Upload
allen-lindsey
View
236
Download
1
Embed Size (px)
Citation preview
Ferromagnetic semiconductors for spintronics Theory concepts and experimental overview
Tomas Jungwirth
University of Nottingham Bryan Gallagher, Tom Foxon,
Richard Campion, Kevin Edmonds, Andrew Rushforth, Chris King et al.
Hitachi Cambridge, Univ. Cambridge Jorg Wunderlich, Andrew Irvine, David Williams,
Elisa de Ranieri, Byonguk Park, Sam Owen, et al.
Institute of Physics ASCR Jan Mašek, Josef Kudrnovský, František Máca,
Alexander Shick, Karel Výborný, Jan Zemen, Vít Novák, Kamil Olejník, et al.
University of Texas Allan MaDonald, et al.
Texas A&MJairo Sinova, et al.
Electric field controlled spintronics
HDD, MRAMcontrolled by Magnetic field
Spintronic Transistorcontrol by
electric gates
STT MRAMspin-polarized charge current
From storage to logic
Magnetic race track memory
Current spintronics with FM metals
FM semiconductors: all features of current spintronics plus much more
Basic magnetic and magnetotransport properties of (Ga,Mn)As and related FS
Hard disk drive Hard disk drive
First hard discFirst hard disc (1956) (1956) - - classical electromagnet for read-outclassical electromagnet for read-out
From PC hard drives ('90)From PC hard drives ('90)to mto miicro-discscro-discs - - spintronispintronic read-headsc read-heads
MBMB’s’s
10’s-100’s 10’s-100’s GBGB’s’s
1 bit: 1mm x 1mm1 bit: 1mm x 1mm
1 bit: 101 bit: 10-3-3mm x 10mm x 10-3-3mmmm
Dawn of spintronicsDawn of spintronics
Anisotropic magnetoresistance (AMR) – 1850’s Anisotropic magnetoresistance (AMR) – 1850’s 1990’s 1990’s
Giant magnetoresistance (GMR) – 1988 Giant magnetoresistance (GMR) – 1988 1997 1997
Inductive read/write element
Magnetoresistive read element
Fert & Grunberg, Nobel Prize 07
MRAM – universal memoryMRAM – universal memory fast, small, low-power, durable, and non-volatile
2006- First commercial 4Mb MRAM
RAM chip that actually won't forget instant on-and-off computers
Spin-orbit couplingSpin-orbit coupling
nucleus rest frame electron rest frame
vI Q rE3
04 r
Q
3
0
4 r
rIB
EvEvB 200
1
c EvSS
2B
mc
egH B
SO
Lorentz transformation Thomas precession
2 2
ee--
Spintronics: it’s all about spin and chargeof electron communicating
quantum mechanics & special relativity Dirac equation
E=p2/2mE ih d/dtp -ih d/dr
E2/c2=p2+m2c2
(E=mc2 for p=0)
& HSO (2nd order in v/c around the non-relativistic limit)
Spin
Anisotropic Magneto-ResistanceAnisotropic Magneto-Resistance
Current sensitive to magnetization direction
~ 1% MR effect~ 1% MR effect
SO coupling from relativistic QM SO coupling from relativistic QM
FerromagnetismFerromagnetism = Pauli exclusion principle & Coulomb repulsion = Pauli exclusion principle & Coulomb repulsion
total wf antisymmetric = orbital wf antisymmetric * spin wf symmetric (aligned)
• RobustRobust (can be as strong as bonding in solids)(can be as strong as bonding in solids)
• Strong coupling to magnetic fieldStrong coupling to magnetic field (weak fields = anisotropy fields needed (weak fields = anisotropy fields needed only to reorient macroscopic moment)only to reorient macroscopic moment)
DOS
DOS
ee--
ee--
ee--
Giant Magneto-ResistanceGiant Magneto-Resistance
~ 10% MR effect~ 10% MR effect
DOS
AP
P
>
SO-coupling not utilized
Tunneling Magneto-ResistanceTunneling Magneto-Resistance
~ 100% MR effect~ 100% MR effect
DOS DOS
More direct link between transport and spin-split bands
Spin Transfer Torque writingSpin Transfer Torque writing
Slonczewski JMMM 96
Current spintronics with FM metals
FM semiconductors: all features of current spintronics plus much more
Basic magnetic and magnetotransport properties of (Ga,Mn)As and related FS
Mn
Ga
AsMn
Dilute moment ferromagnetic semiconductors
GaAs - GaAs - standard III-V semiconductorstandard III-V semiconductor
Group-II Group-II Mn - Mn - dilute dilute magneticmagnetic moments moments & holes& holes
(Ga,Mn)As - fe(Ga,Mn)As - ferrromagneticromagnetic semiconductorsemiconductor
More tricky than just hammering an iron nail in a silicon wafer
Ohno et al. Science 98
Mn-d-like localmoments
As-p-like holes
Mn
Ga
AsMn
Strongly spin-split and spin-orbit coupled carriers in a semiconductor
LSdr
rdV
err
mc
p
mc
