Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites

206 Int. J. Nanotechnology, Vol. 10, Nos. 3/4, 2013

Copyright © 2013 Inderscience Enterprises Ltd.

Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites

Nguyen Thi Minh Hong, Nguyen Huu Duc and Pham Duc Thang* Laboratory for Micro and Nanotechnology, Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University, Hanoi, Building E3, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam Email: [email protected] Email: [email protected] Email: [email protected] *Corresponding author

Abstract: Magnetoelectric effect has attracted considerable interest due to the promising approach towards novel spintronics device. In this work, we study the converse magnetoelectric properties of PZT/NiFe/CoFe nanocomposites in which the piezoelectric substrate is transversely polarised and the thickness of the NiFe layer is varied. In these structures, the magnetic behaviour significantly changes under an applied voltage thanks to magnetoelectric coupling between the layers. By changing the NiFe thickness, this change in magnetisation reaches up to 245% at low bias magnetic field. Besides, the investigation of voltage controlled magnetisation switching is expounded. These results are discussed in term of a stress-induced anisotropy field model and magnetoelectric coupling between two phases.

Keywords: nanocomposites; magnetoelectric effect; voltage induced magnetisation.

Reference to this paper should be made as follows: Minh Hong, N.T., Duc, N.H. and Thang, P.D. (2013) ‘Converse magnetoelectric effect in PZT/NiFe/ CoFe nanocomposites’, Int. J. Nanotechnology, Vol. 10, Nos. 3/4, pp.206–213.

Biographical notes: Nguyen Thi Minh Hong is working as the researcher at the Laboratory for Micro and Nanotechnology and the Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University (VNU) in Hanoi, Vietnam. She obtained her MSc degree in Electromagnetic Physics from the University of Natural Science, VNU. At present, she is pursuing PhD programme and her current research interests are in the field of nanomagnetics, multifferroics and micro-nano ferroelectrics.

Nguyen Huu Duc obtained his PhD degree in Physics from the University of Hanoi in 1988. He has received the French Habilitation (DHR) in Physics at the Joseph Fourier University of Grenoble in 1997, became a full professor of the VNU in 2004 and professor of merit in 2008. He serves as the head of the Laboratory for Micro and Nanotechnology at University of Engineering and Technology, VNU. He extended his research on various aspects of magnetism. He is author of 100 scientific papers in various international journals and of

Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites 207

five monographs published in recent volumes of the Handbook of Magnetic Materials and the Handbook on the Physics and Chemistry of Rare Earths (Elsevier Science Publisher).

Pham Duc Thang obtained the PhD degree in Experimental Physics from the University of Amsterdam in 2003. From 2003 to 2006 he worked as a postdoctoral researcher at the University of Twente. In 2006 he joined the University of Engineering and Technology, VNU as a research staff at the Faculty of Engineering Physics and Nanotechnology. He became an associate professor of the university in 2011. His current research interests are focused on nanostructured magnetic materials, functional ferroelectrics, piezoelectrics and multiferroics, micro-nano fabrication and devices.

This paper is a revised and expanded version of a paper entitled ‘Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites’ presented at the ‘3rd International Workshop on Nanotechnology and Application (IWNA’2011)’, Vung Tau, Vietnam, 10–12 November 2011.

1 Introduction

The magnetoelectric effect can be classified as direct magnetolectric effect (DME) and converse magnetoelectric effect (CME) that are characterised as magnetic field induced polarisation and electric field induced magnetisation, respectively [1]. To date, most of the published papers are devoted to investigations of the DME effect. However, there are only several works, reporting on the CME effect in the recent years. For data storage applications, it is indeed both physically interesting and technologically important to quantitatively characterise the CME effect. Thus, the multiferroic materials, which include single phase magnetoelectric system and two phase magnetoelectric system, are studied widely to obtain large CME effect [2–5]. In this paper, we investigate the CME effect of PZT/NiFe/CoFe nanocomposites with various NiFe ferromagnetic thicknesses. Besides, the voltage controlled magnetisation switching process will be discussed.

2 Experimental procedures

In this work, the ferromagnetic layers of NiFe/CoFe were directly grown in sequence by an rf magnetron sputtering (2000F, AJA International Inc.) on polycrystalline PZT substrate with transverse polarisation (APC-855, American Piezoceramics Inc.). Before deposition, the sputtering chamber was vacuumed to a base pressure of 2×10–7 Torr. A power of 50 W and Ar gas pressure of 2.2×10–3 Torr have been used for deposition of NiFe and CoFe. In this hybrid structure, sputtering time for CoFe layer is fixed of 30 minutes. Meanwhile, thickness of NiFe layer was changed by varying sputtering time from 10, 20, 40 and 60 minutes. These samples are denoted as MN13, MN23, MN43 and MN63, respectively. For the structure of the present study, the thickness of NiFe/CoFe is up to 100 nm and a 500 μm thick PZT substrate is used. Finally, a Ta thin layer was sputtered on CoFe layer to prevent oxidisation for ferromagnetic layers.

