Download ppt - Spintronics (1).ppt

national laboratory for advanced Tecnologies and nAnoSCience

Material and devices for spintronics

•What is spintronics?•Ferromagnetic semiconductors

Physical basisMaterial issues

•Examples of spintronic devicesElectric field control of magnetismSpin injectorsSpin valves

Trieste, 20.10.06


Spintronics = spin-based electronics

Information is carried by the electron spin,not (only) by the electron charge.

1. Ferromagnetic metallic alloys- based devices

Transport in FM metals is naturally spin-polarized

Ideal, fully polarized case, only spin down states are available

silvia


1988: discovery of GMR (Giant Magnetoresistive effect):

In alternateFM/nonmagnetic layered system, R is low when the magnetic moments in the FM layers are aligned,R is high when the magnetic moments in the FM layers are antialigned.

(Baibich et al, PRL61, 2472 (88) Binach et al, PRB39, 4828 (89))


GMR based Spin Valves and Magnetic tunnel junction

Prinz, Science 282, 1660 (98)Wolf et al, Science 294, 1488 (01)

AF layer (A) or AF/FM/Ru/ trilayer (B)to pin the magnetization of the top FM layer


GMR based Spin Valves for read head in hard drives

Prinz, Science 282, 1660 (98)Wolf, Science 294, 1488 (01)

Standard geometry for GMR based Spin Valves

But also MRAM



1. Ferromagnetic metal - based devices

2. Semiconductor based spin electronics

Courtesy C.T. Foxon



1. Ferromagnetic metal - based devices

3. Devices for the manipulation of single spin (quantum computing).The idea:

Electron spins could be used as qubits.They can be up or down, but also incoherent superpositions of up and down states

2. Semiconductor based spin electronics devices


How can we measure the magnetic state of a thin epilayer:SQUID measurements but also Anomalous Hall effect

Md

RB

d

RR SHall 0

R0=1/pe

Ordinary Hall effect contribution, negligible.

RHall is proportional to M.


Two main issues in semiconductor spintronics:

1. Avaiability of suitable materialsIdeal material should be

• Easily integrable with ‘‘electronic’’ materials• Able to incorporate both n- and p-type dopants• With a TC above room T

2. Understandig and controlling the physical phenomena:• Spin injection• Transport of spin polarized carriers across

interfaces• Spin interactions in solids: role of defects,

dimensionality, semiconductor band structure• .................

Examples: Eu– dichalcogenides (EuS, GdS,EuSe) and spinels CdCr2Se4.

Extensively studied in ’60-’70.Exchange interaction between electrons in the semiconducting band and localized electrons at the magnetic ions.

Interesting properties, but •Crystal structure quite different from Si and GaAs, difficult to integrate•Crystal growth very slow and difficult•Low TC


Magnetic semiconductor, constituted bya periodic array of magnetic ions


As one can obtain n- o p-type semiconductors by doping, one can syntetize new magnetic materials by introducing magnetic impurities in non magnetic semiconductors.

Alloys of a nonmagnetic semiconductor and magnetic elements:Diluted Magnetic Semiconductors (DMS)


II-VI DMS

ZnSe, CdSe and related alloys + Mn

Mn (group II) substitute the cation.Isoelectronic incorporation, no solubility limit.

Easy to prepare both as bulk material and epitaxial layers and etherostructures

ButMagnetic interaction dominated by antiferromagnetic direct exchange among Mn spins. In undoped material paramagnetic, antiferromagnetic and spinglass behavior, no FMInteresting: ‘‘Giant’’ Zeeman splitting !!


III-V DMS

GaAs, InAs and their alloy + Mn. Mn substitute the cation and introduce a hole.

Low solubility of the magnetic element, max 0.1 at % under normal growth condition.

Non-equilibrium epitaxial growth methods (MBE)to overcome the thermodynamic solubility limit.Standard MBE growth condition not sufficiently far from equilibrium

Low temperature MBE 1992 FM InMnAs1996 FM GaMnAs


The mechanism of FM in Mn based Zincblend DMS

• Antiferromagnetic direct coupling between Mn ions. Dominate in undoped materials.

• Ferromagnetic coupling in p-type materials as a result of exchange interaction between substitutional Mn S=5/2 and hole spins.The exchange interaction follows from hybridization between Mn d orbital and valence band p orbital.

Hole mediated FM

See PRB 72, 165204(05)and reference therein


Hole mediated FM

In a mean field virtual crystal approximation

x = substitutional Mn

p = hole density

In III-V DMS the holes comes from Mn !!!x and p are intimately related

Room temperature TC is expected for

Ga0.9Mn0.1As.

See PRB 72, 165204(05)and reference therein

31

xpTC


Know-how learning curve for GaMnAs MBE growth

Why it’s so difficultto rise TC???

Recipe determined by the Nottingham Univ. group (TC=173 K, world record)


GaMnAs structure

To increase TC one has to•Minimize As antisite defects•Minimize interstitial Mn•Get sufficiently high Mn content


To increase Mn content and minimize surface segregation,low growth temperature

R.P. Campion et al, JCG 251, 311 (03)

Ideal temperature vs Mn content identified by monitoring the RHEED : the highest T giving 2D RHEED

Mn incorporation


As antisite

•As flux reduced to the minimum necessary in order to maintain a 2D RHEED pattern at the selected temperature.

