spin galvanic effects and magnetization manipulation in layered … · 2020. 9. 7. · sot...
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Spin Galvanic Effects and
Magnetization Manipulation
in Layered vdW Systems
Simran Singh
Department of Physics
LIQUID Group: https://www.liquidgroupcmu.com/
2D material platform for spintronics
Graphene and Phosphorene:
Possibility for long spin coherence
(spin lifetime and spin diffusion length)
Low intrinsic spin-orbit coupling
Weak hyperfine coupling
High mobility
spin information channel
Han et. al, Nature Nano. (2015)
Possibility to optically excite and control
polarized carriers with long spin coherence
Strong spin-orbit coupling
Giant spin splitting
Spin-valley locking
Transition metal Dichalogenides (WS2, MoS2, WSe2…..):
2D opto-spintronics
Xiao et. al, PRL. (2012)
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
2D material platform for spintronics
2D ferromagnets (CrI3, Cr2Ge2Te6…..) & insulators (h-BN):
Atomically thin memory cells
Efficient spin injection; magnetization
dynamics; magnetization control
2D Ising ferromagnetism
Spin-filtered tunnel barriers
Tunable magnetism
Huang et. al, Nature (2017)
2D material heterostructures :
All-2D component
spintronic devices
Lu et. al, Nano Letters (2017)Song et. al, Science (2018)
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
❑ Interfacial effects for spin-charge interconversion
❑ Emergent spin galvanic effects at Graphene/TMD interfaces
❑ Control the interface quality and transparency
Spin-charge interconversion at Pt/Gra interface
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Chris Hammel Roland Kawakami
The Ohio State University
Generation and detection of spin imbalance
Experimental techniques at hand
Spin polarize the charge carriers of non-magnetic
material using the magnetization dynamic of a
ferromagnet.
Electrical spin injection
Drive spin polarization from a ferromagnet to non-
magnetic material by a bias across the interface
Spins via circularly polarized light
Optically orient the charge carrier spins in
semiconductors.
Magnetization dynamics-based spin pumping
Spin galvanic effect in non-magnetic materials
Spin-orbit interaction driven spin dependent
transverse scattering of chare carriers.
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Pt/Gra inverted vdW heterostructures
Graphene
SiO2 /Si
Graphene on SiO2
Substrate
van der Waal bonded Pt/graphene inverted heterostructures
Graphene on Pt nanowires
Cobalt Cobalt
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
-400 -200 0 200 400
-1.0
-0.5
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NL (
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By (Oe)
-5 -4 -3 -2 -1 0 1 2 3 4 5
0.00
0.25
0.50
RN
L (
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Bx (k Oe)
Standard non-local spin transport and Hanle precession measurements with two
ferromagnetic electrodes
Characterize the graphene channel and electrode properties
- Spin diffusion length of graphene
- Spin relaxation time
- Ferromagnetic electrode (E1) spin polarization
Spin generation at Pt/Gra interface
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Spin generation at Pt/Gra interface
Generation of
spin-polarized
current
“Pure” spin
current diffuses
into graphene
Spin current detection
using a ferromagnetic
electrode
Non-local lateral spin valve device to demonstrate charge to spin conversion
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Spin generation at Pt/Gra interface
-4 -2 0 2 4
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RREE (
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Bx (kOe)
-1
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Sin
Ic = + 10 μA
Ic = - 10 μA
Direction of the spins generated at
the interface
Magnetization direction of ferromagnetic detector
x
y
Experimental demonstration of charge to spin conversion
IC
VREE
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Gra/Pt interface
“Pure” spin
current diffuses
into graphene
Spin current generation
using a ferromagnetic
electrode
Non-local lateral spin valve device to demonstrate spin to charge conversion
Spin detection at Pt/Gra interface
Zero voltageNon-zero voltage
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Spin detection at Pt/Gra interface
IC
V
Experimental demonstration of spin to charge conversion
-4 -2 0 2 4
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=180
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=0
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-4 -2 0 2 4
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VIR
EE (
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B (kOe)
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
φ
0 20 40 60 80 100 120 140 160 180
-4
-2
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IRE
E (
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(degrees)
Spin detection at Pt/Gra interface
Experimental demonstration of spin to charge conversion and vice-versa
Detailed angualr dependance of spin to charge signals
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Rashba effect driven spin to charge conversion
- Scales with applied charge current
- Robust upto near room temperature
- Shows a weak charge carrier dependence
-40 -30 -20 -10 0 10 20 30 400
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Vg= -20 V
Vg= 0 V
Vg= +20 V
Vg= +40 V
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RE
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Temperature (K)
-45 -30 -15 0 15 30 450
500
1000
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Re
sis
tan
ce
(
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Gate Voltage (V)
Charge current dependence Gate voltage dependence Temperature dependence
Spin generation at Pt/Gra interface
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Spin-charge interconversion at Pt/Gra
1D- drift diffusion model of spin transport in Pt/Graphene/FM non-local spin valve
Tiancong Zhu
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Accounts for :
❑ Electrical spin injection (detection) with ferromagnetic electrode
❑ Spin detection (injection) with Pt electrode via ISHE (SHE)
❑ Spin diffusion in the graphene channel
Spin-charge interconversion at Pt/Gra
Some interfacial phenomena (Rashba effect) at Pt/Gra ??