SeBH effSO
)(1
Strong SO due to the As p-shell (L=1) character of the top of the valence band
V
BBeffeff
pss
Beff Bex + Beff
AMR, TMR, …
Dietl et al., Abolfath et al. PRB 01
GaAs Mn
Mn
10-100x smaller Ms
One
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 currents for switching
Dilute moment nature of ferromagnetic semiconductorsDilute moment nature of ferromagnetic semiconductors
Low-voltage gating (charge depletion) of ferromagnetic semiconductors
Owen, et al. arXiv:0807.0906
(Ga,Mn)As p-n junction FET
Low-voltage dependent R & MR
Switching by short low-voltage pulses
Mag
neti
zati
on
Ga
As Mn
Mn
Tc below room-temperature issue
• Low-Tc inherent feature of dilute moments but Tc 200K for 10% (Ga,Mn)As compared to Tc~300K in the 100% MnAs, i.e., Tc’s are already remarkable and the quest is still on
• New spintronics paradigms applicable to conventional ferromagnets or semiconductors
increasing Mn-doping
Wang, et al. arXiv:0808.1464Olejnik et al., PRB 08
AMRAMR TMRTMR
TAMRTAMR
) vs.( ~ IMvg
M
FM exchange int.:
Spin-orbit int.:
FM exchange int.:
)()( TDOSTDOS
)(MTDOS
Au
Discovered in GaMnAs Gould et al. PRL’04
ab intio theoryTAMR is generic to SO-coupled systems including room-Tc FMs
experiment
Bias-dependent magnitude and sign of TAMR
Shick et al PRB ’06, Moser et al. PRL 07,Parkin et al PRL ‘07, Park et al PRL '08
Park et al PRL '08
Consider uncommon TM combinationse.g. Mn/W voltage-dependent upto ~100% TAMR
spontaneous momentmag
netic su
sceptib
ility
spin
-orb
it cou
plin
g
Optimizing TAMR in transition-metal structures
Shick, et al PRB ‘08
GM
MGG
C
C
e
MV
MVVCQC
QQU
)(&
)]([&2
)(0
20
electric && magneticmagnetic
control of CB oscillations
Source Drain
GateVG
VDQ
Devices utilizing M-dependent electro-chemical potentials: FM SET
SO-coupling (M)
[010] M[110]
[100]
[110][010]
(Ga,Mn)As nano-constriction SET
~ 1mV in GaMnAs~ 10mV in FePt
Low-gate-voltage controlled huge magnitude and sign of MR very sensitive spintronic transistor
SO-coupling (M)
[010] M[110]
[100]
[110][010]
Wunderlich et al, PRL '06
Complexity of the relation between SO & exchange-split bands and
transport
SET
Resistor
Tunneling device
Chemical potential CBAMR
Tunneling DOS TAMR
Group velocity & lifetime AMR
Complexity of the device design
Magnitude and sensitivity to electric
fields of the MR
Datta-Das transistor
Spintronics in conventional semiconductorsSpintronics in conventional semiconductors
Datta and Das, APL ‘99
Karplus&Luttinger intrinsic AHE mechanism revived in Ga1-xMnxAs
Experiment AH 1000 (cm)-1
TheoryAH 750 (cm)-1
Anomalous Hall effect
intrinsic AHE in pure Fe:
I
_ FSO
FSO
_ __
V
Jungwirth et al. PRL ‘02,APL ’03
Yao et al. PRL ‘04
Karplus&Luttinger PR ‘54
I
_ FSO
FSO
_ __
Spin Hall effectspin-dependent deflection transverse edge spin polarization
I
_ FSO__
V
Anomalous Hall effect
M
Spin Hall effect
n
n
p
SHE mikročip, 100A supercondicting magnet, 100 A
Spin Hall effect detected optically in GaAs-based structures
Same magnetization achievedby external field generated bya superconducting magnet with 106 x larger dimensions & 106 x larger currents
Murakami et al Science 04, SInova et al. PRL 04,Wunderlich et al. PRL ‘05
Current spintronics with FM metals
FM semiconductors: all features of current spintronics plus much more
Basic magnetic and magnetotransport properties of (Ga,Mn)As and related FS
(Ga,Mn)As material(Ga,Mn)As material
5 d-electrons with L=0 S=5/2 local moment
moderately shallow acceptor (110 meV) hole
- Mn local moments too dilute (near-neighbors couple AF)
- Holes do not polarize in pure GaAs
- Hole mediated Mn-Mn FM coupling
Mn
Ga
AsMn
Mn-d-like localmoments
As-p-like holes
Mn
Ga
AsMn
EF
DO
S
Energy
spin
spin
Ferromagnetic semiconductor GaAs:Mn
valence band As-p-like holes
As-p-like holes localized on Mn acceptors
<< 1% Mn ~1% Mn >2% Mn
onset of ferromagnetism near MIT
Exchange-split, SO-coupled, & itinerant holes
Mn
Ga
AsMn
Mn–hole spin-spin interaction
hybridization
Hybridization like-spin level repulsion Jpd S shole AF interaction
Mn-d
As-p
Equivalence between microscopic hybridization (weak) picture and kinetic-exchange model
Microscopic (Anderson) Hamiltonian
Schrieffer-Wolf transformation
k=0 approx.
