown for half a rotation. Right:The reference measurement is normalized to the cobalt measurement. The cobalt measurementitself is normalized as well. The normalized measurements are shown for a full rotation. Thedifference between both normalized curves is also shown.

We now observe that the angle dependent effect of the cobalt device can be seenas the sum of a ’block’ function and the intrinsic angle dependence as measured inthe reference devices. Furthermore, the angle dependence in the reference devicehas a larger magnitude, which might be related to the difference in applied biasvoltage.

We normalize the reference measurements to the features observed in the cobaltmeasurements for angles between 30 and 150 degrees. The cobalt measurementsare also normalized to the maximum value observed at 400 mT. The normalizedreference signal is then subtracted from the measurements performed on the cobaltdevice. The resulting signal is now considered to be caused solely by the cobaltlayer. An example of these processing steps is shown in the right hand side offigure 5.13, showing the data for a full rotation of measurements performed at 400mT. The normalized reference data is shown in black, a factor 1.4 smaller thanthe direct measurement as shown in the left hand side. The subtracted signal isshown using a thick orange line, showing an almost block-like function.

The maximum in the subtracted signal is denoted as the (normalized) currentshift, while the width between the block functions at half maximum is denotedby FWHM. From all measurements, we can now plot the normalized current shiftand the FWHM for all measured field strengths, see figure 5.14.


0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

0

6

12

18

24

30

FW

HM

ste

p w

idth

(deg

rees

)

Nor

mal

ized

cur

rent

shi

ft (a

.u.)

Field (mT)

Figure 5.14: The normalized current shift and FWHM step width as a function of the externallyapplied magnetic field.

From this figure, we see that the current shift shows the same general shape ofδMC as presented figure 5.7. Note that the exact shape does not match, because δIis normalized to the current at an angle of 0 degrees at the external filed while δMCis normalized to MC‖,500mT, which in turn is normalized to the current as measuredwithout an external field. The results do, however, show that an additional effectis induced by the presence of the cobalt layer instead of modifying the intrinsicOMAR effect.

We finally note that the FWHM step width increases with the applied magneticfield. The increased field strength thus induces a change in the magnetizationsuch that larger angles around the perpendicular orientation are influenced by thecobalt layer. This is in agreement with the observation that the magnetizationis pulled out of plane. For larger fields, the magnetization is pulled out of planefurther, as shown in chapter 4. A small change in angle only slightly affectsthe component of the external field perpendicular to the device. With increasingfield, the magnetization is less sensitive to changes in the external field, allowingthe change in angle to be larger before the magnetization returns to an in-planeorientation.

5.6 Voltage dependence

Up until this section, we have only discussed MC measurements performed at onespecific bias voltage, without explicitly discussing the influence of changing thebias voltage. We will first discuss the voltage dependence of a device with a cobaltlayer at the bottom in comparison to that of a reference device. Devices wherecobalt is located either at the bottom of the device structure will be analyzedafterwards.

5.6. Voltage dependence 71

5.6.1 Results

First we inspect the voltage dependence of devices with a cobalt layer at thebottom of the device structure. Using equation 3.2 we calculate δMC for MCmeasurements performed in the parallel and perpendicular orientations. Thesemeasurements are performed for a large number of bias voltages. The results for asubset of these voltages are shown in the left part of figure 5.15. The right part ofthe same figure shows the results of similar measurements performed on a referencedevice.

100 101 102

-2

0

2

4

6

8

100 101 102

Reference

MC

(%)

Field (mT)

2.7 V 3.2 V 3.6 V 4.2 V 5.0 V

Cobalt

Field (mT)

2.5 V 3.0 V 3.5 V 4.0 V 4.5 V 5.0 V

Figure 5.15: δMC as a function of the applied field, displayed on a log scale. Left: Resultsobtained for a device with a cobalt bottom electrode. Right: Results obtained for a referencedevice.

We note that the cobalt data at 5 V shows some deviation, which can be at-tributed to deviations in the original measurements which are used to calculateδMC. For low field strengths (∼10 mT), |δMC| seems to get smaller with in-creasing voltage for both the modified and the reference devices. Apparently thebehaviour of the cobalt device as a function of the applied voltage is not influencedfor these low field strengths, indicating that the fringe fields do not have a signif-icant contribution to the MC in one particular orientation. Furthermore, a clearmaximum of the high field contribution is found at a field strength of 400 mT forthe cobalt device.

The voltage dependence at high field strengths has an opposite sign in thecobalt and reference devices, as indicated by the arrows. This opposite sign showsthat the fringe fields caused by the cobalt layer are causing an additional voltagedependence. This is understood by looking at the definition of δMC, which showsthe difference in parallel and perpendicular orientations. In the parallel orientation,the fringe fields are absent for fields larger than a few mT. This is a result ofreaching the saturation magnetization, as shown in chapter 4. In the perpendicularorientation, the magnetization is shown to reach saturation at approximately 1.6T. As such, the fringe fields are changing with the applied magnetic field up tothis magnitude.


The difference in behaviour for fields up to a few mT can be explained bythis observation. For these fields, the magnetization is not significantly pulledout-of-plane in the perpendicular orientation and thus more or less matches themagnetization in the parallel orientation. Fringe fields resulting from the magneticdomains are then similar. If the MC at this field scale is mainly caused by thefringe fields, δMC is then close to zero as the MC in both orientations is similar.Note that we will discuss the results of the reference device further in chapter 6.

5.6.2 Cobalt induced voltage dependence

Plotting the maximum in δMC at 400 mT as a function of the voltage will showthe voltage dependence of the cobalt induced contribution. This graph is shownin figure 5.16. The device layout is also shown in this graph, indicating the 45 nmspacer layer present between the cobalt and organic layers. Inspection of the graphshows an apparent linear relation between the maximum in δMC and the voltage.The guide to the eye displayed in the graph emphasizes this trend, showing thatonly small deviation from this linear relation occur. One exception should bementioned at 4.6 V, which clearly does not follow the trend. As δMC is calculatedby subtracting two independent measurements, it can be understood that somedeviations can occur.

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

4

6

8

10

12

14

16

MC

400m

T (%

)

Voltage (V)

50 nm spacer50 nm spacer50 nm spacer50 nm spacer50 nm spacer45 nm spacer

50 nm spacer50 nm spacer

50 nm spacer

Figure 5.16: The maximum value of δMC at 400 mT is shown as a function of the applied biasvoltage (black squares). The results of a device with a cobalt layer at the bottom is also shown(grey circles). The lines function as a guide to the eye, indicating a linear voltage dependencefor both devices.

The graph also shows the results obtained from a device with a cobalt layerpositioned on top of the device structure. The spacer layer thickness of this devicewas 50 nm, see section 3.1.2 for the exact device layout. The MC was measuredin the parallel and perpendicular orientations at a range of voltages. The shape ofδMC for this device is similar to that of devices with a cobalt layer at the bottom

5.6. Voltage dependence 73

and will not be shown here. The maximum in δMC is analyzed as a function ofthe applied bias voltage as shown in figure 5.16.

Inspection of the graph shows that the maximum in δMC decreases as a func-tion of the voltage. Again, a line is shown as a guide to the eye, also indicatinga linear relation to the applied voltage. The effect caused by the cobalt layer isnoticeably larger in the device with the cobalt layer on top. We observe that δMCdecreases with increasing magnitude, independently of the location of the cobaltlayer. Note that similar results have been obtained for devices with different spacerlayer thicknesses.

5.6.3 Discussion

Comparing the results of the previous section, we note that the peak in δMC(δMCmax) decreases as a function of the voltage, independently of the location ofthe cobalt layer. The difference in magnitude of δMCmax might be attributed to anumber of factors (difference in cobalt growth speed - and thus roughness, changein recombination location - see below), however, no definitive conclusion is drawn.If we consider the e-h pair mechanism discussed in section 2.2.3, we note thatthe formed singlet excitons recombine by emitting light. If the fraction of singletexcitons is influenced by the fringe fields, the location where most recombinationoccurs could be of importance.

In a simple drift-diffusion model, this recombination location is shifted by ap-plying a bias voltage. In a typical organic semiconductor, where the electron andhole mobilities are unequal, the recombination location shifts to one electrode.This shift of the (normalized) recombination location is shown in figure 5.17 for a100 nm thick organic layer with a band gap of 2.8 eV and µh = 2µe = 1 · 10−10

m2/Vs, where µh(e) is the hole (electron) mobility. For the full details of the sim-ulated system we refer to [42].

2.0 2.5 3.0 3.5 4.0 4.5 5.050

60

70

80

90

100

0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

1.0

Rec

ombi

natio

n lo

catio

n (%

)

Voltage (V)

Nor

mal

ized

reco

mbi

natio

n (a

.u.)

Position (nm)

2.0 V 2.3 V 2.6 V 3.0 V 3.5 V 4.0 V 5.0 V

Figure 5.17: Left: the normalized recombination as a function of the position for several biasvoltages. Right: maximum in the relative recombination location as a function of the appliedbias voltage.

When the voltage dependence of our δMC measurements is linked to a shift in


the recombination location, the voltage dependence would have an opposite signwhen comparing devices with a cobalt layer at the bottom and devices with a cobaltlayer at the bottom. This can be understood by thinking of the fringe fields andnoting that their gradient degrades into the device. The recombination locationmoves to one electrode, independently of the cobalt layer location. In one devicetype, this location will move to a region with larger gradients, while the other willmove to a region with smaller gradients. As a result, δMC is thus expected toincrease in the first case and decrease in the latter. A shift in the recombinationlocation thus cannot be the cause of the voltage dependence observed in deviceswith a cobalt layer.

Alternatively, it can be shown that the hopping frequency of the charge carriersis increased by applying a bias voltage, and thus an electric field, to the device.The relative hopping rate r defined in section 2.2.4 thereby increases as well.In the context of the ∆B model discussed in the same section, this increase inhopping rate can have significant non-linear effects on the magnetic field effects,see figure 2.12. We do note that the model presented there shows a MFE(r)line shape which is calculated for a static value of ∆B equal to ten times thehyperfine field strength. Because the fringe field gradient is not static, the valuefor ∆B should be taken as a distribution, which will result in a broadening of thetransition from slow to fast hopping. Additionally, the hyperfine fields are takento be random and thus isotropic, which does not apply to the fringe fields. Topresent any definitive conclusions on this voltage dependent effect, more thoroughsimulations on the exact shape of the MFE as a function of the relative hoppingrate should therefore be performed.

5.7 Summary

In this chapter, we have introduced a ferromagnetic layer to a regular OLED device.The addition of such a layer is shown to influence the magnetoconductance (MC).The angle between the device normal and the external magnetic field is shown tobe of importance on the field scale and magnitude of this effect.

For a cobalt-platinum multilayer stack with an out-of-plane magnetization,changes are observed when the magnetic field is oriented perpendicular to thedevice surface. A change in MC has been found to occur for field strengths between10 and 70 mT, having a magnitude of up to 13 %. This field range is associated withthe switch in magnetization of the ferromagnetic layer. The switching behaviourof the used multilayer stack is hysteretic. The MC measurements confirm this, asthe forward and backward field sweeps show the change in MC at opposite signsof the field.

Introducing a relatively thick cobalt layer of 15 nm with an in-plane mag-netization is also shown to change the MC when the magnetic field is orientedperpendicular to the device surface. The layer is positioned at the top or at thebottom of the device, showing similar changes in MC. The change in MC is found at

5.7. Summary 75

100 mT and larger, which is attributed to fringe fields occurring caused by pullingthe magnetization of the cobalt layer out-of-plane. As a result of increasing fringefields, the spin mixing is thought to increase again, resulting in a reduction of theMC and hence the observed change in line shape.

