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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

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Spintronics spin-based electronics Information
is carried by the electron spin, not (only) by
the electron charge.

• 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 Magnetic
Resistance) 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
Prinz, Science 282, 1660 (98) Wolf et al, Science
294, 1488 (01)
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Standard geometry for GMR based Spin Valves
GMR based Spin Valves for read head in hard drives
But also MRAM
Prinz, Science 282, 1660 (98) Wolf, Science 294,
1488 (01)
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Spintronics spin-based electronics

• Ferromagnetic metal - based devices

• Semiconductor based spin electronics

Courtesy C.T. Foxon
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Spintronics spin-based electronics

• Ferromagnetic metal - based devices

• Semiconductor based spin electronics devices

• 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
R01/pe Ordinary Hall effect contribution,
negligible. RHall is proportional to M.
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• Two main issues in semiconductor spintronics
• 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

• 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
1992 FM InMnAs 1996 FM GaMnAs
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• 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 S5/2 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
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• 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
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Know-how learning curve for GaMnAs MBE growth
Why its so difficult to rise TC???
Recipe determined by the Nottingham Univ. group
(TC173 K, world record)
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GaMnAs structure

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

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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
R.P. Campion et al, JCG 251, 311 (03)
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• 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 !!
C.T.Foxon, private comm.
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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 T180C. 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)
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Energy formation of interstitials depend on the
Fermi energy of the material !!!
Magnetization data in three p-type AlGaAs/GaMnAs/A
lGaAs 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
Yu et al, APL84, 4325 (04)
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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!
Kawakami et al, APL 77, 2379 (00)
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n- and p-type doping of DFH by doping the GaAs
spacers!! independent control of magnetism and
free carriers
Johnston-Halperin et al, PRB 69, 165328 (03)
Fermi Energy effect?
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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)
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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)
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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)
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II-VI Spin injectors

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

Popt (I(s)-I(s- ))/ (I(s)I(s- )) 1/2
Pspin
B?0, low T
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Fiederling et al, Nature 402, 787 (99)
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III-V Spin injectors

• FM GaMnAs as spin aligner
• Spin-polarization measured
• from el-emission polarization

Below TC polarization survive also at H0
Ohno et al, Nature 402, 790 (99)
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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

?R/R0.2
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Akiba et al. JAP 87, 6436 (00)
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TMR in semiconductor magnetic tunnel junction

• In plane magnetic field
• Optimal barrier thickness 1.6 nm
• Antiparallel configuration is stable
• ?R/R70

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Tanaka et al, PRL 87, 026602 (01)
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Large Magnetoresistance in GaMnAs nanoconstriction

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

Rüster et al, PRL 91, 216602 (03)
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• GMR
• 1.5 when R48kO
• further etching,
• 8 when R78kO

further etching, 2000 when R4MO!!! TMR!
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Rüster et al, PRL 91, 216602 (03)
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Tunneling anisotropic magnetoresistance -TAMR New
physics!

• Single GaMnAs magnetic layer
• AlOx tunnel barrier

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

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Gould et al, PRL 93, 117203 (04)
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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)
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In plane Field Angular dependence! Sensor of B
orientation?
F 95 T1.7K and low bias
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