Magneto-transport anisotropy phenomena in GaMnAs and beyond

1
Magneto-transport anisotropy phenomena in GaMnAs
and beyond
Tomas Jungwirth
University of Nottingham
Bryan Gallagher, Richard Campion, Kevin Edmonds,
Andrew Rushforth, Tom Foxon, et al.
Institute of Physics ASCR Karel Výborný,
Alexander Shick. Jan Zemen, Jan Mašek, Vít
Novák, Kamil Olejník,, et al.
University Hitachi Cambridge Jorg
Wunderlich, Andrew Irvine, Elisa de Ranieri,
Byonguk Park, et al.

• Texas AM
• Jairo Sinova, et al.

University of Texas Allan MaDonald, Maxim
Trushin,et al.
University of Wuerzburg Charles. Gould, Laurens
Molenkamp, et al.

2
Observations made from studies of AMR phenomena
in GaMnAs (outline)
1. More than just bulk AMR in ohmic devices
TAMR, CBAMR 2. In DMSs bulk AMR has the
simplest intuitive picture 3. TAMR and CBAMR
are transferable to room-T metal FMs AFMs
3
Experimental observation of (ohmic) AMR
magnetization
Lord Kelvin 1857
current
AMR sensors dawn of spintronics
Inductive read elements
Magnetoresistive read elements
1980s-1990s
Now often replaced by GMR or TMR but still
extensively used in e.g. automotive
industry Problems with small magnitude and scaling
4
Theory of AMR current response to magnetization
via spin-orbit coupling
Model for transition metal FMs
Smit 1951
?
Miscroscopic theory relativistic LDA
Kubo formula
theory
experiment
FeNi
BanhartEbert EPL95
5
Renewed research interest in AMR due to FS like
(Ga,Mn)As
Ohno. Science 98
MnGa acceptor electrical conduction similar to
conventional p-doped GaAs
metallic
0.1 Mn
insulating
ltlt0.1 Mn
Jungwirth et al. PRB 07
6
Renewed research interest in AMR due to FS like
(Ga,Mn)As
(Ga,Mn)As
Mn moment Ferromagnetism reminiscent of
conventional metal band FMs (Fe, Co, Ni,..)
d?/dTcv
Ni
Tc
(Ga,Mn)As
ferromagnetic
Tc
Novak et al. PRL 08
7
Renewed research interest in AMR due to FS like
(Ga,Mn)As
AMRs of order 1-10 - routine
characterization tool - semi-quantitatively
described assuming scattering of valence-band
holes
Baxter et al. PRB 02, Jungwirth et al. APL02,
03
8
Magnetic anisotropies in (Ga,Mn)As valence band
degenerate HH bands and LH bands in
GaAs anisotropic surface and spin-texture due
to crystal and SO coupling in As(Ga) p-orbitals
j3/2
HH
HH LH Fermi surfaces
exchange-split HH bands and LH bands in
(Ga,Mn)As anisotropic due to crystal, SO
coupling and FM exchange field
HH
HH
M
Dietl et al. PRB 01, Abolfath et al. PRB 01
9
Simple direct link between band structure and
transport
Complexity of the device design
Magnitude, control, and tuneability of MR
SET
Chemical potential ? CBAMR
micro-structures
MTJ
Tunneling DOS ? TAMR
heterostructures
bulk
Scattering lifetimes ? ohmic AMR
Resistor
10
TAMR spectroscopy of tunneling DOS anisotropy
k - resolved tunneling DOS
electrode
barrier
GaMnAs
Vbias
Binpl
M
Giddings et al. PRL 04
M
Selectivity tuned by choice of barrier,
counter-electrode, or external fields
11
TAMR spectroscopy of tunneling DOS anisotropy
Gould et al. PRL 04
M
M
Non-selective barrier and counter-electrode ?
only a few TAMR
12
TAMR spectroscopy of tunneling DOS anisotropy
M
M
Giraud et al. APL 05, Sankowski et al. PRB07,
Ciorga et al.NJP07, Jerng JKPS 09
Very selective p-n Zener diode MTJs
Binpl
Giraud et al. Spintech 09
13
TAMR spectroscopy of tunneling DOS anisotropy
M
M
Extra-momentum due to Lorentz force during
tunneling
Very selective p-n Zener diode MTJs
Binpl
Giraud et al. Spintech 09
14
CBAMR M-dependent electro-chemical potentials in
a FM SET
Wunderlich et al. PRL 06
110
M
?
100
110
010
magnetic
electric
control of CB oscillations
15
Huge MRs controlled by low-gate-voltage likely
the most sensitive spintronic transistors to date
Wunderlich et al. PRL 06
Schlapps et al. PRB 09
16
Simple direct link between band structure and
transport
Chemical potential ? CBAMR
SET
Tunneling DOS ? TAMR
MTJ
Scattering lifetimes ? AMR
Resistor
17
Simplicity of the microscopic picture of AMR in
(Ga,Mn)As
SET
CBAMR,TAMR SO FM polarized bands
M
MTJ
MnGa
MnGa
ohmic AMR main impurities FM polarized random
MnGa ? can consider bands with SO coupling only
Resistor
18
Simplicity of the microscopic physical picture in
(Ga,Mn)As
current
SET
CBAMR only el.-chem potentials ? no M vs current
term
M
cryst. axis
TAMR current direction is cryst. distinct ?
inseparable M vs current term
current
M
MTJ
cryst. axis
current
M
AMR M vs current (non-crystalline) term can be
separated and dominates in (Ga,Mn)As
cryst. axis
Resistor
19
Key mechanism for AMR in (Ga,Mn)As FM
impurities SO carriers in non-cryst.-like
spherical bands
KL Hamiltonian in spherical approximation
MGa
Heavy holes
current
Electro-magnetic impurity potential of MnGa
acceptor
Rushforth PRL07, Trushin et al. PRB 09,
Vyborny et al. PRB 09
20
Pure magnetic MnGa impiruties positive AMR,
Backward-scattering matrix elements
current
21
Electro-magnetic MnGa impiruties negative AMR,
current
Backward-scattering matrix elements
22
Electro-magnetic MnGa impiruties negative AMR,
? screened Coulomb potential ?
current
all scatt.
backward scatt.
23
Electro-magnetic MnGa impiruties negative AMR,
? screened Coulomb potential ?
current
all scatt.
backward scatt.
24
Negative and positive and crystalline AMR in RD
2D system
Dresselhaus
Rashba
25
AMR in 2D RD and 3D KL system from exact
solution to integral Boltzmann eq.
contains only cos? and sin? harmonics
analytical solution to the integral Boltzmann eq.
26
AMR in transition/noble metals
Model for transition metal FMs
Smit 1951
?
Miscroscopic theory relativistic LDA
Kubo formula
theory
experiment
FeNi
BanhartEbert EPL95
27
TAMR and CBAMR predictions for metals
ab intio theory Wunderlich et al., PRL
06,Shick, et al, PRB '06
Anisotropy in DOS
Anisotropy in chemical potential
28
Experimental observation of large and bias
dependent TAMR
Shick et al PRB 06, Parkin et al PRL 07, Park
et al PRL '08
ab intio theory
TAMR in SO-coupled FMs
experiment
Park et al PRL '08
29
Experimental observation of CBAMR in metals
Bernand-Mantel et al Nat. Phys.09
30
Optimizing TAMR/CBAMR in transition-metal
structures
Consider TM combinations containing Mn e.g. FM
Mn/W ? upto 100 TAMR
Shick, et al PRB 08
But most transition/noble metals with Mn are AFMs!
31
AFM spintronics
Zero stray field in compensated AFMs
Ultrafast dynamics of spin excitations
32
Mn2Au
spin-dn
spin-up
Predicted strong AFM with no frustration
33
MnIr
spin-dn
spin-up
Conventional AFM
34
Magnetic moments (mB)
Element specific MAE (meV)
MAE accuracy 0.01 meV
Local Mn-atom moment contributes only little to
the MAE
Most of the MAE comes from zero moment Au, Ir
atoms
35
Each of localized 3d(Mn)- sublattices ? induces
the magnetic moment on 5d-site
Strong 5d-SOC produces the MAE
Summing over 3d(Mn)- sublattices ?
- non-zero!
0
complies with t-reversal symmetry of AFM
Strong 5d-SOC x 3d(Mn)-exchange filed x local
susceptibility produce the MAE
36
TAMR and CBAMR
ADOS(q,f-q,f) DOSq, fDOSq,f/
DOSq,f
and ATDOS TDOSq, fTDOSq,f/TDOSq,f
Hard 001-to-easy 110
ADOS(001-110) 50
ATDOS(001-110) 20
??Ef001-Ef110-2.5 mV
Sizable TAMR and CBAMR in AFMs
37
Effect of in-plane strain moment reorientations
and TAMR
010
Easy 110 ? Easy 010 at lt1 strain
100
1 strain
Strain-induced TAMR
ADOS(110-010) 20
ATDOS(110-010) 20
38
GMR/TMR and spin-torque relay on coherence
quality of interfaces ? in principle possible but
likely very difficult to build AFM spintronics on
these effects
Instead bulid AFM spintronics on a set of
magnetic anisotropy phenomena
Piezo- (or other) electric control of AF moment
orientation TAMR (CBAMR)
39
Observations made from studies of AMR phenomena
in GaMnAs (summary)
1. More than just bulk AMR in ohmic devices
TAMR, CBAMR 2. In DMSs bulk AMR has the
simplest intuitive picture 3. TAMR and CBAMR
are transferable to room-T metal FMs AFMs