When the external field is oriented parallel to the device surface, the in-planeswitch of the magnetization can be observed at relatively low field strengths of1-2 mT. At even smaller field scales of ∼ 0.5 mT, the USFE has been shown todisappear with respect to a regular OLED device, independently of the orientationof the magnetic field. This is attributed to the constant presence of fringe fields,even without an externally applied field.

For devices with a cobalt layer on top of the organic layer, the thickness of thespacer layer has been varied. Decreasing the spacer layer thickness is shown toincrease the change in MC from 5 % at 100 nm to 58 % at 10 nm. The magnitudeof the change in MC has been shown to obey a r−2 relation with respect to thespacer layer thickness. This behaviour corresponds to the found change in thegradient of the simulation fringe fields of the previous chapter. This observationconfirms that only two sites have to be considered when inspecting the hopping ofcharge carriers.

Gradually changing the angle between the device surface and external field hasshown that the regular change in MC due to the changing angle is still presentin devices with an additional cobalt layer. The magnitude of the effect has beenreduced slightly by the cobalt layer, however the shape is exactly the same. Inaddition to this regular angle dependence, an additional component caused by thecobalt layer has been observed. This component has an almost block-like shapeand its magnitude is shown to have a similar shape as the magnetoconductancewhen plotted as a function of the applied field. For a perpendicular orientationof the field with respect to the device, the current is found to be lower due tothe cobalt layer causing fringe fields For increasing field strengths, an increasinglylarger angle around the current minimum is found to have a lower current. Thiseffect has been attributed to the fact that an increasing angle achieves the sameperpendicular component of the external field for increasing field strengths.

Increasing the applied bias voltage has shown to influence the relative change inMC at large fields. The sign of this relative change in MC for devices with a cobaltlayer at the bottom is found to be opposite to that of reference devices. This hasshown that the cobalt layer induces an additional voltage dependence. The sign ofthe voltage dependence is also found to be negative for devices where the cobaltlayer is positioned at the top of the device. It has therefore been concluded thatthe voltage dependence cannot be caused by a changing recombination locationin the organic layer. A change in hopping rate has been suggested to cause avoltage dependence, however additional simulations need to be performed to makea conclusive statement on this voltage dependence.


6 Angle Dependence of intrinsicOMAR in SY-PPV

This chapter will focus on a relatively unexplored feature in the magnetoconduc-tance of organic devices. The angle between the applied magnetic field and theorganic device is shown to be of influence on the magnitude of the magnetocon-ductance. In the previous chapter, this effect was of great importance due to theanisotropy of the additional ferromagnetic layer. Even without such a ferromag-netic layer, an angle dependence is still present. In the literature a basic under-standing of this angle dependence has been developed, however, it is still not clearwhat the exact mechanism is. In this chapter we will perform magnetoconductancemeasurements on standard SY-PPV devices as a function of the angle between theexternally applied field and the device. The chapter will be concluded by a summaryin which the main findings are repeated.

6.1 Introduction

Changing the angle between the sample and the externally applied magnetic fieldhas recently been shown to have an effect on the magnitude of the magnetocon-ductance in an organic semiconductor [36]. A basic understanding of this angledependence has recently been developed, however, several possibilities remain toexplain the angle dependence. Even more recently, the angle dependence in or-ganic semiconductors has been shown to accurately determine the magnitude anddirection of the earth magnetic field [7]. Due to this recent development, it is inter-esting to investigate the angle dependence more closely. To that end, a standardSY-PPV device will be measured and analysed.

The angle dependence mentioned before is attributed to spin-spin interactionsbetween polarons, which were neglected in models up until that point [6]. Withsimulations, it was shown that anisotropy must be present in either the spin-spininteractions or in the hyperfine fields in order to explain the angle dependence.

Two types of measurements are performed on a standard SY-PPV device. Seesection 3.1.1 for more information about the device layout. First the currentis measured at a constant voltage of 3 and 5 V at various field strengths. Thesemeasurements are discussed in section 6.2. The second set of measurements focuseson the magneto electroluminescence (MEL) measured at a constant current of

77

78 Chapter 6. Angle Dependence of intrinsic OMAR in SY-PPV

0.33 mA, corresponding to a voltage of approximately 5 V. The results of thesemeasurements are discussed in section 6.3.

6.2 Electrical measurements

The current through the device is measured at a constant bias voltage, accordingto the method outlined in section 3.2.1 in the section Angle dependence at constantfield. While measuring the current, the sample is rotated in a constant appliedmagnetic field.

6.2.1 Results

A first set of measurements is performed at 5 V, with an applied magnetic fieldof 20, 100, 200 and 500 mT. A second set of measurements is performed at 3 V,with an applied magnetic field of 20, 100, 300 and 500 mT. The results of thesemeasurements are shown in figure 6.1. Note that 5 lines are indicated in the legendof the figure, while only 4 are shown per measurement. This is due to the differentmagnetic fields used at the two voltages.

0 30 60 90 120 150-0.10

-0.05

0.00

0.05

30 60 90 120 150 180

I (%

)

Angle (degrees)180/0

300 mT 500 mT

Angle (Degrees)

20 mT 100 mT 200 mT

3V 5V

Figure 6.1: The relative change in current as a function of the angle between the normal ofthe device and the externally applied magnetic field. These measurements are performed ona reference device for different external magnetic fields. The left graph shows measurementsperformed at 3 V, the right graph shows measurements performed at 5 V.

A sign change is visible in the angle dependence as a function of the externalmagnetic field, which is especially clear when inspecting the peak at 90◦ in themeasurements performed at 5 V. Furthermore, the shape of the angle dependencechanges as a function of the external magnetic field. Two contributions appear tobe present in the angle dependence, one at low magnetic fields and one at highermagnetic fields.

6.2. Electrical measurements 79

We also note the fact that the magnitude appears to be voltage dependent. Anincreased voltage shows a decreased effect at low magnetic field strengths, whileat high magnetic field strengths the effect becomes larger for an increased voltage.Increasing the voltage also appears to shift the magnetic field required for thenegative and positive contributions at 90◦ to cancel, which can be observed byinspecting the measurements performed at a field strength of 100 mT.

As stated in the introduction of this section, anisotropy is thought to cause theangle-dependence in the OMAR effect. Assuming anisotropy in the hyperfine fieldsin combination with isotropic spin-spin interactions is shown to lead to a cos2θangle dependence [6], which clearly does not conform to the angle dependence wemeasured. Therefore, a closer look at anisotropic spin-spin interactions is required.The most likely interactions between two polarons are the exchange and dipoleinteractions. The exchange coupling will be isotropic under the assumption of apoint approximation of the polaron wave function. The dipole interaction in apolaron pair is anisotropic as it strongly depends on the orientation of the spinsand their displacement vector ~R.

6.2.2 Low field effects

Spin mixing is thought to cause OMAR, therefore pairs of spins should be con-sidered when inspecting the angle dependence. As the mixing between the singletand triplet configurations is determined by their energy levels, these energy levelsshould be inspected in some more detail. Both the exchange and dipole contri-butions considered in the previous paragraph should be taken into account wheninspecting the energy difference between the singlet and triplet energy levels.

Upon inspection of the injection and transport of charge carriers in a device,we noted in section 2.1 that opposite charge carriers move in opposite directionsthrough the device. This movement is determined by the electric field induced bythe bias voltage applied to the device. The injection and transport is indicatedin figure 6.2a. In addition to the charge carriers, the electric and magnetic fieldsare also indicated, as is the displacement vector of an exciton. Because the chargecarriers move along the electric field, the displacement vector is aligned parallel tothe electric field. When the device is rotated with respect to the magnetic field,the displacement vector also rotates with respect to the magnetic field. The anglebetween ~B and ~R is indicated with φ. We now refer to φ = 0 as the parallelorientation and φ = π/2 as the perpendicular orientation.

The energy associated with the dipole interaction between two spins dependson their relative orientation, because they reside in each other’s magnetic field.The externally applied field ~B and the displacement vector ~R between the twospins determine their mutual orientation and consequently the dipole energy ofthe two spins. We will now consider a simplified classical picture concerning twospins in a magnetic field to intuitively explain the angle dependence. Figure 6.2bshows the singlet configuration for a parallel and perpendicular orientation of ~Band ~R. The magnetic fields caused by the spins are schematically drawn, showing


R

E

B

φ

(a)

R R

B B

(b)

R R

B B

(c)

T0

T-1

T+1

gµBB

∆S

φ=π/2

φ=0φ

(d)

Figure 6.2: (a) The displacement vector is displayed in a device structure, where ~R is parallelto ~E. Figure adapted from [36]. (b,c) A spin pair is separated by the displacement vector ~R. Thespins are precessing around a magnetic field ~B. The fields associated with the spin are drawn fora parallel and perpendicular orientation between the magnetic field and displacement vector. Thisis shown for the singlet configuration (b) and the triplet configuration (c). (d) Energy diagramshowing the three triplet states and the singlet state. The singlet state is shown to change withrespect to the T0 state with a changing angle φ.

matching field lines in the parallel configuration. The field lines in the perpendicu-lar case do not match, which indicates that the dipole energy is lower in the parallelconfiguration. Figure 6.2b now shows the triplet configurations for a parallel andperpendicular orientation of ~B and ~R. The situation is now exactly reversed, asthe field lines match in the perpendicular orientation while they do not in theparallel orientation. Consequently, the energy of the triplet state is lower in theperpendicular orientation.

The energy levels as discussed here are shown in figure 6.2d. As mentioned insection 2.1.5, only T0 and S are important in spin mixing due to Zeeman splittingof the triplet states. We noted that both the singlet and triplet energies shift inenergy while rotating the magnetic field, however in the figure we only show therelative shift of the singlet state with respect to the T0 state. The energy difference∆ between the singlet and triplet state energies is also indicated,

After this qualitative introduction, we now present the results of a quantitativequantum mechanical analysis of the energy levels for the singlet and triplet con-figurations. In this description, we include the exchange interaction between thetwo spins, caused by the wave function overlap of two polarons. Note that unlikedipole coupling, the exchange interaction does not depend on the displacementvector ~R but only on the separation distance |~R|. For large magnetic fields, it canbe shown that the energy difference between S and T0 can be expressed as:

∆ = 12∣∣∣D (

1− 3cos2θ)− 4J

∣∣∣ , (6.1)

where D is the dipole coupling strength and J is the exchange coupling strength.


These coupling strengths are defined as [43]:

D = µ0γ2eµB

4π1R3 (6.2)

J = J0e−2 R

R0 , (6.3)where γe is the gyromagnetic ratio of the charge carrier, µ0 the vacuum perme-ability and µB the Bohr magneton. J0 and R0 are determined by the inherentstrength of the exchange interaction and the decay of the wave functions.

In the case that the exchange interaction is larger than a quarter of the dipoleinteraction, a cos2θ relation is retrieved. Otherwise more maxima are found inthe energy difference between the two states, as can be seen in figure 6.3a inwhich equation 6.1 is shown for D = 2 and J = 0. The change in triplet excitonformation between B 6= 0 and B = 0 can be calculated using a numerical densitymatrix approach of the e-h pair mechanism as discussed in section 2.2.3. Oneof the parameters of these simulations is the energy difference between the singletand triplet states, thereby allowing the angle dependence to be taken into account.Because the triplet exciton formation is an essential part of spin mixing, it is agood measure for the magnetic field effects. From these calculations, an angledependence with the same shape is retrieved [6].