For electrical measurement, we use silver adhensive glue, electrodes according to the polarisation direction of PZT substrate as the schematic shown in Figure 1. The dimensions of this structure were 5×5 mm2. The magnetic and CME measurements were

208 N.T. Minh Hong, N.H. Duc and P.D. Thang

carried out by using a vibrating sample magnetometer (VSM 7404, Lakeshore). The sample was subjected to an external bias magnetic field and the applied voltage was also varied from –700 V to 700 V using a voltage amplifier. Studies on the morphology and crystallographic structure have been analysed in [6].

Figure 1 Geometry and working principle for converse magnetoelectric effect (see online version for colours)

Electrode

Hbias

3 Results and discussion

The magnetisation (M) measured as a function of angle α between the applied magnetic field and film normal direction is shown in Figure 2 for α = 0°, 45° and 90°. One observes that in-plane magnetic anisotropy dominates for all samples due to the contribution of NiFe/CoFe ferromagnetic layers. Magnetisation measured along the film plane, MS//, increases when increasing NiFe thickness. It reaches a maximum for sample MN43 (as presented in Figure 3). Meanwhile, coercivity µoHC// has opposite tendency, showing a minimum value for MN43 sample. This rule alters coincidental when measuring at various α angles. In addition, the anomalous changing of MS// and µoHC// for MN43 sample indicates that magnetic properties can be optimised by choosing suitable buffer layer thickness. It is noteworthy that an optimised thickness ratio of the magnetic and ferroelectric components is a prerequisite for obtaining large CME [7].

Figure 2 Magnetic hysteresis loop of PZT/NiFe/CoFe composites with different NiFe thickness measured at various angles α (see online version for colours)

-0.0025

0.0000

0.0025 90o

45o

0o

M(e

mu)

MN13

90o

45o

0o

MN23

90o

45o

0o

-10000 -5000 0 5000 10000-0.0025

0.0000

0.0025

M

(em

u)

μoH(Oe)

90o

45o

0o

MN43

-10000 -5000 0 5000 10000

MN63

90o

45o

0o

μoH(Oe)


Figure 3 Saturation magnetization MS// and coercivity μo HC// of samples (see online version for colours)

0 20 40 600

50

100

μoHC//(tNiFe)MS//(tNiFe)

tNiFe (minutes)

μ oH

c// (

Oe)

1500

2000

2500

MS// (μem

u)

For study CME, an external voltage (V), varied from –700 V to 700 V, is applied to these samples. The experiment is carried out at different magnetic field aligned parallel to sample plane. The results, presented in Figure 4, display linear increasing tendency of M(V) curves are observed for all samples. That corresponds closely to the ferroelectric hysteresis loop P(V) curve (see in Figure 5). This indicates that elastic stress transferred from piezoelectrics to ferromagnetic layers of NiFe/CoFe, resulted in an increase of magnetisation. The change in magnetisation, ΔM, measured at 700 V and 50 Oe are listed in Table 1 for all samples. In the range of applied voltage, the relative change in magnetisation is actually large, up to 245, 220, 170 and 225%, for sample MN13, MN23, MN43 and MN63, respectively.

Figure 4 Dependence of magnetisation on the applied voltage M(V) at different magnetic field (see online version for colours)

-600 -400 -200 0 200 400 600-600

0

600

1200

1800

2400

3000

V(V)

μoH=5 Oe μoH=50 Oe μoH=200 Oe μoH=1000 Oe μoH=2000 Oe

M(μ

emu)

MN23

-600 -400 -200 0 200 400 600-600

0

600

1200

1800

2400

3000

M(μ

emu)


MN43

V(V)

-600 -400 -200 0 200 400 600


V(V)

MN63

-600

0

600

1200

1800

2400

3000

M(μ

emu)


MN13


Figure 5 Dependence of polarization on the applied voltage of PZT substrate (see online version for colours)

-300 -200 -100 0 100 200 300-0.10

-0.05

0.00

0.05

0.10

P(μC

/cm

2 )

V(V)

Table 1 The magnetisation change ΔM, the relative change in magnetisation ΔM/M and the magnetisation reversed voltage Vrev (taken at 5 Oe)

Sample ΔM (µemu) ΔM/M (%) Vrev (V)

MN13 760 245 –300 MN23 700 220 –500 MN43 540 170 –600 MN63 570 225 –250

Furthermore, we observed the magnetisation switching under the applied voltage for each sample. At a magnetic field, magnetisation reversal appears at a certain voltage which is called as the magnetisation reversed voltage Vrev (listed in Table 1 for the magnetic field of 5 Oe). In principle, the application of an electric field can induce a stress and therefore a change in magnetic moment orientation, thanks to the magnetostriction of the magnetic materials. The NiFe layer plays a role in affecting voltage induced magnetisation switching process. For MN43 sample, the value of Vrev is larger than that of others and this result also reflects the above mentioned magnetic hysteresis measurements.

Figure 6 shows the voltage induced magnetisation susceptibility χVIM measured on sample MN13 at various magnetic field from 5 Oe to 2000 Oe. Firstly, χVIM has a positive value at high applied voltage. When decreasing applied voltage, χVIM increases to a maximum, then cancels at fixed voltage (magnetisation reversed voltage, Vrev) and finally changes its sign. For different magnetic fields, these values are different. The higher bias magnetic field is, the higher Vrev requires. Consequently, magnetisation switching can be decided by the competition between the bias magnetic field energy and applied electric field energy.

Recently, future spintronic devices based on new materials with voltage controllable magnetic properties are very attractive for practical applications because of lower power consumption compared to the present conventional magnetic-controlled counterparts. Through voltage induced stress/strain by the piezoelectric effect, the voltage controllable magnetisation can be realised. As shown in Figure 7, the sample MN13 expressed an


obvious influence on the magnetic hysteresis loops while applying the voltage along the sample plane. It clearly reveals the increases in coercivity, from 63 Oe to 80 Oe (manipulated by 21%) at applied voltage of 600 V.

Figure 6 Electric field induced magnetisation susceptibility (χVIM) of sample MN13 (see online version for colours)

-600 -400 -200 0 200 400 600-6

-3

0

3

6χ V

IM (a

bs.u

nit)

V(V)


MN13

Figure 7 In-plane magnetic hysteresis loops of sample MN13 at different applied voltage (see online version for colours)

-200 -100 0 100 200-0.0010

-0.0005

0.0000

0.0005

0.0010

M(e

mu)

μoH(Oe)

600V 0V

MN13

The change in coercivity reflects the dependence of magnetoelastic anisotropy in the magnetic layers on stress, originated from out-of-plane lattice distortions in PZT crystal. This dependence can be caused by the following factors: the direction of the stress-induced anisotropy field Hσ regarding that of the external field H; the coercivity mechanism and the contribution to the effective magnetic anisotropy field energy. In this case, the analysis uses the Stoner-Wohlfrauth relation between coercivity and stress-induced anisotropy field. In order to analyse the effective magnetic anisotropy field we consider first the magnetoelastic energy which is given by [8]:

2me meE K sin= θ (1)


where the stress anisotropy constant Kme is expressed as:

me3

K2

= λσ (2)

λ is the average magnetostriction coefficient quantifying the relative length change of the sample between the demagnetised and magnetised states, σ is the induced stress and θ is the angle between the magnetisation M and the direction of σ (as illustrated in Figure 8). Apparently, Kme>0 favours θ=0° which means the parallel alignment of the magnetisation direction relative to the stress axis, meanwhile Kme<0 favours perpendicular alignment with θ=90°. For our samples, PZT substrate has transverse polarisation and NiFe/CoFe ferromagnetic layers have in-plane anisotropy which means the parallel alignment of magnetisation relative to stress axis, thus Kme>0. NiFe/CoFe layers have the magnetostriction coefficient λ>0 and therefore σ>0, indicating the stress in those layers is tensile. This is in accordance with the above increase in magnetisation when changing applied voltage.

Figure 8 Scheme of stress-induced magnetic anisotropy for sample with an in-plane easy axis (see online version for colours)

The effective magnetic anisotropy constant, Keff, of ferromagnetic layer NiFe/CoFe in contact with PZT is the sum of several anisotropy contributions and can be expressed as:

2S oeff V S me

NiFe/CoFe

2KK K M K

t 2μ

= + − + (3)

where KV is the volume magnetocrystalline anisotropy constant, the second term is the surface anisotropy with its constant KS for a film thickness of tNiFe/CoFe, the third term is the shape anisotropy which favours an easy in-plane magnetisation, and the last term is the magnetoelastic anisotropy term. Besides, we have:

effo C

S

2KH

Mμ = (4)

Substituting equations (2) and (4) into equation (3), one can get the formula of the stress as:

2S oo C S V S

NiFe/CoFe

2KH M 2 K Mt 2

3

⎛ ⎞μμ − + +⎜ ⎟

⎝ ⎠σ =λ

(5)


Hence, the sign of stress allows us determining if it is compressive or tensile. Our recent study indicates that the sign of stress is positive. This can be the main cause of an increase in magnetisation when applying an external voltage. Theoretical work related to these results is under investigation and will be published elsewhere.

4 Conclusion

We have presented in this paper the new results on converse magnetoelectric effect in the nanocomposite PZT/NiFe/CoFe. A large change in magnetisation induced by voltage has been observed. This indicates that the elastic stress transfers from ferroelectric substrate to ferromagnetic layers. Based on the model of stress-induced magnetic anisotropy and the coupling between two phases, we also demonstrate that the stress, which is imposed by the PZT substrate on NiFe/CoFe layer, is tensile. Furthermore, the effect of applied voltage and bias magnetic field on magnetisation switching process is also discussed. These results are promising for future spintronic applications.

Acknowledgements

This research was supported by project 103.02.87.09 of the Vietnam National Foundation for Science and Technology Development (NAFOSTED).

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Converse magnetoelectric effect in PZT/NiFe/CoFe nanocomposites