•2 Ga cell to maintain the exact stoichiometry during both GaAs and GaMnAs growth.

•Use of As2 instead of As4


As antisite cannot be eliminated by post-growth treatments !!

C.T.Foxon, private comm.


Interstitial Mn

Evidences (by RBS and PIXE) of the presence of interstitial Mn in as grown GaMnAs.

Low T annealing reduce the interstitials density that diffuse toward the surface, rise TC and p

Yu et al, PRB 65,201303R (02)Edmonds et al, PRL 92, 037201 (04)

Interstitial Mn are detrimental for FM:• are double donor• are attracted by substitutional Mn and coupled with them antiferromagnetically reduce the effective Mn moments concentration xeff


Long annealing at T=180C.

TC increases with annealing

p increases with annealing,no compensation in annealed samples

TC increase nearly

linearly with xeff

RT TC expected at

xeff = 0.10.

Jungwirth et al, PRB72, 165204 (05)


Eid et al, APL86, 152505 (05)Nanoengineered TC

by lateral patterning

50 nm Ga0.94Mn0.06As+ 10 nm GaAs capannealing is uneffective!

Lateral patterningTc + 50K with annealing!Free surface is important for interstitials passivation


Energy formation of interstitials depend on the Fermi energy of the material !!!

Yu et al, APL84, 4325 (04)

Magnetization data in three p-typeAlGaAs/GaMnAs/AlGaAsmodulation dopedheterostructures (MDH):N-MDH: Be above GaMnAsI-MDH: Be below GaMnAs.

Lower TC and more interstitials in GaMnAs grown on p-type semicondctor!! This may be a limit for TC


Alternative to bulk GaMnAs growth: Digital ferromagnetic heterostructure (DFH)

Kawakami et al, APL 77, 2379 (00)

Alternate deposition of GaAs and MnAs

Max TC = 50 Kbut also a single MnAs layer is FM!


n- and p-type doping of DFH by doping the GaAs spacers!! independent control of magnetism and free carriers

Johnston-Halperin et al,PRB 68, 165328 (03)

Fermi Energy effect?


Alternative to bulk GaMnAs growth: Mn doping = -like doping profile along the growth direction.Holes/Mn not enough to get FM.

+ p selectively doped heterostructure (p-SDHS) FM!!!

ds is the critical parameter

no FM for ds≥ 5nm

Nazmul et al, PRB 67, 241308R(03)


Nazmul et al, PRL 95, 017201 (05)

Mn -doping and heterostructue design

Record TC = 190 K after annealing

Record TC = 250 K after annealing !

EF effect on Mn interstitial density?


Electric field control of ferromagnetism

Ohno et al, Nature 408, 944 (00)

The idea: in hole mediated FMDecrease/increase of hole densityDecraese/increase exchange interaction between Mn

Metal insulator FETInMnAs with TC above 20K

Isothermal and reversible change of the magnetic state


II-VI Spin injectors

•Giant Zeeman splitting in II-VI•Spin polarization detected from light polarization

Fiederling et al, Nature 402, 787 (99)

B≠0, low TPopt= (I(σ+)-I(σ- ))/ (I(σ+)+I(σ- ))

=1/2 Pspin


III-V Spin injectors

Ohno et al, Nature 402, 790 (99)

Below TC polarization survive also at H=0

•FM GaMnAs as spin aligner•Spin-polarization measuredfrom el-emission polarization


First observation of spin-dependent MRin all-semiconductor heterostructure

Akiba et al. JAP 87, 6436 (00)

•InGaAs buffer to get tensile strain and out of plane easy axis •Two different Mn x to get different coercitive field

ΔR/R=0.2%


Large TMR in semiconductor magnetic tunnel junction

Tanaka et al, PRL 87, 026602 (01)

•In plane magnetic field•Optimal barrier thickness 1.6 nm•Antiparallel configuration is stable•ΔR/R=70%


Large Magnetoresistance in GaMnAs nanoconstriction

Rüster et al, PRL 91, 216602 (03)

•Large MR expected in transport trough domain wall•Constrictions pin domain walls


Rüster et al, PRL 91, 216602 (03)

GMR:(a)1.5% when R=48kΩfurther etching,(b) 8% when R=78kΩ

further etching, 2000% when R=4MΩ!!!TMR!


Tunneling anisotropic magnetoresistance -TAMRNew physics!

•Single GaMnAs magnetic layer•AlOx tunnel barrier

•Two resistance states•Position and sign of the switch depend on Φ•Interplay of anisotropic DOS with Φ and a two step magnetization reversal process

Gould et al, PRL 93, 117203 (04)


Tunneling anisotropic magnetoresistance -TAMRHuge effects and new physics

H perpendicular to the film (hard axis)

No histeresis!!

Related to the absolute and not relative orientation

Rüster et al, PRL 94, 27203 (05)


In plane FieldAngular dependence!Sensor of B orientation?

Φ = 95°T=1.7K and low bias