0 30 60 90 120 150 1800.0

0.5

1.0

1.5

2.0

(a.u

.)

Angle (degrees)

(a)

0 30 60 90 120 150 180-0.10

-0.08

-0.06

-0.04

-0.02

0.00

3 V 5 V

I (%

)

Angle (degrees)

(b)

Figure 6.3: (a) A plot of equation 6.1 for D = 2 and J = 0, showing multiple maxima inthe energy difference between the S and T0 states. (b) Measurements at an external magneticfield of 20 mT at 3 and 5 V, fitted with equation 6.4 showing that J = 0.02D and J = 0.05D,respectively.

The measurements as presented in figure 6.1 do not show the sharp peaksobserved in figure 6.3a. The rounding of the peaks in equation 6.1 can be explainedby incorporating one of several mechanisms. Changing the hopping rate from slowhopping to intermediate hopping, decreasing the external magnetic field strengthand introduction of a Gaussian distribution in the direction of the displacementvector ~R with a very small standard deviation all lead to rounding.

Because the ultra small field effect present in these samples is attributed to in-termediate hopping rates, this is a realistic way to incorporate rounding. Further-more, we use finite magnetic field strengths which can also realistically introduce


rounding. Finally, it is physically realistic to assume a Gaussian distribution of thedisplacement vector, as this vector points along the direction of the hopping path.Because this hopping path is percolating through the device, the displacementvector will not be directed along one particular direction, but will have a distribu-tion around a general direction (pointing from one electrode to the other). Thisis schematically shown in figure 6.4a for a constant and a disordered displacementvector. Simulations on the change of the triplet exciton fraction ∆χT as a func-tion of the angle are shown in figure 6.4b. See section 2.2.3 for more details aboutthese simulations. The figure shows simulations assuming a static displacementvector (black squares) and a Gaussian distribution in the displacement vector (redcircles). As the change in ∆χT is a measure for the MC, we show that roundingis introduced by assuming disorder in the displacement vector. Furthermore, therounding through such a mechanism is in agreement with our experiments. Notethat the sign of ∆χT is opposite to that of the MC.

(a)

0 30 60 90 120 150 180-5

-4

-3

-2

-1

Static value Gaussian distribution Fit

T (%)

Angle (degrees)

(b)

Figure 6.4: (a) A number of displacement vectors are drawn in an ideal situation (left) andwhen a certain disorder is assumed (right). (b) The change in triplet fraction due to a changingangle is calculated using a static displacement vector (black squares) and using a Gaussian spreadin the angle of the displacement vectors with a standard deviation of 0.05 (red circles). A fit usingequation 6.4 shows that the rounding is fitted perfectly.

In addition to this rounding effect, an analytical function for the angle depen-dence for delayed fluorescence in dye-sensitized anthracene crystals is given in theliterature:

δI ∝(∆2 + c2

)−1, (6.4)

in which ∆ represents the energy difference between the S and T0 states and c aconstant independent of spin-spin interactions [44].

This equation perfectly describes the line shape measured at low magnetic fieldstrengths. This is demonstrated in figure 6.3b, which shows the measurementsperformed at a field strength of 20 mT, which are fitted with this equation. Theexchange interaction strength with respect to the dipole coupling is found to berelatively small, J = 0.02D at 3 V and J = 0.05D at 5 V, confirming that dipolecoupling is the dominant spin-spin interaction mechanism in SY-PPV devices. The


weak exchange interaction can be explained by either a large polaron pair sepa-ration distance or a small polaron wave function overlap. These statements arerelated, because the wave function overlap decays with distance. Note that theexchange interaction decays at a higher rate compared to the dipole interaction.We also observe that the rounding caused by a Gaussian distribution in the dis-placement vector can be perfectly fitted using equation 6.4, which is demonstratedin figure 6.4b.

6.2.3 High field effects

The results for low field strengths can thus be understood. As indicated earlier, asign change of the angle dependence occurs for increasing magnetic field strengths,see figure 6.1. Furthermore, the shape of the angle dependence at high fields differsfrom the shape at low fields in a way that cannot be explained by the previouslydescribed system.

To get a better image of the magnetic field strength dependence, we decidedto look at the difference in magnetoconductance between the perpendicular (0◦)and parallel (90◦) orientations. Full magnetic field sweeps were measured at theseorientations according to the method outlined in section 3.2.1 in the section Mag-netic field dependence at constant angle. We then use equation 3.2 to calculateδMC. This way, we retrieve information about the angle dependence as measuredearlier. Because we normalized to the MC measured at 500 mT in the parallel ori-entation, we can compare the difference in line shape as a function of the appliedbias voltage.

These measurements are performed at several voltages. The results are shownin figure 6.5. From these results, it becomes clear that a distinction between lowfield and high field effects can be made. This observation is reinforced by lookingback at the angle dependent measurements performed at 5 V in figure 6.1. Inthis figure, a cancellation of the feature at 90◦ at a field of approximately 100 mTresults from the low field and high field contributions with an opposite sign. Forlow field strengths almost no voltage dependence is present, while for high fieldstrengths a clear increase with voltage is observed.

The difference in voltage dependence and shape we observe for the low and highfield strengths indicates that the underlying mechanism must differ. Several op-tions for a HFE mechanism are suggested in the literature, i.e. the ∆g-mechanism,the reaction of triplet excitons with polarons or the reaction of triplet excitons witheach other. In the ∆g-mechanism, spin mixing is increased by a difference in Landefactor of electron and hole polarons. This mechanism scales with the field strengthand therefore becomes more important at high field strengths. Because dipole cou-pling causes the anisotropy in the ∆g mechanism, just as for the low field effect,the shape of the angle dependence at low and high field strengths should not dif-fer. Additionally, the sign of the HFE caused by this mechanism should be equalto that of the LFE. As our measurements do show a difference in shape, the ∆gmechanism is not likely to cause the observed high field effect.


10-1 100 101 102 103

-1.5

-1.0

-0.5

0.0

0.5

MC

(%)

Field (mT)

2.5 V 3.0 V 3.5 V 4.0 V 4.5 V 5.0 V

Figure 6.5: The relative difference in MC between parallel and perpendicular orientations.

The triplet-exciton reactions also have a high field contribution, which is at-tributed to the zero field splitting (ZFS) of the triplet excitons. Zero field splittingis caused by spin-spin interactions between two nearby charge carriers, i.e. in anexciton, separating the degenerate energy levels of the triplet states. This effectis typically larger in magnitude than the hyperfine interaction [45]. Note thattriplet exciton based models have been suggested to be responsible for magneticfield anisotropy in the fluorescence and photo induced currents in anthracene andtetracene crystals [46, 47]. Anisotropy in the zero field splitting of triplet excitonscan result in a different shape of the angle dependence. Furthermore, the num-ber of triplet excitons in a device increases with the recombination current andthus with the voltage [48]. As such, the effects of a ZFS mechanism will increasewith an increasing voltage. The different behaviour at high fields might thus beunderstood using triplet exciton based mechanisms.

6.3 Luminescence measurements

Besides measuring the electrical signal, it is also possible to measure the electroluminescence while rotating the device in the magnetic field. This magneto electroluminescence (MEL) effect is measured at a constant current of 0.33 mA, corre-sponding to a bias voltage of approximately 5 V. The measurement technique isoutlined in section 3.2.2. Again, the sample is rotated in a magnetic field whilemeasuring the voltage and at the same time measure the photocurrent generatedby the photodiode. It should be noted that the used photodiodes were not checkedfor a magnetic field dependence. As such, a deviation in the electroluminescencemeasurements can occur.

6.3. Luminescence measurements 85

6.3.1 Results

The measurements are performed at a constant current of 0.37 mA and exter-nally applied fields of 500, 250, 100 and 20 mT. The changes in voltage (δV ) andluminescence (δL) as a function of the angle are displayed in figure 6.6.

0 60 120 180 240 300 360-0.01

0.00

0.01

V %

)

Angle (degrees)

20 mT 100 mT

250 mT 500 mT

0 60 120 180 240 300 360

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

L (%

)

Angle (degrees)

Figure 6.6: The relative change in voltage (left) and luminescence (right) as a function of theangle between the normal of the device and the externally applied magnetic field.

Looking at the change in voltage as a function of the angle, we note thatthe same shapes are found in comparison to the measurements performed at aconstant voltage, as shown in the section 6.2. The sign is inverted due to the factthat we now measure the voltage instead of current. The magnitude of the angledependence is 5 times as low due to the I ∝ V 3.7 relation found in section 5.2.Also note that the noise level is somewhat higher compared to the constant voltagemeasurements.

The angle dependent effects in the luminescence are found to be in the anorder of magnitude larger and having a different shape compared to the effectsin the voltage. A direct comparison between the shape of δV and δL are shownin figure 6.7a. Sharp features are observed at 35◦ and 145◦ in the luminescencewhereas they are observed at 60◦ and 120◦ in the voltage. The local maximumobserved in the voltage between these sharp features is smaller than those in thephotocurrent. At field strengths from 100 mT and higher, we observe a transitionat 60◦ and 120◦ in the luminescence. Also note that the sign change with increasingmagnetic field strength occurring in the voltage is not present in the luminescence.From these observations, it is clear that a different mechanism is causing the angledependence in the electro luminescence. Note that fitting the angle dependence ofthe luminescence with equation 6.4 is no longer possible due to the difference inshape.

Applying a magnetic field of 100 mT leads to the maximum angle dependenteffect in the luminescence. Inspection of the full magnetic field behaviour at theangle of this maximum effect, 34.2◦, shows a different line shape compared to the


same measurement performed at an angle of 0◦. The results of these measurementsare shown in figure 6.7b.

0 60 120 180 240 300 360

0.00

0.01

Angle (degrees)

V (%

)

0.00

0.02

0.04

0.06

0.08

0.10

L (%

)

(a)

-400 -200 0 200 400-1

0

1

2

3

4

[L-L0] / L0

[V-V

0] / V

0 / [L

-L0]

/ L0 (

%)

Field (mT)

0 degrees 34.2 degrees

[V-V0] / V0

(b)

Figure 6.7: (a) Angle dependence in the device current and luminescence measured at anexternally applied field of 20 mT. The device current was kept constant at 0.37 mA. The devicevoltage is shown in black squares while the luminescence is shown in red circles. (b) Full magneticsweep measurements of the device voltage and luminescence at a fixed current of 0.37 mA. Thesweeps were performed at an angle of 0◦ (black squares) and 34◦ (red circles).

The change in voltage at a certain magnetic field strength compared to thevoltage at zero external field is plotted as a function of the magnetic field strength,indicated by [V − V0]/V0 or MV. The change in luminescence is expressed in thesame way, using [L−L0]/L0 or MEL. The MEL and MV are shown for the deviceangles of 0◦ and 34.2◦.

As mentioned before, the line shape of the MEL measured at 34.2◦ significantlydiffers from the measurement at 0◦. The change in line shape has an onset atapproximately 10 mT, it increases to a maximum at 100 mT and decreases again forlarge fields. No such change is observed in the MV, indicating that some differentprocess causes the difference in line shape. The angle dependence as displayed infigures 6.6 and 6.7b can be directly related by looking at the change between themeasurements at the two angles. It is clear that the large angle dependent effectat 100 mT corresponds to the change in line shape in these magnetic field sweepmeasurements.

6.3.2 Discussion

We conclude this section by observing that the shape of the angle dependence inthe electro luminescence is different compared to that of the device current anddevice voltage. The effects measured are also larger in the electro luminescencecompared to the device current and device voltage.

