Spintronics in metals and semiconductors

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Spintronics in metals and semiconductors
Tomas Jungwirth
University of Nottingham
Bryan Gallagher, Tom Foxon, Richard
Campion, Kevin Edmonds, Andrew
Rushforth, Chris King et al.
Institute of Physics ASCR Alexander Shick,
Karel Výborný, Jan Zemen, Jan Masek, Vít
Novák, Kamil Olejník, et al.
Hitachi Cambridge Jorg Wunderlich, Andrew
Irvine, David Williams, Elisa de Ranieri, Sam
Owen, et al.

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Outline 1. Tunneling anisotropic
magnetoresistance in transition metals 2.
Ferromagnetism in (Ga,Mn)As and related
semiconductors 3. Spintronic transistors
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Spintronics Spin-orbit exchange interactions
nucleus rest frame
electron rest frame
Thomas precession
? spin-orbit interaction
DOS
Coulomb repulsion Pauli exclusion principle ?
exchange interaction
? ferromagnetism
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AMR
TMR
1 MR effect
100 MR effect
Exchange int.
Spin-orbit int.
magnetic anisotropy
TAMR
Exchange int.
AFM-FM exchange bias
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TAMR in CoPt structures
ab intio theory Shick, et al, PRB '06, Park,
et al, PRL '08
experiment Park, et al, PRL '08
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TAMR in TM structures
Consider uncommon TM combinations Mn/W ? 100
TAMR Consider both Mn-TM FMs AFMs
Shick, et al, unpublished
spontaneous moment
magnetic susceptibility
spin-orbit coupling
exchange-spring rotation of the AFM Scholl et al.
PRL 04
Proposal for AFM-TAMR first microelectronic
device with active AFM component
Shick, et al, unpublished
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Outline 1. Tunneling anisotropic
magnetoresistance in transition metals 2.
Ferromagnetism in (Ga,Mn)As and related
semiconductors 3. Spintronic transistors
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TM-based ? semiconducting multiferroic
spintronics sensors memories ? transistors
logic
Magnetic materials
spintronic magneto-sensors, memories
Semiconductors
Ferroelectrics/piezoelectrics
transistors, logic, sensitive to doping and
electrical gating
electro-mechanical transducors, large
persistent el. fields
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Ferromagnetic semiconductors
Need true FSs not FM inclusions in SCs
GaAs - standard III-V semiconductor Group-II Mn
- dilute magnetic moments
holes (Ga,Mn)As - ferromagnetic
semiconductor
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GaAsMn extrinsic p-type semiconductor
EF
spin ?
1 Mn
ltlt 1 Mn
gt2 Mn
DOS
Energy
spin ?
onset of ferromagnetism near MIT
As-p-like holes localized on Mn acceptors
valence band As-p-like holes
As-p-like holes
FM due to p-d hybridization (Zener
local-itinerant kinetic-exchange)
Mn-d-like local moments
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Strong spin-orbit coupling
As-p-like holes
Strong SO due to the As p-shell (L1) character
of the top of the valence band
Beff
Bex Beff
Note TAMR discovered in (Ga,Mn)As Gold et al.
PRL04
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(Ga,Mn)As synthesis
high-T growth

• Low-T MBE to avoid precipitation
• High enough T to maintain 2D growth
• need to optimize T stoichiometry
• for each Mn-doping
• Inevitable formation of interstitial Mn-donors
• compensating holes and moments
• ? need to anneal out

optimal-T growth
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Interstitial Mn out-diffusion limited by
surface-oxide
Polyscrystalline 20 shorter bonds
O
GaMnAs-oxide
x-ray photoemission
MnI
GaMnAs
Olejnik et al, 08
10x shorther annealing with etch
Optimizing annealing-T another key factor
Rushforth et al, 08
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Tc limit in (Ga,Mn)As remains open
Indiana California (03) .. Ohnos 98
Tc110 K is the fundamental upper limit .. Yu
et al. 03
Nottingham Prague (08) Tc up to 185K so far
California (08) Tc 150-165 K independent of
xMngt10 contradicting Zener kinetic exchange ...
Mack et al. 08
Combinatorial approach to growth with fixed
growth and annealing Ts
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Other (III,Mn)Vs DMSs
Kudrnovsky et al. PRB 07
Delocalized holes long-range coupl.
Weak hybrid.
Mean-field but low TcMF
InSb
d5
Impurity-band holes short-range coupl.
Strong hybrid.
Large TcMF but low stiffness
GaP
(Al,Ga,In)(As,P) good candidates, GaAs seems
close to the optimal III-V host
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Other DMS candidates
III I II ? Ga Li Zn
GaAs and LiZnAs are twin SC (Ga,Mn)As and
Li(Zn,Mn)As should be twin ferromagnetic SC

• But Mn isovalent in Li(Zn,Mn)As
• no Mn concentration limit and self-compensation
• possibly both p-type and n-type ferromagnetic SC
• (Li / Zn stoichiometry)

Masek et al. PRL 07
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Towards spintronics in (Ga,Mn)As FM transport
Dilute-moment MS ?F d?-?
Dense-moment MS ?Fltlt d?-?
Eu? - chalcogenides
Broad peak near Tc disappeares with annealing
(higher uniformity)???
Critical contribution to resistivity at Tc
magnetic susceptibility
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Critical contribution at Tc to d?/dT like TM FMs
Fe
Fisher Langer 68
Ni
Novak et al., 08
d?/dT cv
?F d?-?
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FisherLanger 68
d?/dT
Scattering off short range correlated
spin-fluctuation
Tc
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Outline 1. Tunneling anisotropic
magnetoresistance in transition metals 2.
Ferromagnetism in (Ga,Mn)As and related
semiconductors 3. Spintronic transistors
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Gating of the highly doped (Ga,Mn)As p-n
junction FET
p-n junction depletion estimates
25 depletion feasible at low voltages
Olejnik et al., 08
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Increasing ? and decreasing AMR and Tc with
depletion
AMR
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Persistent variations of magnetic properties with
ferroelectric gates
Stolichnov et al., Nat. Mat.08
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Electro-mechanical gating with piezo-stressors
Strain SO ?
Rushforth et al., 08
Electrically controlled magnetic anisotropies
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(Ga,Mn)As spintronic single-electron transistor
Wunderlich et al. PRL 06
Huge, gatable, and hysteretic MR
Single-electron transistor
Two "gates" electric and magnetic
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AMR nature of the effect
Coulomb blockade AMR
normal AMR
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magnetic
electric
SO-coupling ? ?(M)
control of Coulomb blockade oscillations
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Theory confirms chemical potential anisotropies
in (Ga,Mn)As predicts CBAMR in SO-coupled
room-Tc metal FMs

• CBAMR if change of ??(M) e2/2C?
• In our (Ga,Mn)As meV ( 10 Kelvin)
• In room-T ferromagnet change of ??(M)100K
• Room-T conventional SET
• (e2/2C? gt300K) possible

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Nonvolatile programmable logic
Variant p- or n-type FET-like transistor in one
single nano-sized CBAMR device
1
1
0
0
0
1
1
0
OFF
ON
OFF
ON
ON
OFF
ON
0
1
1
1
0
0
1
1
OFF
ON
OFF
1
0
1
0
OR
OFF
ON
ON
OFF
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Nonvolatile programmable logic
Variant p- or n-type FET-like transistor in one
single nano-sized CBAMR device
OR
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Physics of SO exchange
Device design
Materials
Chemical potential ? CBAMR
TM FMs, MnAs, MnSb
SET
(III,Mn)V, I(II,Mn)V DMSs
Tunneling DOS ? TAMR
Tunneling device
Mn-based TM FMsAFMs
Group velocity lifetime ? AMR
TM FMs
Resistor
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END
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Dawn of spintronics
Magnetoresistive read element
Inductive read/write element
Anisotropic magnetoresistance (AMR) 1850s ?
1990s Giant magnetoresistance (GMR) 1988 ?
1997
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MRAM universal memory fast, small, low-power,
durable, and non-volatile
2006- First commercial 4Mb MRAM
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Based on Tunneling Magneto-Resistance (similar to
GMR but insulating spacer)
RAM chip that actually won't forget ? instant
on-and-off computers
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DOS
?? ? ??
Giant Magneto-Resistance
?AP
gt
?P
10 MR effect
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Tunneling Magneto-Resistance
DOS? ? DOS?
100 MR effect
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Spin Transfer Torque writing
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Dilute moment nature of ferromagnetic
semiconductors

• Key problems with increasing MRAM capacity (bit
  density)
• Unintentional dipolar cross-links
• External field addressing neighboring bits

10-100x weaker dipolar fields
10-100x smaller Ms
10-100x smaller currents for switching
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Magnetism in systems with coupled dilute moments
and delocalized band electrons
(Ga,Mn)As
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Hole transport and ferromagnetism at relatively
large dopings
conducting p-type GaAs - shallow acc. (C, Be)
1018 cm-3 - Mn 1020 cm-3
Non-equilibrium growth - technological
difficulties
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Magnetization
Magnetization
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Variable controlled strain using a Piezo stressor

A.W. Rushforth, J. Zemen, K. Vyborny, et al.
arXiv0801.0886
Strain induced by piezo voltage /- 150V 2
10-4 (at 50K)
M. Overby, et al., arXiv0801.4191
Easy axis rotation by 50 deg for Vpiezo
-150V ? 150V
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Fast Precessional switching via gatevoltage
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Spintronics with spin-currents only
Magnetic domain race-track memory
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Spin Hall effect detected optically in
GaAs-based structures
Same magnetization achieved by external field
generated by a superconducting magnet with 106 x
larger dimensions 106 x larger currents
SHE mikrocip, 100?A
supercondicting magnet, 100 A
SHE edge spin accumulation can be extracted and
moved further into the circuit
SHE detected elecrically in metals
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Spintronics in nominally non-magnetic materials
Datta-Das transistor
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Spin Hall effect spin-dependent deflection ?
transverse edge spin polarization
skew scattering
intrinsic
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Spintronics explores new avenues for
?

• Information reading

• Information reading storage

Tunneling magneto-resistance sensor and memory
bit

• Information reading storage writing

Current induced magnetization switching

• Information reading storage writing
  processing

Spintronic transistor magnetoresistance
controlled by gate voltage

• New materials

Ferromagnetic semiconductors, Multiferroics Non-ma
gnetic SO-coupled systems