Heff
= Jpd
<shole> || -x
Mn
As
Ga
heff
= Jpd
<S> || x
Hole Fermi surfaces
Mean-field ferromagnetic Mn-Mn coupling mediated by holes
holes
Fluctuations around the MF state
MF
= - Jpd Ss
Antiferromagnetic coupling (Jpd > 0) STOT
= S - s
GS = Jpd /2 [ (S-s)(S-s+1) - S(S+1) -s(s+1) ] = - Jpd (Ss+s)
GS
< MF
H = Jpd S . s = Jpd /2 ( S2TOT
- S2 - s2)
Magnetism in systems with coupled dilute moments and delocalized band electrons
(Ga,Mn)As
cou
pli
ng
str
eng
th /
Fer
mi
ener
gy
band-electron density / local-moment density
Jungwirth et al, RMP '06
d4
d
Weak hybrid.Delocalized holeslong-range coupl.
Strong hybrid.
More localized holesshorter-range coupl.
no holes
InSb, GaAs
GaN
GaP
d5
Nature of Mn-impurity in III-V host
Kudrnovsky et al. PRB 07
hole-Mn exchange = hybridization & splitting between Mn d-level and valence band edge
Weak hybrid.
Strong hybrid.
InSb
GaP
d5
GaAs seems close to the optimal III-V host
Mean-field butlow Tc
MF
Large TcMF but
low stiffness
Hole-mediated Mn-Mn exchange in III-V host
MIT in p-type GaAs:- shallow acc. (30meV) ~ 1018 cm-3
- Mn (110meV) ~1020 cm-3
Mobilities:- 3-10x larger in GaAs:C- similar in GaAs:Mg or InAs:Mn
> 1-2% Mn: metallic but strongly disordered
Model:SO-coupled, exch.-split Bloch VB & disorder
- conveniently simple and increasingly meaningful as metallicity increases
- no better than semi-quantitative
Random Mn disorder
Short-range ~ M . s potential
Together with central-cellshifts MIT to ~1% Mn (1020 cm-3)
(Ga,Mn)As growth
Low-T MBE to avoid precipitation & high enough T to maintain 2D growth need to optimize T & stoichiometry for each Mn-doping
high-T growth optimal-T growth
Annealing also needs to be optimized for each Mn-doping
Detrimental interstitial AF-coupled Mn-donors need to anneal out (Tc can increase by more than 100K)
Tc in (Ga,Mn)As semiquantitative theory understanding (within a factor of ~2)
No saturation seen in theory and in optimized (Ga,Mn)As samples yet
Material synthesis becomes increasingly tedious for >6% MnGa
MnGa doping
t=(Tc-T)/Tc
4.03.0
~
tM
1.5% 8%
Optimized (Ga,Mn)As materials
Wang, et al. arXiv:0808.1464Olejnik et al., PRB 08, Novak et al. PRL 08
• I-II-Mn-V ferromgantic semiconductors (so far in theory only)
III = I + II Ga = Li + Zn
• GaAs and LiZnAs are twin semiconductors
• Prediction that Mn-doped are also twin ferromagnetic semiconductors
• No limit for Mn-Zn (II-II) substitution
• Independent carrier (holes or electrons) doping by Li-Zn stoichiometry adjustment
Masek, et al. PRL 07
VB-CB
VB-IB
Mn-acceptor level (IB)
Short-range ~ M . s potential
Together with central-cellshifts MIT to ~1% Mn (1020 cm-3)
Transport in (Ga,Mn)As: MIT
GaAs VB
GaMnAs disordered VB
2.2x1020 cm-3
Jungwirth et al, PRB '07
MIT in GaAs:Mn at order of magnitude higher doping than quoted in text books
Curie point transport anomaly
Ordered magnetic semiconductors
Eu - chalcogenides
Disordered DMSs
Sharp critical contribution to resistivity at Tc ~ magnetic susceptibility
Broad peak near Tc and disappeares with annealing (higher uniformity)???
~)( dF
Eu0.95Cd0.05S
][~),(~)( 002 SSSSJTRT iipdi
Fisher&Langer, PRL‘68
singular
UdF ~)~(
vcdTdUdTd /~/
Tc
Ni, Fe
singular
Scattering off correlated spin-fluctuations
Optimized GaMnAs materials with x~4-12% and Tc~80-185K: very well behaved FMs
Annealing sequence
In GaMnAs F~d- sharp singularity at Tc in d/dT
Novak et al., PRL ‚08
T/Tc-1
• New paradigms for spintronics applicable to conventional FM and SC
• Spintronic field-effect transistors
• Well behaved ferromagnet compatible with standard SC technologies
[010] M[110]
[100]
[110][010]
Conclusions
CBAMR
(Ga,Mn)As and related FS: