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Material and devices for spintronics
•What is spintronics?
•Ferromagnetic semiconductors
Physical basis
Material issues
•Examples of spintronic devices
Electric field control of magnetism
Spin injectors
Spin valves
national laboratory for advanced
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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
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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))
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GMR based
Spin Valves and Magnetic tunnel junction
AF layer (A) or AF/FM/Ru/ trilayer (B)
to pin the magnetization of the top FM
layer
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Prinz, Science 282, 1660 (98)
Wolf et al, Science 294, 1488 (01)
Standard geometry for
GMR based Spin Valves
GMR based Spin Valves for
read head in hard drives
But also MRAM
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Prinz, Science 282, 1660 (98)
Wolf, Science 294, 1488 (01)
Spintronics = spin-based electronics
1. Ferromagnetic metal - based devices
2. Semiconductor based spin electronics
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Courtesy C.T. Foxon
Spintronics = spin-based electronics
1. Ferromagnetic metal - based devices
2. Semiconductor based spin electronics 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 in
coherent superpositions of up and down states
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How can we measure the magnetic state of a thin epilayer:
SQUID measurements but also
Anomalous Hall effect
RHall
R0
RS

B
M
d
d
R0=1/pe
Ordinary Hall effect
contribution,
negligible.
RHall is proportional
to M.
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Two main issues in semiconductor spintronics:
1. Avaiability of suitable materials
Ideal 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
• .................
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Magnetic semiconductor, constituted by
a periodic array of magnetic ions
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
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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)
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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
But
Magnetic interaction dominated by antiferromagnetic
direct exchange among Mn spins.
In undoped material paramagnetic, antiferromagnetic
and spinglass behavior, no FM
Interesting: ‘‘Giant’’ Zeeman splitting !!
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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
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1992 FM InMnAs
1996 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
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See PRB 72, 165204(05)
and reference therein
Hole mediated FM
See PRB 72, 165204(05)
and reference therein
In a mean field virtual crystal approximation
TC  xp
1
3
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.
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Know-how learning curve for GaMnAs MBE growth
Why it’s so difficult
to rise TC???
Recipe determined by the Nottingham Univ. group
(TC=173 K, world record)
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GaMnAs structure
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To increase TC one has to
•Minimize As antisite defects
•Minimize interstitial Mn
•Get sufficiently high Mn content
Mn incorporation
To increase Mn content
and minimize surface
segregation,
low growth temperature
Ideal temperature vs
Mn content identified
by monitoring the
RHEED :
the highest T giving
2D RHEED
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R.P. Campion et al, JCG 251, 311 (03)
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
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As antisite cannot be eliminated by
post-growth treatments !!
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C.T.Foxon,
private comm.
Interstitial Mn
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
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)
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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.
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Jungwirth et al, PRB72, 165204 (05)
Nanoengineered TC by
lateral patterning
50 nm Ga0.94Mn0.06As
+ 10 nm GaAs cap
annealing is uneffective!
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Eid et al, APL86, 152505 (05)
Lateral patterning
Tc + 50K with annealing!
Free surface is important for
interstitials passivation
Energy formation of interstitials depend on the Fermi
energy of the material !!!
Magnetization data in three
p-type
AlGaAs/GaMnAs/AlGaAs
modulation doped
heterostructures (MDH):
N-MDH: Be above GaMnAs
I-MDH: Be below GaMnAs.
Lower TC and more
interstitials in GaMnAs grown
on p-type semicondctor!!
This may be a limit for TC
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Yu et al, APL84, 4325 (04)
Alternative to bulk GaMnAs growth:
Digital ferromagnetic heterostructure (DFH)
Alternate deposition
of GaAs and MnAs
Max TC = 50 K
but also a single MnAs layer is FM!
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Kawakami et al, APL 77, 2379 (00)
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)
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Fermi Energy effect?
Alternative to bulk GaMnAs growth:
Mn d-doping = d-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
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Nazmul et al, PRB 67, 241308R(03)
Mn d-doping and heterostructue design
Record TC = 190 K
after annealing
Record TC = 250 K
after annealing !
EF effect on Mn
interstitial density?
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Nazmul et al, PRL 95, 017201 (05)
Electric field control of ferromagnetism
The idea: in hole mediated FM
Decrease/increase of hole density
Decraese/increase exchange interaction between Mn
Metal insulator FET
InMnAs with TC above 20K
Isothermal and reversible
change of the magnetic state
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Ohno et al, Nature 408, 944 (00)
II-VI Spin injectors
•Giant Zeeman splitting in II-VI
•Spin polarization detected
from light polarization
Popt= (I(σ+)-I(σ- ))/ (I(σ+)+I(σ- ))
=1/2 Pspin
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B≠0, low T
Fiederling et al, Nature 402, 787 (99)
III-V Spin injectors
•FM GaMnAs as spin aligner
•Spin-polarization measured
from el-emission polarization
Below TC polarization survive
also at H=0
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Ohno et al, Nature 402, 790 (99)
First observation of spin-dependent MR
in all-semiconductor heterostructure
•InGaAs buffer to get
tensile strain and out of
plane easy axis
•Two different Mn x to
get different coercitive
field
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ΔR/R=0.2%
Akiba et al. JAP 87, 6436 (00)
Large TMR in semiconductor
magnetic tunnel junction
•In plane magnetic field
•Optimal barrier thickness 1.6 nm
•Antiparallel configuration is stable
•ΔR/R=70%
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Tanaka et al, PRL 87, 026602 (01)
Large Magnetoresistance in GaMnAs nanoconstriction
•Large MR expected
in transport trough
domain wall
•Constrictions pin
domain walls
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Rüster et al, PRL 91, 216602 (03)
GMR:
(a) 1.5% when R=48kΩ
further etching,
(b) 8% when R=78kΩ
further etching,
2000% when R=4MΩ!!!
TMR!
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Rüster et al, PRL 91, 216602 (03)
Tunneling anisotropic magnetoresistance -TAMR
New 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
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Gould et al, PRL 93, 117203 (04)
Tunneling anisotropic magnetoresistance -TAMR
Huge effects and new physics
H perpendicular to the film
(hard axis)
No histeresis!!
Related to the absolute and
not relative orientation
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Rüster et al, PRL 94, 27203 (05)
In plane Field
Angular dependence!
Sensor of B orientation?
Φ = 95°
T=1.7K and low bias
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