expecting the unexpected in the spin hall effect: from...
TRANSCRIPT
Research fueled by:
8th International Workshop on Nanomagnetism & Superconductivity
Coma-ruga July 2nd, 2012
Expecting the unexpected in the spin Hall effect: from fundamental to practical
JAIRO SINOVA
Texas A&M UniversityInstitute of Physics ASCR
Hitachi CambridgeJoerg Wünderlich, A. Irvine, et al
Institute of Physics ASCRTomas Jungwirth, Vít Novák, et al
1
U. of WurzbergLaurens Molenkamp, E. Hankiewicz, et al
2
Talk dedicated to “THE” dream team
CAMPEONES! OLE !
Nanoelectronics, spintronics, and materials control by spin-orbit coupling 3
I. Introduction:•Basics of AHE: SOC origins and mechanism•SHE phenomenology
II. Spin Hall effect: the early days •First proposals: from theory to experiment•First observations of the extrinsic and intrinsic (optical)
III. Inverse spin Hall effect: SHE as a spin current detector•Direct iSHE in metals•Spin pumping and iSHE•Intrinsic mesoscopic SHE
IV.SHE-FET: first steps towards practicality (but perhaps not)•Spin Hall injection and spin precession manipulation•iSHE device with spin-accumulation modulation
V. FMR measurement of SHE angle: giant SHE as a spin current generator•FMR and SHE angle •Giant intrinsic SHE and STT: Future MRAM technology?
VI.Conclusion
Expecting the unexpected in the spin Hall effect:
from fundamental to practical
⇢xy
= � �xy
�2xx
+ �2xy
⇡ ��xy
�2xx
⇡ ��xy
⇢2xx
⇡ �A⇢xx
� B⇢2xx
Simple electrical measurement of out of plane magnetization (or spin polarization ~ n↑-n↓)
InMnAs
4
Spin dependent “force” deflects like-spin particles
ρH=R0B ┴ +4π RsM┴
Anomalous Hall Effect: the basics
I
_ FSO
FSO
_ __majority
minority
V
�AHxy
⇡ B + A�xx
M⊥
AHE is does NOT originate from any internal magnetic field created by M⊥; the field would have to be of the order of 100T!!!
Nagaosa, Sinova, Onoda, MacDonald, Ong, Rev. Mod. Phys. 2010
e-
Nanoelectronics, spintronics, and materials control by spin-orbit coupling 5
(one of the few echoes of relativistic physics in the solid state)
HSO = �~µB · ~Beff
This gives an effective interaction with the electron’s magnetic moment
Classical explanation (in reality it arises from a second order expansion of Dirac equation around the non-relativistic limit)
~E = �1erV (~r)• “Impurity” potential V(r) Produces
an electric field
∇V
Beff
ps
In the rest frame of an electronthe electric field generates an effective magnetic field
• Motion of an electron ~Beff= � ~~k
cm⇥ ~E
Consequence #1: Spin or the band-structure Bloch states are linked to the momentum. Coupled multi-band system.
Internal communication between spin and charge:spin-orbit coupling interaction
Consequence #2: Mott scattering
6
Cartoon of the mechanisms contributing to AHE �AHxy
⇡ B + A�xx
independent of impurity density
xc =@"(~k)~@
~
k
+e
~~
E ⇥ ~⌦
⌦z
(~k, n) = 2Im⌧
@
@y
n~k
����@
@x
n~k
�
Electrons have an “anomalous” velocity perpendicular to the electric field related to their Berry’s phase curvature which is nonzero when they have spin-orbit coupling.
Electrons deflect to the right or to the left as they are accelerated by an electric field ONLY because of the spin-orbit coupling in the periodic potential (electronics structure)
E
SO coupled quasiparticles
Intrinsic deflection B
Electrons deflect first to one side due to the field created by the impurity and deflect back when they leave the impurity since the field is opposite resulting in a side step. They however come out in a different band so this gives rise to an anomalous velocity through scattering rates times side jump.
independent of impurity density
Side jump scatteringVimp(r) (Δso>ħ/τ) or ∝ λ*∇Vimp(r) (Δso<ħ/τ)
B
Skew scattering
Asymmetric scattering due to the spin-orbit coupling of the electron or the impurity. Known as Mott scattering.
~σ~1/niVimp(r) (Δso>ħ/τ) or ∝ λ*∇Vimp(r) (Δso<ħ/τ) A
Spin Hall effectTake now a PARAMAGNET instead of a FERROMAGNET:
Spin-orbit coupling “force” deflects like-spin particles
I
_ FSO
FSO
_ __
V=0
non-magnetic
Transverse spin-current generation in paramagnets without external magnetic fields by spin-depedent deflection of electrons
Carriers with same charge but opposite spin are deflected by the spin-orbit coupling to opposite sides.
7
Spin Hall Effect(Dyaknov and Perel 1971)
InterbandCoherent Response
∼ (EFτ) 0
Occupation # Response
`Skew Scattering‘
[Hirsch, S.F. Zhang] 2000
Intrinsic`Berry Phase’
[Murakami et al, Sinova et al]
2003
Influence of Disorder[Inoue et al, Misckenko et
al, Chalaev et al…]
8
First experimental observations at the end of 2004
Wunderlich, Kästner, Sinova, Jungwirth, cond-mat/0410295PRL January 05
Experimental observation of the spin-Hall effect in a two dimensional spin-orbit coupled semiconductor system
Co-planar spin LED in GaAs 2D hole gas: ~1% polarization
Kato, Myars, Gossard, Awschalom, Science Nov 04 Observation of the spin Hall effect bulk in semiconductors
Local Kerr effect in n-type GaAs and InGaAs: ~0.03% polarization (weaker SO-coupling, stronger disorder)
9
I
FSO
FSO
_majority
minority
V
I
_ FSO
FSO
_ __
V=0non-magnetic
Ispin
FSO
FSO _
Vnon-magnetic
Mz
Mz=0
I=0
Mz=0
magnetic
optical detection
AHE
SHESHE-1
10
✓
✓
Completing the spin dependent Hall family: SHE-1
Valenzuela, S. O. & Tinkham, M, Nature‘06
xy
zBz
Electrical non-local spin valve detection by FM and by iSHE
extrinsic SHE-1 in metals
iSHENL spin detection
11
SHE-1 magnetoresistance measurement
12
Kimura et al PRL 98, 156601 (2007)
(SHE angle HIGHLY underestimated)
Magnetoresistance signals from SHE and inverse SHE θSH ~ 0.0037
Originally proposed by Shufeng Zhang 2000
4 nm thick Pt
80 nm thick Cu
100 nm junction width
Reason for the underestimate of θSH by Kimura et al.
The Cu shunts most of the charge current, so the charge current density in the Pt was much smaller than assumed.
Courtesy of D.C. Ralph13
Spin pumping and SHE-1
14
Saitoh et al APL 06
Theory based on ref: Silsbee, Janossy, and Monod, PRB 19, 4382 (1979)
sample layout
Brune et al, Nature Physics
insu
lating
p-co
nduc
ting
n-conducting
Mesoscopic intrinsic SHE-1 in HgTe
theory
15
16
I. Introduction:•Basics of AHE: SOC origins and mechanism•SHE phenomenology
II. Spin Hall effect: the early days •First proposals: from theory to experiment•First observations of the extrinsic and intrinsic (optical)
III. Inverse spin Hall effect: SHE as a spin current detector•Direct iSHE in metals•Spin pumping and iSHE•Intrinsic mesoscopic SHE
IV.SHE-FET: first steps towards practicality (but perhaps not)•Spin Hall injection and spin precession manipulation•iSHE device with spin-accumulation modulation
V. FMR measurement of SHE angle: giant and SHE as a spin current generator•FMR and SHE angle •Giant intrinsic SHE: Future MRAM technology?
VI.Conclusion
Expecting the unexpected in the spin Hall effect:
from fundamental to practical
17
Problem: Rashba SO coupling in the Datta-Das SFET is used for manipulation of spin (precession) BUT it dephases the spin too quickly (DP mechanism).
From DD-FET to new paradigm using SO coupling
1) Can we use SO coupling to manipulate spin AND increase spin-coherence?
2) Can we detect the spin in a non-destructive way electrically?
3) Can this effect be exploited to create a spin-FET logic device?
Use the persistent spin-Helix state or quasi-1D-spin channels and control of SO coupling strength (Bernevig et al 06, Weber et al 07, Wünderlich et al 09, Zarbo et al 10)
Use AHE to measure injected current polarization electrically (Wünderlich, et al Nature Physics. 09, PRL 04)
Spin-Hall AND-gate device (Wünderlich, Jungwirth, et al Science 2010)
DD-FET
VH2I
VbVH1
x
VH2
VbVH1
x
Spin Hall effect transistor:Wunderlich, Jungwirth, et al, Science 2010
SiHE
inverse SHE
18
iSHE transistor
Spin-FET with two gates → logic AND function
Wunderlich et al., Science.‘10
SHE transistor AND gate
19
Electrical spin modulator
Bx=0
Olejník, K. et al. arxiv.org/abs/1202.0881, to appear in PRL (2012).20
21
I. Introduction:•Basics of AHE: SOC origins and mechanism•SHE phenomenology
II. Spin Hall effect: the early days •First proposals: from theory to experiment•First observations of the extrinsic and intrinsic (optical)
III. Inverse spin Hall effect: SHE as a spin current detector•Direct iSHE in metals•Spin pumping and iSHE•Intrinsic mesoscopic SHE
IV.SHE-FET: first steps towards practicality (but perhaps not)•Spin Hall injection and spin precession manipulation•iSHE device with spin-accumulation modulation
V. FMR measurement of SHE angle: giant and SHE as a spin current generator•FMR and SHE angle •Giant intrinsic SHE: Future MRAM technology?
VI.Conclusion
Expecting the unexpected in the spin Hall effect:
from fundamental to practical
T. Kimura et al.PRL 98, 156601 (2007)
Magnetoresistance signals from SHE and inverse SHE
θSH ~ 0.0037
K. Ando et al.PRL 101, 036601 (2008)Effect of inverse SHE on
magnetic dampingθSH ~ 0.08
SHE angle measurements in Pt Vary by a Factor of 20
O. Mosendz et al.PRL 104, 046601 (2010), PRB
82, 214403 (2010)Magnetically-excited Py
precession produces voltage by inverse SHEθSH ~ 0.013
(assumes λSF= 10 nm in Pt)
Courtesy of D.C. Ralph
22
DC-Detected Spin-Transfer-Driven Ferromagnetic Resonance (ST-FMR)
Resonant resistance oscillations generate a DC voltage component by mixing
S
VmixDCcircuitry
Main source of signal at low bias:
Related work: Tulapurkar et al., Nature 438, 339 (2005)€
V (t) = I(t)R(t) ~ IRF cos(ωt)ΔRcos(ωt + δ)
=12IRFΔR cos(δ) + cos(2ωt + δ)[ ]
Vmix ~ 12IRFΔRcos(δ) ∝ d(Torque)
dV
Courtesy of D.C. Ralph23
Slonczewski torque: symmetric Lorentzian
A. A. Tulapurkar, et al., Nature 438, 339 (2005).J. N. Kupferschmidt et al., PRB 74, 134416 (2006).A. A. Kovalev et al., PRB 75, 104403 (2007).
Vmix Vmix
If both Slonczewski and out-of-plane spin-torque components are present then the FMR response is a simple sum of two contributions.
Out-of-Plane torque: antisymmetric Lorentzian
Out-of-Plane Torque
Slonczewski Torque
Mfree
Mfixed
Accurate measurement of SHE angle
€
Vmix ∝A
1+[( f − f0) /Δ0]2
€
Vmix ∝B[( f − f0) /Δ0]1+[( f − f0) /Δ0]
2
Courtesy of D.C. Ralph 24
FMR Peak Shape Analysis
Spin torque FMR measurement of the SHE
Spin current in plane torque τST symmetric peak
Oersted field perpendicular torque τH antisymmetric peak
The two driving forces induce oscillations with 90° phase differenceDC readout of the FMR signal using the anisotropic magnetoresistance of Py
Courtesy of D.C. Ralph
25
Pt(1.5-15 nm)/Py(2-15 nm), room temperature
+=
Luqiao Liu et al., PRL 106, 036601 (2011)
Spin torque FMR measurement of the SHE
Results: θSH = 0.068 ± 0.005 for Pt
This is big!control tests
Courtesy of D.C. Ralph26
Why is JS/JC ~ 0.06 is Big?
The spin Hall angle is a relationship between current densities.
To calculate the efficiency of total spin current generation, must take into account a difference in areas
A = Lw
t
a = tw
can be >> 1 even with θSH ~ 0.06
With the spin Hall effect, the traversal of one electron through the sample can transfer more than angular momentum to a magnet!
€
θSH =JS /( /2)J /e
Courtesy of D.C. Ralph
27
spin Hall effect in Ta
•ab initio calculation: θSH(Ta) has opposite sign compared to θSH(Pt)
•for highly resistive case, θSH(Ta) can be very large
Tanaka, T. et al, Phys. Rev. B 77, 165117 (2008)
INTRINSIC spin Hall conductivity calculated for 4d, 5d elements
28
ST-FMR induced by the SHE in Ta
CoFeB/Ta CoFeB/Pt
• antisymmetric peak, same sign (Oersted field)
• symmetric peaks, opposite sign (spin torque)
JS/JC = 0.15 ± 0.04! Narrower linewidth – less added damping from Ta compared to Pt
Courtesy of D.C. Ralph29
Liu et al. Science 336, 555 (2012)
SHE as a source for spin current
Jc Js
FM
FM
FM
NM
Spin Hall Device Conventional Magnetic Tunnel Junction
JS and JC travel perpendicular paths JS and JC travel the same path
What is in various metals?
Courtesy of D.C. Ralph30
Text
Using the Spin Hall Torque to Switch In-Plane-Polarized Magnetic Layers -- A 3-Terminal Device
Ta strip 1 µm wide
MTJ 100×300 nm2
DC current in Ta strip to write
Resistance measurement across the MTJ to read
Liu et al. Science 336, 555 (2012)
31
• Switch the magnetic moment using the SHE via an anti-damping mechanism• Use a magnetic tunnel junction to read out the magnetic orientation
DC current induced switching
•Ramp-rate measurement of critical currents:
Ic0 = 2.0 mA E0 ~ 46 kBT
JS/JC ≈ 0.12 ± 0.03 agrees with ST-FMR and perpendicular switchingmeasurements
No barrier degradation, better read-out signal compared to conventional devices.Switching currents well below 100 µA should be possible.
Liu et al. Science 336, 555 (2012)May 4th 2012
Courtesy of D.C. Ralph32
Spin Hall Effect: from fundamentals to applications
33
SOC Effects
SHE
AHEAMR
CITCurrent induced
torque
SHE-1
IntrinsicExtrinsic
Optical
Electrical
Spin current detectorSpin current generator
FMRSpin Pumping
Spin Caoloritronics STT
SHE-MRAM??Jungwirth, Wunderlich, Olejnik, Nat. Mat. 11,382 (2012)