The mechanism responsible for the electro luminescence, recombination of sin-glet excitons to the ground state, might provide an additional contribution on top

6.4. Summary 87

of the magnetic field dependence of the device current. Assuming that the mag-netic field dependence of the luminescence is the sum of field dependences in thedevice current and the recombination, the latter must then have a stronger fielddependence compared to the device current. This conclusion can be supported bythe fact that angle dependent effects are up to 5 times as large in the luminescencecompared to the device current.

On the other hand, if the recombination is assumed to be independent of themagnetic field, triplet excitons do not play a direct role in the luminescence asthe radiative recombination of triplet excitons is negligible compared to that ofsinglet excitons. Recombination of singlet excitons causes light emission, howeverthese singlet excitons can be mixed with triplet excitons, still allowing a mag-netic field dependence in the luminescence. As the singlet and triplet excitons arenow closely related in such a mechanism, combined with the observation that theluminescence shows an angle dependence with a larger magnitude, an additionalmagnetic field dependent mechanism must be present. As a result, it is unlikelythat the recombination process is independent of the orientation of the magneticfield. An anisotropic field dependence might thus be the cause of the observedangle dependence in the luminescence.

6.4 Summary

In this chapter we have introduced a new, automated way of measuring the angledependence. We have performed angle dependent measurements on SuperYellow-PPV devices, measuring both the device current or voltage and the luminescence.From the device current, we have determined that a clear distinction can be madebetween a high field effect (HFE) and a low field effect (LFE). We have shown thatthe LFE can be explained by assuming a dipole coupling which is significantlylarger than the exchange coupling in these devices. An equation used in theliterature to describe the angle dependence in anthracene crystals is found tocorrectly fit the angle dependence at low field strengths.

The HFE is found to have a significant voltage dependence, which has a muchsmaller magnitude in the LFE. Furthermore, the HFE has an opposite sign com-pared to the LFE. The ∆g mechanism has thereby been discarded as a cause ofthe different HFE effect, because no sign change would occur in this mechanism.Triplet exciton based models have been proposed in the literature to account formagnetic field anisotropy in certain organic crystals. These models also show avoltage dependence and thereby are good candidate to explain this high field ef-fect. Simulations of these triplet models could be performed in order to prove thevalidity of the models in our devices.

Angle dependent measurements of the luminescence have shown a differentshape compared to the device current. The relative effects in the luminescencewere also observed to be larger. A possible explanation can be found in assumingan anisotropic magnetic field dependent recombination. The details of such a


mechanism have not been analysed and as such, no verification of such a mechanismhas been provided.

7 Conclusion and Outlook

The first section of this chapter provides a summary of the conclusions from thisreport. An outlook for further research is provided in the second section.

7.1 Conclusions

In this section we will summarize the main conclusions of the work presented inthis thesis. The conclusions are grouped per chapter.

Chapter 4: Ferromagnetic structures for fringe field MRIn this chapter, several analysis techniques have been applied to characterize theferromagnetic layers used in chapter 5. Additionally, a cobalt layer has been sim-ulated in order to get insights in the fringe fields caused by such a layer. Thesewere our main conclusions:

• The surface of a number of materials has been inspected using AFM, showingthat a certain roughness is required for successful spin coating.

• The magnetization of a cobalt-platinum multilayer stack is shown to have anout-of-plane magnetization. The magnetization can be switched from −MS

to +MS by applying an external field perpendicular to the multilayer stack.Domains are shown to be formed in the transition using Kerr microscopy.

• A thick cobalt layer is shown to have an in-plane magnetization. This mag-netization switches when fields of approximately 1.5 mT are applied parallelto the layer. The magnetization is also shown to be pulled out-of-plane whena significant field is applied perpendicular to the layer. Two regimes can bedistinguished, for small fields the magnetization changes quickly, while itchanges slowly for large fields.

• Simulations on a cobalt layer have been performed, showing that fringe fieldscan occur from domains. On average, the fringe fields decay as r−1 while thegradient in the fields decays as r−2.

Chapter 5: Fringe Field MagnetoresistanceIn this chapter we introduced a ferromagnetic layer to a regular OLED structure.The magnetoconductance was measured in two orientations of the external fieldwith respect to the device. These were our main conclusions:

89

90 Chapter 7. Conclusion and Outlook

• Adding a ferromagnetic layer to a regular OLED device structure is shownto have a significant effect when sweeping the external magnetic field. Thedirection of the magnetization of the ferromagnetic layer as well as the ori-entation of the external field with respect to the device is found to be ofsignificant importance.

– A multilayer stack with an out-of-plane magnetization changes the MCwhen the field is perpendicular to the device structure. The MC is onlychanged when the magnetization of the multilayer is switched.

– Switching the in-plane magnetization of a cobalt layer in the parallelorientation is observed in the MC as a small effect.

– Pulling this in-plane magnetization out-of-plane by applying an externalfield perpendicular to the device has shown effects of up to 71 % in theMC at large field strengths.

• The ultra small field effect has been shown to disappear by adding a cobaltlayer. Basic simulations performed in this work support this observation,assuming that fringe fields are present even in the absence of an externalfield.

• The change in MC has been shown to scale as r−2 by varying the distancebetween the organic and cobalt layers.

• The change in MC caused by a cobalt layer has been shown to be voltagedependent. Analysis has shown that this voltage dependence cannot becaused by a change in the recombination location in the organic layer.

Chapter 6: Angle Dependence in SY-PPVIn this chapter we measured the effect of gradually changing the orientation ofthe external field with respect to a regular OLED device. These were our mainconclusions:

• The angle dependence of SY-PPV consists of the sum of low field and highfield effects.

• The low field effect can be explained by dipole coupling, which is assumedto be significantly larger than exchange interaction in this material.

• The HFE is shown to have a larger voltage dependence than the LFE. Addi-tionally, the HFE has been found to have an opposite sign compared to theLFE.

• The HFE cannot be explained by the ∆g mechanism, triplet exciton mech-anisms are suggested to cause the HFE angle dependence.

• The angle dependence in the luminescence has been shown to have a differentshape and a larger magnitude compared to the angle dependence in thecurrent. An anisotropic magnetic field dependent recombination has beensuggested to cause these differences.

7.2. Outlook 91

7.2 Outlook

When working on a research project, questions are answered but new ones alsoarise. Here we will discuss possible future experiments to improve the understand-ing of OMAR and the application of ferromagnetic layers in devices.

We have presented simulations showing the fringe fields for a simulated domainstructure of a ferromagnetic layer. In order to realistically model the influence ofthese fringe fields on the MC, the change in magnetization of the ferromagneticlayer by applying an external field should be taken into account. By changingthe magnetization, the fringe fields will be modified, resulting in a non-constantinfluence on the MC. The magnetization of the cobalt layers used in this thesishave shown to behave in two regimes when their magnetization is pulled out-of-plane. Due to these regimes, simulation will likely not be straight forward, becausethe exact domain structure of these real life devices is unknown.

By adding a ferromagnetic layer, we have shown that the MC can be signifi-cantly altered. Because the distance between the organic and ferromagnetic layershas been shown to be of great importance, future experiments should be performedwhile minimizing this separation distance. Because fringe fields decay relativelyquickly, a large range of fringe field gradients are present in the organic layer.Changing the thickness of the organic layer can be a good way to limit the fieldgradient present in the SOC. When this is combined with changing the distancebetween the organic and ferromagnetic layers, the field gradient present in the OSCcan selectively be chosen. The influence of the magnitude of the field gradient canthereby be controlled and examined more closely.

The ferromagnetic layers used in this thesis have typical anisotropies. Thecobalt layer has an in-plane magnetization, while the cobalt platinum multilayerhas an out-of-plane magnetization. Changing the anisotropy of the ferromagneticlayer can give additional information on the effect of fringe fields. To this end, thesurface structure below a cobalt layer can be changed to influence the behaviourof the magnetization, as mentioned in Appendix F. The anisotropy of a multilayerstack can also be modified, usually by changing the composition of the stack.Changes can be made in the thickness of the layers, but also in the used materials.From the literature, it is also known that the domain structure can be influencedby changing the composition of a multilayer.

An alternative to adding a ferromagnetic layer to a standard device structure isthe addition of superparamangetic beads to the organic material. Such beads willcause more homogeneous fringe fields throughout the organic layer. Alternativedevice structures can also be considered to maximize the influence of fringe fields.Recently, a very large magnetoconductance has been measured in molecular wires.An effect of up to 93 % has been measured at fields of only 15 mT [49]. Applyinga ferromagnetic layer to such a molecular wire might allow a functional memorydevice to be fabricated. The fringe fields of this layer should be controllable toallow or prohibit spin mixing, such that two states can be discerned.

92 Chapter 7. Conclusion and Outlook

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http://www.sigmaaldrich.com/catalog/product/aldrich/443263?lang=en&region=NL

http://www.sigmaaldrich.com/catalog/product/aldrich/443263?lang=en&region=NL

A Sample preparation

In order to create working devices, several steps have to be completed. First thesubstrate needs to be cleaned. This process depends on the type of substrate and thedeposition methods to be used after cleaning. An overview of the cleaning processis given in section A.1.

After the substrate has been cleaned, the actual thin films and electrodes can bedeposited. Section A.2 is therefore written to provide an overview of the techniquesused to deposit thin layers of certain materials.

A.1 Cleaning methods

The devices used in this project were created using various deposition techniques.Before depositing layers however, the used substrate has to be cleaned.

Most samples were created on glass substrates, which were often pre-patternedwith an ITO layer. These samples were cleaned using the process described insection A.1.1. When more complex electrodes are required, i.e. ferromagneticmultilayers, substrates are cleaned as described in section A.1.2.

A.1.1 (ITO) Glass substrates

The glass and pre-patterned ITO substrates are cleaned in several steps. Firstthe substrates are put in a beaker with acetone, which in turn is placed in anultrasonic bath for fifteen minutes. The substrates are then rinsed with soap(Dodecyl sulfate sodium salt 99%) by applying friction and afterward put in anultrasonic bath for fifteen minutes. The soap is removed by letting water flowby the substrates for twenty minutes. In the final step, the substrates are put ina beaker with isopropanol and are put in an ultrasonic bath for twenty minutes.The substrates are then dried by blowing nitrogen over them. If PEDOT:PSS isdirectly spincoated (see section A.2.1) after the cleaning process, the substratesare also given a thirty minute UV-Ozone treatment.

A.1.2 Substrates used for sputtering

The glass substrates used for sputtering are cleaned in a different way. The sub-strates are first put in a beaker with ammonia and put in an ultrasonic bath for

99

100 Appendix A. Sample preparation

five minutes. Afterwards, the substrates are put in a beaker with acetone and areagain put in an ultrasonic bath for five minutes. Finally the substrates are put ina beaker with isopropanol and are put in an ultrasonic bath for five minutes. Thesubstrates are then dried by blowing nitrogen over them.

A.2 Deposition methods

Once the substrates are cleaned, as described in Appendix A.1, and a bottomelectrode is present, a PEDOT:PSS layer is usually deposited by spin coating,which is described in section A.2.1. This process is also applicable to deposit apolymer-based organic layer. An organic layer consisting of small molecules canbe evaporated in high vacuum, as described in section A.2.2. Finally, metalliclayers (usually functioning as bottom or top electrodes) can be evaporated in highvacuum as well, see section A.2.3. During both evaporation steps, the substrateholder rotates to increase the homogeneity of the deposited layer. The pressureduring evaporation is kept as low as possible in order to increase the purity ofthe deposited material. To create more complex bottom electrodes, multilayerstructures can be deposited using sputtering as described in section A.2.4.

A.2.1 Spin coating

Some layers, usually consisting of polymers, need to be deposited using a processcalled spin coating. An example of such a layer is the PEDOT:PSS layer commonlyused in organic devices. To spin coat a material on a substrate, first a solution ofthat material has to be prepared. Details of the solutions and related spin coatparameters of the materials used in this thesis are provided in table A.1.

Table A.1: Parameters of used solutions and related parameters to spin coat certain materials.

Material Solvent Concentration Spin speed Spin duration CommentsPEDOT:PSS H2O ? 3000 rpm 120 sec Already disolved, only needs filtering

Super-yellow PPV Chlorobenzene 6 mg/ml 1200 rpm 60 sec

The substrate is placed on a centrally rotating plate, after which an appropri-ate amount of the solution is applied to the substrate. By rotating the plate, acentrifugal force acts on the fluid, causing the fluid to spread outward. The solventused to create the solution evaporates during rotation, changing the viscosity andhalting the flow of the solution. To make sure all solvent has been evaporated,an extra heating step is often included after spin coating. The thickness of thefinal layer is dependent on several parameters, mainly the viscosity of the createdsolution and the spin speed. A schematic representation of the process is shownin figure A.1a.

A.2. Deposition methods 101

rotating platesubstratesolution

rotation

centrifugal

force

1)

2)

3)

(a)

shutter

vacuum chamber

organic materialmetal

rotation

+ -

substrate

heater

metal atom /

organic molecule

(b)

substrate

+ -

target

plasma

Ar ion

shutter

target

atom

vacuum chamber

table

(c)

Figure A.1: (a) The process of spin coating a material by applying a solution on a substrate(1), rotating the sample to spread the solution due to the centrifugal force (2) resulting in a thinlayer of the desired material (3). Image adapted from [50]. (b) Schematic representation of athermal evaporation setup, showing the substrate holder, mask and shutter above the metal ororganic source(s). (c) Schematic representation of a sputtering setup. A plasma is generated torelease ions from a target material which can land on the substrate. Image adapted from [50].

A.2.2 Organic evaporation

When using small molecules as the basis for an organic layer, they are usuallyevaporated onto the substrate in high vacuum. To evaporate small molecules,the material must be placed in a quartz crucible that can be safely heated toseveral hundred degrees. The material can be used in either solid, powder orliquid form. The crucible is placed in a heating pocket, regulated by a thermostat,which is manually operated. The substrates are placed in a substrate holder abovethe heating pockets. This substrate holder can also be used to hold a mask forpatterning of the deposition layer.

Before the organic material can be evaporated, the system must be pumpeddown to high vacuum levels (typically 3 · 10−7 mbar). The organic material shouldthen be slowly heated to the point it starts evaporating. Heating the material tooquickly can cause the temperature to locally rise to such temperatures that thematerial starts breaking down. The deposition rate is linked to the evaporationtemperature and is monitored by measuring the resonance frequency of a calibratedcrystal, which depends on the amount of material deposited on the crystal. Oncea sufficient rate is achieved, the shutter can be opened to deposit the material.

Before the vacuum can be broken, the organic material should be cooled downconsiderably so it will not reach critical temperatures when the pressure is in-creased. The used setup for thermal evaporation is shown in figure A.1b, inwhich the organic material holder is drawn on the right side. Details for thesmall molecules used in this thesis are provided in table A.2.

102 Appendix A. Sample preparation

Table A.2: Parameters used to evaporate certain materials.

Material Appearance Tevap CommentsAlq3 green-yellow 190-210 Temperature depends on the ammount of material in the crucibleTPD green-grey / transparent 120 The material becomes transparent after the first evaporation

A.2.3 Metal evaporation

Metallic layers can be evaporated in a similar way, using almost the same setupas for organic evaporation. Instead of using a heating pocket, the metal is placedon a metallic boat. This boat is clamped between two poles and can be heated byrunning a current through it. This way, the desired metal is evaporated.

As with the organic evaporation procedure, a high vacuum level should beattained (typically 3 · 10−7 mbar). Usually the metal is heated in two steps, firstgradually increasing to a certain level to let the metal heat up. After that, themetal is heated further until a sufficient rate is achieved.

Once the desired layer thickness has been achieved, the current can be stoppedand the system can be filled with nitrogen. The used setup for thermal evaporationis shown in figure A.1b, in which the boat containing the metal is drawn on theleft side.

A.2.4 Sputtering

To create more complex electrodes, consisting of multiple thin metallic layers, onecan use sputter deposition. Like thermal evaporation as described in the previoustwo sections, this process requires a high vacuum system. Sputtering however, isbased on creating a plasma which bombards a target of the desired material withparticles. As a result, target atoms are ejected and deposited on the substrate.

To create the plasma, an inert gas is injected near the target, after which alarge potential difference is applied between the target and a shielding ring. Theplasma ions will have a large energy and will bombard the target, thereby ejectingtarget atoms. The ejected atoms will move away from the target, headed towardthe table supporting the substrate. As with the thermal evaporation techniquefrom the previous sections, a shutter is used to control the actual deposition onthe substrate. The rate at which material is deposited depends on the targetmaterial, the pressure and the power used to create the plasma [50]. The systemas described here is schematically shown in figure A.1c.

To create a stack of multiple layers, the table holding the substrate can beplaced under the targets in the order of the desired stack. The combination ofmultiple thin layers can create an electrode with specific properties. For example,a cobalt nickel multilayer stack results in an out-of-plane magnetized electrode.

B Used materials

Several materials can be used in making organic based devices. The materials usedin this thesis, including their properties and some general information are discussedin the sections below. At the end of this section, a diagram is presented showingthe concerning energy levels for these materials. These energy levels are crucial indesigning a working device.

B.1 Organic materials

The materials discussed in this section are organic in origin, indicating that theymainly consist of carbon and hydrogen atoms. Small molecules like Alq3 andTPD are usually thermally evaporated, while polymers like SuperYellow-PPV andPEDOT:PSS are generally spin coated. These respective techniques are discussedin section A.2.

The material properties of these organic materials will be briefly discussed inthe following subsections.

B.1.1 Alq3

Alq3, or tris(8-hydroxy-quinoline)aluminum is a small molecule with the chemicalformula Al(C9H6NO)3, the structure is shown in figure B.1a. Several parametersare important for depositing an Alq3 layer, which are described in section B.1.1.

Alq3 has a much larger electron mobility than hole mobility, which is differentfrom most organic semiconductors. The electron mobility has been reported to bein the order of 10−6-10−5 cm2V−1s−1, while the hole mobility is reported to be inthe order of 10−8-10−7 cm2V−1s−1 [51]. Even though the electron mobility is muchlarger than the hole mobility, it is still much smaller than the mobility in silicon.The electron mobility shows a strong dependence on the electric field, which is acharacteristic of hopping transport. On a side note, degradation of the material ismainly caused by the injection of holes in the organic layer [52].

Electronically, the HOMO and LUMO levels of bare Alq3 are located at 5.7eV and 3.0 eV respectively. These values are determined by cyclic voltammetry,based on the reduction and oxidation of the material [53].

103

104 Appendix B. Used materials

N

O

A l

O

N

O

N

(a)

RO

OR

OR

OR

np

m

(b)

Figure B.1: Chemical structure of the organic light emitting materials used in this thesis. (a)Alq3. (b) SuperYellow PPV (SY-PPV).

Deposition parameters

Because the quinoline groups in Alq3 can be oriented in two distinct ways, twogeometrical isomers (mer idional and facial) exist. Both isomers are present in thinfilms deposited by sublimation in high vacuum. Furthermore, thin films createdthis way are amorphous [54].

When no PEDOT:PSS layer is used, the Alq3 layer thickness is of influenceon the luminescence of the device. This is attributed to the position where re-combination takes place in the device. When the layer has its optimal thickness,recombination takes place at the centre of the organic layer. When the layer getsthinner, recombination will take place near the Alq3/ITO interface. For thickerlayers, electrons and holes are trapped before recombining [55].

The deposition rate of the Alq3 layer is of influence on the morphology of theorganic layer and thus on its electronic properties. Low deposition rates (<0.15nm/s) lead to local pinholes in the material and resistive-like electrical behaviourwithout luminescence. High deposition rates (>0.4 nm/s) lead to dense, uniformsurfaces and diode-like electrical properties, with luminescence [56].

B.1.2 SuperYellow PPV (SY-PPV)

In contrast to Alq3 as discussed in the previous section, the poly(p-phenylenevinylene) derivative ’Super Yellow’ is not a small molecule but a (co)polymer.The structure of this polymer is shown in figure B.1b. In order to use it, it isgenerally dissolved in chlorobenzene after which it can be spin coated.

The hole mobility of SY-PPV is much larger than the electron mobility, incontrast to Alq3. The mobility is reported to be in the order of 10−6 cm2V−1s−1

[57]. The HOMO and LUMO levels of the material are located at 5.2 and 2.8 eV,respectively [20].

B.2. Metals and isolators 105

B.1.3 PEDOT:PSS

The material poly(3, 4 − ethylenedioxythiophene) with poly(styrenesulfonate)dopant (also known as PEDOT:PSS) is a hole transporting layer used to lowerthe hole injection barrier, resulting in an easier hole injection. This yields a lowerthreshold voltage at which the device starts emitting light, as well as an increasedmaximum luminescence [9, 55]. The material electronically behaves semi-metallic,and is characterized by a work function of 5.1 eV [58].

Furthermore, the PEDOT:PSS is thought to reduce the roughness of the ITOsurface [59]. As the PSS dopant is water-based, the PEDOT with its PSS dopantis easy to process [60]. The structure of PEDOT is shown in figure B.2a while thestructure of PSS is shown in figure B.2b.

B.1.4 TPD

Another material, N,N ′−Bis(3−methylphenyl)−N,N ′−diphenylbenzidine orTPD, can also be used as a hole transporting layer. In contrast with PEDOT:PSS,this material is a small molecule and not a polymer. As a result, TPD can beevaporated in high vacuum just like Alq3. Energy-wise, TPD can be used incombination with PEDOT:PSS, making for an easier hole injection. As a result,the current density greatly increases, as well as an increase of electroluminescenceefficiency and brightness [61].

The HOMO and LUMO levels of TPD are located at 5.5 and 2.3 eV, respectively[62]. The structure of the small molecule is shown in figure B.2c.

O O

S

OO

S

n

(a)SO3H

n

(b)

CH3

N N

H3C

(c)

Figure B.2: Chemical structure of the organic hole transporting materials used in this thesis.(a) PEDOT. (b) PSS. (c) TPD.

B.2 Metals and isolators

Besides the organic materials, metallic and even isolating materials are used aselectrodes and injection improving layers. As these materials are an essential partof any working device, they are also briefly discussed.


B.2.1 Aluminum

Aluminum is commonly used as the top electrode in organic light emitting diodes.The material is suitable as an electron injecting material, especially in combina-tion with lithium fluoride (see section B.2.5). The work function of aluminum isreported to be 4.1 eV [63].

In the facilities available to us, aluminum can either be thermally evaporatedor sputtered, see section A.2 for more details about these processes. Thermalevaporation is used when creating devices at the chemistry department, either tocreate a top electrode or as a seeding layer for an additional cobalt layer at thebottom of the device. Sputtering is used when creating multilayer stacks, whereit is deposited as a capping layer. Oxidation of this sputtered layer is thought toincrease the attachability of PEDOT:PSS during the spin coating process.

B.2.2 Cobalt

As cobalt is one of the ferromagnetic elements, it is an interesting material to usewhen investigating fringe field effects in organic semiconductors. Due to its workfunction of 5.0 eV, it is less suitable as an electron injector and more as a holeinjector [63]. However, when using conductive spacer layers between the cobaltand organic layers, the work function is no longer important.

Cobalt can be thermally evaporated and sputtered, just like aluminum. Ther-mal evaporation requires a relatively thick tungsten boat, as the cobalt and tung-sten react before the cobalt is evaporated. Sputtering is typically used in thecreation of multilayer stacks, however it can also be used to create better definedflat layers.

B.2.3 Platinum

Platinum is a non-magnetic metal with a work function of 5.6 eV in a polycrys-talline layer [63]. The material can be used in creating multilayer stacks in con-junction with cobalt.

In this thesis, platinum is purely sputtered in multilayer stacks and as a spacerlayer. In such multilayer stacks, interface effects between the cobalt and platinumlayer cause the magnetization of the cobalt layer to be pulled out-of-plane. Byvarying the layer thickness of these layers, the magnetic properties can be influ-enced.

B.2.4 Indium Tin Oxide (ITO)

Indium tin oxide (ITO) acts as a hole injection layer, and is often used in OLEDfabrication due to its optical transparency. ITO is however known to have arelatively large surface roughness (measured using AFM) [64]. This roughness has

B.3. Energy levels 107

also been qualitatively determined using a scanning electron microscope (SEM), seefigure B.3a. This roughness leads to a relatively ill-defined interface. Additionally,the edge of the pre-patterned ITO layer is found to be very rough.

The work function of ITO is reported to be 4.7 eV [65].

B.2.5 Lithium Fluroide (LiF)

LiF is an ionic compound and as such, in principle, it does not have any mobileelectrons or ions for conduction while in solid state. It should be noted though, thatno material is purely ionic as some degree of covalent binding is always present.As a result, LiF is at best a poor conductor and as such theoretically this materialwill not improve devices.

The exact function of the presence of a LiF layer still is a matter of discussion,however it is known to enhance device performance [66, 67]. Practically, whencombined with an aluminum electrode, the work function at the interface of theresulting layer is reduced, corresponding to an easier electron injection.

It is suggested that the LiF layer functions as a buffer between the organic layerand the top electrode, which would improve electron injection. Another suggestionis that the LiF prevents the top electrode from diffusing into the organic layerduring evaporation and thereby creating a neater interface [18, 64].

B.3 Energy levels

The energy levels of all discussed materials are displayed in figure B.3b.

(a)

Vacuum level

-4.7

ITO

-5.1

PE

DO

T:P

SS

-5.5

-2.3

TP

D

-5.8

-3.1

Alq

3

-5.2

-2.8

SY

-PP

V

LiF

-4.1

Alu

min

um

-5.0

Co

ba

lt

-5.6

Pla

tin

um

(b)

Figure B.3: (a) A SEM image showing the roughness of the ITO surface. (b) Energy levels ofall materials used in the creation of organic-based devices.


C Additional measurement tech-niques

In addition to the electrical and optical measurement techniques we discussed insection 3.2, some additional techniques were used to characterize certain aspectsof the devices used in this thesis. We will first consider the techniques used todetermine the magnetic properties of the ferromagnetic layers used in our devices.In the second section, we will discuss the techniques used to determine the surfaceproperties of layers used in our devices.

C.1 Magnetic characterization

In order to characterize the magnetic properties of a ferromagnetic layer, severaltechniques can be used. Various techniques have different applications, howeverin this thesis we are only interested in determining the magnetization of a certainlayer as a function of the applied magnetic field.

Generally, the magnetization of a ferromagnetic material changes when an ex-ternal field is applied. Depending on the material, the magnetization can be di-rected in-plane or out-of-plane, which has to be kept in mind when positioningthe sample. To probe the magnetization, an external field ~H is generally sweptforward and backward to measure the hysteresis loop for the probed material.By sweeping the external field, the remanence magnetization (the magnetizationwhen no external magnetic field is present), the coercive field (the field at which themagnetization is zero) and the saturation magnetization can be determined. Anexample of such a hysteresis loop obtained by using MOKE is shown in figure C.1.

The first technique we will discuss is based on the so-called magneto opticalKerr effect (MOKE). This effect is based on the fact that linearly polarized lightis rotated when it interacts with a magnetized material. The second subsectionwill consider the superconductive quantum interference device, which uses theJosephson effect to measure the magnetic flux caused by a material.

C.1.1 Magneto optical Kerr effect (MOKE)

When considering a ferromagnetic material, the spin system is polarized, causinga net magnetization. In order to measure the magnetization, the spin system thus

109

110 Appendix C. Additional measurement techniques

-15 -10 -5 0 5 10 15

-1

0

1

- 0HC

-MR

M /

MS (a

.u.)

0H (mT)

0HC

MR

Figure C.1: An example of a hysteresiscurve obtained by using MOKE, showing the remanencemagnetization MR and the coercive field µ0H.

needs to be probed. When no spin-orbit (SO) coupling is present in a system, thedipole selection rules for interaction with EM fields (light) only allow transitionsin the angular moment L with ∆l = ±1. In order to probe the spin system withlight, a coupling between the total spin ~S and angular moment ~L thus needs tobe present. This can be caused by spin-orbit coupling, which couples the angularmomentum and the total spin through the total angular momentum ~J = ~L + ~S.The dipole selection rules will now apply to the total angular momentum ~J insteadof the angular moment ~L, allowing interactions with ∆j = ±1. The angularmomentum of light can thus interact with the total spin of the magnetic materialwhen SO-coupling is present.

In a practical setup, linearly polarized light is shone on a magnetic material.Due to the interaction described above, the polarization of the reflected light isrotated. This rotation is called the Kerr rotation and is proportional to the mag-netization of the material. By using a linear polarizing filter, a rotation of thepolarization will induce a change in the light intensity. The magnetization of amagnetic material can now be investigated as a function of an externally appliedmagnetic field by measuring the light intensity.

Several geometries with respect to the magnetization direction and light direc-tion can be distinguished. When the magnetization is out-of-plane, the incidentlight beam has the largest interaction when its incidence is also perpendicular tothe surface. This geometry is shown in figure C.2a. When the magnetization isin-plane, two situations can be distinguished: longitudinal and transverse (figuresC.2b and figure C.2c), in which the magnetization points in the direction of theplane of incidence or perpendicular to that plane. When measuring the magnetiza-tion of a sample, one thus has to distinguish in-plane and out-of-plane orientationsof the magnetization.

C.1. Magnetic characterization 111

Mx

y

z i r

(a)M

i r

(b)

M

i r

(c)

Figure C.2: Various geometries available for MOKE measurements. (a) Polar mode. (b)Longitudinal mode. (c) Transverse mode. Figures adapted from [68].

C.1.2 Superconductive Quantum Interference Device (SQUID

The magnetization of a layer can also be inspected using a superconductive quan-tum interference device (SQUID) magnetometer. An advantage of the SQUID overthe MOKE technique is that the absolute value of the magnetic moment can bedetermined. In this thesis however, we are not interested in the magnetic momentbut in the change in magnetization as a function of the applied field. We will givea brief overview of the working of the SQUID, for a more detailed description andpossible applications we refer to [69].

The SQUID is based on a superconductive ring with one or more Josephsonjunctions. Such a junction is an insulating interruption in the superconductingring, also referred to as the weak link. The weakly bound electrons in a supercon-ductor (Cooper pairs) are able to tunnel through such a junction if the coherencelength of such a pair is larger than the thickness of the junction. A superconduc-tive ring with such a Josephson junction will thus remain superconductive whenthe current is lower than a critical current Ic. When the current becomes larger,the superconductive behaviour is lost. The V-I curve for a Josephson junction isshown in the left part of figure C.3.

Vo

lta

ge

Superconducting

State

Normal

State

CurrentIB

1 Φ0

Flux Quantum: Φ0 = 2.068 x 10-15 Wb

Vo

lta

ge

Magnetic Field

Figure C.3: Left: The V-I characteristic of a Josephson junction. Right: The voltage os-cillates as a function of the field with a periodicity of the quantum flux. Image adapted from[69].


If we now take a look at the magnetic flux in the superconductive ring, wefind that it is quantized. Changes in the flux are only allowed in discrete steps ofthe flux quantum Φ0. Inductively coupling a magnetic flux into the ring createsa screening current to make sure the flux remains quantized. These screeningcurrents effectively increase or decrease the critical current Ic depending on thedirection of the induced flux. A fixed bias current Ib larger than the critical currentis now applied. This current is chosen such that it is halfway the transition fromsuperconductive to normal resistive behaviour, as indicated in figure C.3. Whenthe flux is now increased or decreased, the critical current is changed and thus themeasured voltage is also changed as can be observed from the figure. Increasing themagnetic flux thus induces an oscillation voltage with the period of a flux quantum,as the flux in the ring is only added in quantized amounts. This behaviour isindicated in the right part of figure C.3. By measuring the change in voltage, theflux coupled into the SQUID ring can thus be determined.

The setup used in this thesis makes use of a MPMS SQUID VSM by QuantumDesign, where VSM is an abbreviation for vibrating sample magnetometer. Asimplied by the name, the sample is vibrated in the z-direction. The vibrationinduces a changing flux in the detection coils, which are so-called second-ordergradiometers. Such coils are insensitive to uniform and linearly changing magneticfields and thereby are more sensitive in combination with the vibrating sample.The current in these detection coils is now inductively coupled to the SQUID ringwhich allows a sensitive determination of the flux caused by the sample.

C.2 Thickness and surface characterization

Several techniques are available to inspect the surface and thickness of variouslayers. Techniques like scanning tunnelling microscopy (STM) and x-ray diffraction(XRD) are powerful and sensitive but limited to the upper most layer and arerelatively complex to use. The materials used in this thesis have relatively largedimensions, both the size of the organic molecules and the thickness of the usedlayers, allowing the use of less sensitive techniques.

The thickness of organic and metallic layers created using thermal evaporationand spin coating are measured using profilometry. The surface roughness of certainmaterials is investigated using atomic force microscopy (AFM). These techniquesare briefly discussed in the following sections.

C.2.1 Stylus-based profilometry

Layer thicknesses can be measured for multiple purposes, for example calibrationor verification of the desired thickness. By making sure an edge is present inthe material which needs to be measured, usually achieved by making a scratchthrough the material or using a mask during deposition, the step height at theedge can be measured. By scanning some region around the edge, the height of

C.2. Thickness and surface characterization 113

the edge can be reliably measured.This is achieved by moving a stylus over the edge and making sure a constant

force is applied on the stylus during this process. Due to the fact that a constantforce acts on the stylus, the position of the stylus closely follows the surface. Asa result, the height of the stylus is measured as a function of the position ofthe stylus. By determining the difference in height between the two sides of theedge, the thickness of the deposited layer is accurately measured. A schematicrepresentation of a measurement setup is shown in figure C.4a.

Deposited layer

Substrate

(a)

Sample

Piezo controlled

movement (x, y, z)

Laser diode

Lens

Cantelever

Tip

Photodetector

Mirror

Controlling

electronics

(b)

Figure C.4: Schematic representations of setups for: (a) profilometry and (b) atomic forcemicroscopy.

Several parameters are of importance on the accuracy of the measurement. Thediameter at the end of the stylus limits the resolution in the lateral and perpen-dicular directions. At the same time the cleanness of the substrate is also limitingthese resolution. The resolution in the perpendicular direction of the device itselfis claimed to be 1 A, however the repeatability of a step height measurement isclaimed to be 6 A at most in a one sigma certainty region [70]. Realistically, a fewnanometres resolution is thus achievable.

C.2.2 Atomic Force Microscopy

In order to measure the surface of a certain material, atomic force microscopy(AFM) can be used. A bendable cantilever, equipped with a sharp tip, is broughtin the vicinity of the surface. This tip is then scanned across the surface. Byallowing the tip to (almost) touch the surface, van-der-Waals forces between thesurface and tip can be measured. In order to do this, the deflection of the cantileveris measured using a laser, which is reflected on the cantilever. The reflected beam isthen measured using a photodiode which is sensitive in two dimensions. This way,not only vertical movement can be detected, but also lateral forces, which rotatethe cantilever in the xy-plane instead of the xz-plane. A schematic representationof an AFM setup is shown in figure C.4b.

Several modes of operation are available in AFM. Contact mode is the fastestmode, keeping contact between the tip and sample at all times. Due to contact withthe sample, surface features can be distorted by lateral forces. Furthermore, soft


materials can be damaged by the lateral forces. In tapping mode, the cantileveris oscillated at its resonance frequency. While scanning the tip over the surface,changes in height are measured due to a change in the amplitude of the oscillation.Additionally, the phase difference between the oscillation of the cantilever and thepiezo drive can be measured. This phase difference is dependent on various sampleproperties, including composition, adhesion and friction. Additional features of thesurface can thus be measured this way.

D Relation between AD and MCmeasurements

In order to relate the angle dependent measurements to the measurements wherewe compare MC sweeps in the parallel and perpendicular orientation, we need tolook at the definition of these effects.

The angle dependence is expressed using equation 3.4, expressing the changein current as a function of the current in the perpendicular orientation (θ = 0◦).This equation can alternatively be written as:

δIB (θ) = IB (θ)− IB (0◦)IB (0◦) . (D.1)

In comparison, the magnetoconductance as measured at a constant angle is definedusing equation 2.8. Again, we introduce an alternative way of writing the equation:

MCB (θ) = IB (θ)− I0 (θ)I0 (θ) . (D.2)

The difference between the MC as measured in the parallel and perpendicularorientations, δMC, is expressed using equation 3.2. Using the definitions given inequations D.1 and D.2, we can now relate δMC to δIB. To do so, we start bywriting down equation 3.2 and using equation D.2:

δMCB = MCB (90◦)−MCB (0◦)MC500mT (90◦) =

IB(90◦)−I0(90◦)I0(90◦) − IB(0◦)−I0(0◦)

I0(0◦)

MC500mT (90◦) . (D.3)

This equation can be simplified by inspecting the numerator of this fraction:

δMCB =[IB (90◦)− I0 (90◦)

I0 (90◦) − IB (0◦)− I0 (0◦)I0 (0◦)

]· 1MC500mT (90◦) . (D.4)

We now note that I0 (0◦) = I0 (90◦) = IR, allowing us to write:

δMCB =[IB (90◦)− IR

IR

− IB (0◦)− IR

IR

]· 1MC500mT (90◦) . (D.5)

We now simplify this equation:

δMCB = IB (90◦)− IB (0◦)IR

· 1MC500mT (90◦) . (D.6)

115

116 Appendix D. Relation between AD and MC measurements

In this equation, we recognize equation D.1 for θ = 90◦. Using this observation,we can now write:

δMCB = IB (90◦)− IB (0◦)IB (0◦) · IB (0◦)

IR

· 1MC500mT (90◦) = IB (0◦)

IR

· δIB (90◦)MC500mT (90◦) .

(D.7)We finally note that we can replace IR by I0 (0◦). Using this observation, we write:

δMCB = IB (0◦)I0 (0◦) · δIB (90◦)

MC500mT (90◦) . (D.8)

We can now conclude that a linear relation exists between δMCB and δIB.Furthermore, the MC as measured at 500 mT in the perpendicular orientation isof importance, as we use this quantity to normalize δMC. The final term describesthe effect of applying a magnetic field on the current.

E Fringe field calculation

In order to do simulations based on a sample being affected by fringe fields of amagnetic layer on top of the sample, the fringe fields of this magnetic layer shouldbe calculated. The magnetization of the magnetic layer was measured using XMCD.To calculate the fringe fields from this magnetization measurement, a dipole ap-proximation was made, assuming that each measured spot of 10x10 nm contains amagnetic dipole with a certain magnetic moment ~m.

To calculate the fringe fields using the dipole expression, three equations willbe derived for the Cartesian components of the fringe field. These equations willapply for the presence of only one dipole. To get a useful value of the fringe field ata certain point above the magnetized layer, the contributions of each dipole shouldbe taken into account.

E.1 Equations for one dipole moment

Starting with the equation for a magnetic dipole moment, the three componentsof the magnetic flux density can be calculated [71]:

~B(~r) = µ0

4π

(3~r (~m·~r)

r5 − ~m

r3

). (E.1)

Under the assumption that the magnetic moment ~m is out-of-plane, we canwrite ~m = ±m·~ez. The term ~m·~r thereby transforms into m· rz, where rz

denotes the z-component of the position ~r and m can be either positive or negative.By using the basic relation r2 = r2

x + r2y + r2

z , we can write an expression for rz interms of r, rx and ry:

rz = r

√1−

(rx

r

)2−(ry

r

)2. (E.2)

As a result, we can now write an expression for the term ~m·~r:

~m·~r = m· r

√1−

(rx

r

)2−(ry

r

)2, (E.3)

where the magnetic moment m can be either positive or negative, dependingon its orientation.

117

118 Appendix E. Fringe field calculation

Combining equation E.1 and E.3 results in an expression for the magnetic fluxdensity which assumes an out-of-plane magnetization:

~B(~r) = µ0

4π

3~rm· r

√1−

(rx

r

)2−(

ry

r

)2

r5 − m·~ez

r3

. (E.4)

This expression can be simplified further by writing ~r = r·~er:

~B(~r) = µ0m

4πr3

3√

1−(rx

r

)2−(ry

r

)2~er − ~ez

. (E.5)

The unit vector ~er in terms of Cartesian coordinates is given by ~er = r−1 (rx~ex + ry~ey + rz~ez).Using this definition, we can give an expression for the three Cartesian coordinatesof the magnetic flux density ~B of equation E.5. These components can be writtenas:

Bx(~r) = µ0

4πm

r3

3rx

r

√1−

(rx

r

)2−(ry

r

)2 , (E.6a)

By(~r) = µ0

4πm

r3

3ry

r

√1−

(rx

r

)2−(ry

r

)2 , (E.6b)

Bz(~r) = µ0

4πm

r3

3rz

r

√1−

(ry

r

)2−(ry

r

)2− 1

. (E.6c)

E.2 Fringe field for multiple magnetic moments

As stated before, the contributions of many dipoles should be taken into accountin order to get a useful value for the fringe field at a certain position above themagnetized layer. To this end, equations E.6 should be summed for all dipolespresent in the measured magnetization. Assuming a two dimensional magnetized

E.3. Matlab script 119

layer, the equations transform into:

Bx(~r) = µ0

4π∑i,j

mij

r3ij

3rij,x

rij

√√√√1−(rij,x

rij

)2

−(rij,y

rij

)2, (E.7a)

By(~r) = µ0

4π∑i,j

mij

r3ij

3rij,y

rij

√√√√1−(rij,x

rij

)2

−(rij,y

rij

)2, (E.7b)

Bz(~r) = µ0

4π∑i,j

mij

r3ij

3rij,z

rij

√√√√1−(rij,x

rij

)2

−(rij,y

rij

)2

− 1

. (E.7c)

The magnetization layer can be depicted as a two dimensional grid, whichcontains the magnetization information for a certain surface element. A schematicrepresentation is shown in figure E.1a. Each dot in a grid element represents themagnetization information for that grid element.

i

j

10 nm

(a)i’

j’

1.3 nm

(b)

Figure E.1: A schematical representation of a two dimensional grid of dipoles (a) as used incalculating the fringe field in a second grid at a distance h above the dipole grid (b).

When calculating the fringe fields with the purpose of simulating its effect onthe hopping rate of electrons and holes, a certain grid has to be used at whichthe fringe fields are calculated. Looking at Alq3, the size of the molecule is ap-proximately 11.4 A. A grid spacing of 1.3 nm is thus an acceptable value [72]. Acalculation grid is drawn over the dipole grid in figure E.1b.

E.3 Matlab script

The script written in Matlab to calculate the fringe fields of a magnetic layer can bedivided in three parts. First, the main function loads the data file containing the


magnetization information. After that, the data is shifted such that the associateddipole is located in the centre of a 10x10 nm square. Finally, the script calculatesthe fringe field at the desired locations (indicated by a desired horizontal width anda desired vertical with, combined with a lateral resolution). Finally, the calculatedvalues for the three components of the fringe field are saved to a file. For theMatlab code of this function, see section E.3.1.

To calculate the fringe field at a specific point, a second function is used, whichloops over all dipoles which are present in the circle with a radius of rmax. Thecomponents of the fringe fields are then calculated per dipole, and the results ofevery point are added to reach a fringe field due to the presence of many dipoles.The equations in E.7 are used to determine the sum of all dipoles in this function.For the Matlab code of this function, see section E.3.2.

E.3.1 Main function

1 function out = CalculateFringe(filename, center, height, ...width, depth, resolution, rmax)

2 % Input data:3 % Center = array (x,y) points of the center of the desired grid4 % Height: The distance (in nm) of the desired grid above the ...

substrate5 % Width: The width of the desired grid (in nm)6 % Depth: The depth of the desired grid (in nm)7 % Resolution: The resolution of the desired grid (in nm)8 % Rmax: The in−plane radius in which the dipoles should be ...

summed over. Dipoles outside this radius are not taken into ...account.

9

10 tic;11

12 info = importdata(filename);13 dataraw=info.data;14 [m,n] = size(dataraw);15 processed = zeros(m,n);16

17 for i=1:m18 processed(i,1) = dataraw(i,1)−5; % Shift 5 nm to left, to ...

use value as the center instead of the upper right corner19 processed(i,2) = dataraw(i,2)−5; % Shift 5 nm down, to use ...

value as the center instead of the upper right corner20 processed(i,3) = dataraw(i,3);21 end22

23 centerx = center(1);24 centery = center(2);25 startx = round((centerx − width/2)/resolution)*resolution;26 starty = round((centery − depth/2)/resolution)*resolution;27 endx = round((centerx + width/2)/resolution)*resolution;28 endy = round((centery + depth/2)/resolution)*resolution;

E.3. Matlab script 121

29

30 pointsx = (endx − startx) / resolution + 1;31 pointsy = (endy − starty) / resolution + 1;32

33 outputgrid = zeros(pointsx*pointsy,5);34 counter = 1;35

36 for i=startx:resolution:endx37 % Loop over all x−values of the desired grid38 for j=starty:resolution:endy39 % Loop over all y−values of the desired grid40 [BxT, ByT, BzT] = CalculateActualFields(processed, i, ...

j, height, rmax);41

42 outputgrid(counter, 1) = i;43 outputgrid(counter, 2) = j;44 outputgrid(counter, 3) = BxT;45 outputgrid(counter, 4) = ByT;46 outputgrid(counter, 5) = BzT;47

48 counter = counter+1;49 end50 end51

52 [a, filenamesize] = size(filename);53 newfilenamePartA = filename(1:filenamesize−4);54 newfilenamePartB = ' Bx−By−Bz height−';55 newfilenamePartC = 'nm.dat';56 newfilename = strcat(newfilenamePartA, newfilenamePartB, ...

num2str(height), newfilenamePartC);57 dlmwrite(newfilename, outputgrid, 'delimiter', '\t', ...

'precision', 6);58

59 out = toc;60

61 end

E.3.2 Calculation function

1 function [Bx, By, Bz] = CalculateActualFields(data, xpoint, ...ypoint, height, rmax)

2 % Calculate the magnetic field due to all dipoles in the data ...matrix

3

4 [m,n] = size(data);5

6 Bx = 0;7 By = 0;8 Bz = 0;9


10 mu0over4pi = 1E−7;11

12 for i=1:m13 pointx = data(i,1);14 pointy = data(i,2);15 magnet = data(i,3) * (24E−9 * 2E−6 * 2E−6) * 7E5 / (200ˆ2);16

17 vectorx = (xpoint − pointx) * 1E−9;18 vectory = (ypoint − pointy) * 1E−9;19 radius = sqrt(vectorxˆ2 + vectoryˆ2) * 1E9;20

21 if radius ≤ rmax22 vectorz = (height) * 1E−9;23 vectorlength = sqrt(vectorxˆ2 + vectoryˆ2 + vectorzˆ2);24

25 Bx = Bx + mu0over4pi * magnet / (vectorlengthˆ3) * (3 .../ vectorlength * sqrt(1 − (vectorx / ...vectorlength)ˆ2 − (vectory / vectorlength)ˆ2) * ...vectorx);

26 By = By + mu0over4pi * magnet / (vectorlengthˆ3) * (3 .../ vectorlength * sqrt(1 − (vectorx / ...vectorlength)ˆ2 − (vectory / vectorlength)ˆ2) * ...vectory);

27 Bz = Bz + mu0over4pi * magnet / (vectorlengthˆ3) * (3 .../ vectorlength * sqrt(1 − (vectorx / ...vectorlength)ˆ2 − (vectory / vectorlength)ˆ2) * ...vectorz − 1);

28 end29 end30

31 end

F Cobalt in-plane magnetizationswitching

In section section 5.3.3 we noted that the step in δMC can be attributed to the in-plane switching of the cobalt layer. In this appendix we will give a more thoroughargumentation to support this statement. We will first inspect devices where acobalt layer is added at the bottom of a standard device structure, while in thesubsequent section we will also discuss devices where the cobalt layer is added ontop of a standard device structure.

F.1 Devices with a cobalt layer at the bottom

We now return to the measurements performed on a device with a cobalt layerat the bottom of the standard device structure. For convenience, we will showfigure 5.7 again in figure F.1. We take a closer look at the feature observed at1.5 mT in the δMC measurement of a cobalt device. This figure also shows theresults calculated for a reference device.

10-1 100 101 102

-2

0

2

4

6

8

Reference 3.5 V Cobalt 3.6 V

MC

(%)

Field (mT)

Figure F.1: δMC displayed for reference and cobalt devices, measured at 3.5 V and 3.6 Vrespectively.

Inspection of the figure shows that a small step in the δMC measurement ofthe cobalt device is visible at a field of approximately 2 mT. To investigate this

123

124 Appendix F. Cobalt in-plane magnetization switching

step, the drift-corrected data of the measurements performed on this device at abias voltage of 5 V are plotted in figure F.2, for both orientations. Additionally themeasurement performed on a reference device in the parallel orientation is shownas well. By comparing these measurements, we can determine if this step can beattributed to the in-plane switching of the cobalt layer.

0.1 10.1 10.1 1

0.0

0.2

0.4

0.6

Field (mT)

Forward sweep Backward sweep Difference

Field (mT)


Cobalt - PerpendicularCobalt - Parallel

MC

(%)

Field (mT)


Reference - Parallel

Figure F.2: The raw data of measurements performed on cobalt and reference devices at a biasvoltage of 5 V. The measurements are performed in various orientations, as indicated by thesubtexts.

The step observed in figure F.1 can already be seen in the middle graph, whichshows the measurement performed on a cobalt device in the parallel orientation.Inspection shows a step in the difference between both sweep directions. Notethat this step is also visible at the negative field with the same magnitude. Themeasurement performed on a reference device in the same orientation, as shownin the left graph, shows no such step. A shift in the curves is observed in this rawdata, which is normally corrected for in the data processing steps. As a result ofthis shift, a smooth difference between both sweep directions is present. We canthus already conclude that the step at 1.6 mT can be attributed to the additionof the cobalt layer. The measurements performed on the cobalt device in theperpendicular orientation, as shown in the right graph, do not show a step at 1.6mT. The parallel orientation of the cobalt device is thus causing the step in theMC, and thus in δMC as observed before. The cobalt layer has been shown toswitch in-plane at fields between 1.6 and 2.0 mT, depending on the sweep speed,as shown in chapter 4. We can thus conclude that the step at 1.6 mT is caused byin-plane switching of the cobalt layer.

F.2 Devices with a cobalt layer on top

The results from the devices with a cobalt layer on top of the reference devicestructure show a similar feature. At field strengths of approximately 1.5 mT, astep in δMC can be observed for some devices. A more detailed view of this stepis shown in figure F.3a, showing the measurements performed at 5 V. From thisfigure, we can now clearly see that the devices with a spacer layer of 25, 50 and

F.2. Devices with a cobalt layer on top 125

100 nm show a step in δMC at 1.25, 1.5 and 1.5 mT, respectively. Viewing thestep as a deviation from an imaginary smooth line shows this deviation to have awidth of approximately 1 mT.

0.1 1-1.0

-0.5

0.0

0.5

1.0

1.5

MC

(%)

Field (mT)


(a)

-60 -40 -20 0 20 40 60

0.0

0.5

1.0

1.5

2.0

2.5 Forward Backward Difference

MC

(%)

Field (mT)(b)

Figure F.3: (a) δMC as a function of the applied magnetic field around 1.5 mT. The spacerlayer in the cobalt devices is varied between 10, 25, 50 and 100 nm. The measurements areperformed at 5 V. (b) The raw data from measurements on a device with a 10 nm spacer layer,performed at a bias voltage of 2.5 V. The forward and backward sweeps are displayed in blacksquares and red circles, respectively. The gold diamonds show the difference between the twosweeps.

The reference device and the device with a 10 nm spacer layer do not show anystep in δMC around 1.5 mT. For the reference device, this behaviour is expectedas there is no cobalt layer which switches its magnetization. The 10 nm deviceon the other hand does have a cobalt layer and is therefore expected to switch atthe same fields as the other cobalt devices. As can be seen in the figure, no suchswitch occurs between 0.1 and 3 mT. From figure 5.11 we can see that a differencein shape between 10 and 30 mT for this device occurs, while it is not observed forother devices.

A closer inspection of the raw data shows that sweeping the field in the parallelorientation causes a hysteresis loop. The drift corrected measurements performedat a bias voltage of 2.5 V are shown in figure F.3b. The forward field sweep isdisplayed in black squares, the backward sweep in red circles. Additionally, thedifference between the two sweeps is displayed in gold diamonds.

From the forward and backward sweeps, we immediately see a dependence onthe sweep direction. When sweeping the magnetic field forward, a small deviationis noticed at positive fields around 20 mT. When sweeping backward, the samedeviation is observed at negative fields around 20 mT. By subtracting the backwardsweep from the forward sweep, we can visualise this deviation more clearly. Forvery small field strengths and field strengths of 40 mT and larger, the differencebetween the sweeps returns to 0, as expected. For 10 < |B| < 40 mT, a hystereticfeature is found, matching the change in shape we observed in figure 5.11. Notethat we also observe a very small shift of the MC with respect to the magnetic field.

126 Appendix F. Cobalt in-plane magnetization switching

This is attributed to remanence in the measurement setup, which is not correctedfor in these drift-corrected measurements. For completeness, we also inspect themeasurements performed in the perpendicular orientation. No deviations are foundbetween the two sweep directions. We thus conclude that the change in shapeobserved in figure 5.11 can be attributed to the interaction of the cobalt layer andthe external field present in the parallel orientation.

With this finding, we have shown that the change in shape can be attributedto the in-plane switching of the cobalt layer. We also found that no switching oc-curs at fields around 1.5 mT, indicating that the coercive field of the cobalt layerin the device with a 10 nm spacer layer is increased by an order of magnitude.The only difference between the cobalt-based devices in this section is the thick-ness of the spacer layer, indicating that the spacer layer may play an importantrole in the magnetic behaviour of the cobalt layer. In literature we found reportsof an change in the coercive field when the morphology of the underlying layerchanges. A 100 nm thick cobalt layer was deposited on a noodle-like porous sur-face, which increased the coercive field with a factor 6 with respect to a referencelayer deposited on a flat surface. Furthermore, the shape of the hysteresis loop isfound to change, showing a broader switching regime [73]. Keeping this in mindwhen inspecting our devices, we note that the aluminum spacer is deposited on apolymer. Polymers are generally thought to be ordered, however spin coating is afast process which does generally not allow the polymer to form large crystallinedomains. Depending on the solvent used, a noodle-like morphology might still becreated using spin coating [74].

The increase in coercive field is only observed for a relatively thin spacer layerof 10 nm. We hypothesize that such a thin aluminum layer still retains partof a noodle-like morphology caused by the SY-PPV polymer layer. Increasingthe spacer layer thickness might then cause a loss of this noodle-like morphology,causing a more flat surface. As a result, devices with a spacer layer larger than 10nm show a switching effect around 1.5 mT, while at a spacer layer thickness of 10nm a switching effect around 20 mT is observed. The broader switching regime ofapproximately 30 mT, in comparison to the switching regime of 1 mT for thickerlayers, can also be explained using the results found in [73]. Unfortunately, wecannot confirm this behaviour using MOKE, as a 100 nm aluminum capping layeris deposited on top of the cobalt layer.

G PEDOT:PSS Spincoat cali-bration

The thickness of a PEDOT:PSS layer deposited by spin coating is dependent onthe used rotation speed. To determine the rotation speed required to get a certainthickness, a set of 8 samples was spin coated using a range of rotation speeds. Thethickness of the PEDOT:PSS layer deposited on these samples was then measuredusing a DekTak device, see section C.2.1.

The thickness is shown as a function of the rotation speed in figure G.1.

0 1 2 3 4 5 630

40

50

60

70

80

90

100 PEDOT:PSS Thickness Exponential Decay Fit

Mea

sure

d th

ickn

ess

(nm

)

Spin speed (1000 rpm)

Figure G.1: PEDOT:PSS layer thickness as a function of spin coat speed.

The data shown are averages of at least 6 measurements on different scratchesin the layer. The standard deviation in the data has been calculated using equa-tion G.1. A derivation of this equation can be found in ....

Sy =

√√√√∑ (xi − x)2

N (N − 1) (G.1)

The error bars shown in figure G.1 actually show three times the standard devi-ation, which can be justified due to the fact that the scratches are not perfectlymade. The error in this scratching process is therefore at least in the order of the

127

128 Appendix G. PEDOT:PSS Spincoat calibration

error due to statistical measurements of the thickness, making it acceptable thatthe standard deviation is multiplied with 3.

An exponentially decaying function is fitted through the data according toequation G.2.

y = y0 + A1 · e−x/t1 (G.2)The values of y0, A1 and t1 are found to be 30±2, (1.0±0.1) · 102 and (1.7±0.2) · 103,respectively.

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