Spin Hall effect can not fully explain our large spin-charge conversion signals!!!!!
TemperatureLchannel
(m)lGraphene
(m)Rgraphene
()PFM
Rint_FM
(k)Pt Dimension
(L x W x H, in nm)Rint_Pt
()rPt(sPt
-1)(10-6m)
lPt
(nm)RSCC
()Reported
Pt
Calculated Pt
This work 10K 1 1.3 160 0.157 0.42 5000 x 200 x 8 4 0.21 5* 0.3 N.A. 5.86
Savero Torres et al. 300K 4 3.4 1500** 0.16 20 1000 x 150 x 12 10** 0.46 5 0.013 0.15 ± 0.01 0.13
Yan et al.300K 0.635 1.2 2755.9 0.068 15 250 x 198 x 21 8.4 1.34 2.1 0.0112 0.234 ± 0.025 0.23
50K 0.635 1.2 2755.9 0.068 15 250 x 198 x 21 10.6 0.99 2.1 0.0059 0.178 ± 0.02 0.17
Yan et. al (2017) Torres et. al (2017)
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Rashba effect in 2D systems
Two-dimensional electron gas (2DEG) with spin-orbit coupling
• Asymmetric 2DEG – crystal asymmetry or broken inversion symmetry at interface.
• Spin-orbit term in effective 2D Hamiltonian – lifts the spin degeneracy of the bands.
Structural asymmetry Intrinsic electric field Effective magnetic field
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Spin textures in Rashba system
Spin helicity (spin-momentum locking) of the charge carriers
Defines the direction of spin quantization
Spin-momentum locking
ky
kx
Fermi surface
in-plane spin polarization-oriented transverse to the momentum of the electron
Rashba term in
Hamiltonian(±kx , 0)
Pick a k pointSpins along ±ky
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Spin current via Rashba Edelstein effect
Electric current induced spin-polarization in Rashba system
ky
kx
At zero bias electric field: Detailed balance with equal number of spin up and spin down
carriers (zero net spin polarization)
Zero biasA non-zero bias
With a non-zero bias electric field: A charge current displaces the Fermi sea resulting in an
imbalance of spin up and spin down charge carriers (a non-zero net spin polarization).
Charge to spin
conversion
Δk
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Inverse - Rashba Edelstein effect
ky
kx
Js (σy)
A spin-polarization induced electric field in Rashba system
Injection of spin current at Rashba interface: Creates an accumulation of spin up and
depletion of spin down carriers resulting in shifting of the two inequivalent Fermi contours (a
non-zero electric field)
Spin to charge
conversion
Δk
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Rashba effect in graphene
Angle-resolved photoemission of graphene
Brillouin zone
Rashba spin-orbit coupling at graphene/transition metal heterostructures
Spin- and angle-resolved photoemission spectra of
the graphene π-states
Graphene/Platinum heterostructuresGraphene/Gold heterostructures
Marchenko et. al (2012) Shikin et. al (2014)
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Rashba effect in graphene
E
k
Spin-degenerate Dirac dispersion of graphene
E
k
Spin splitting of graphene bands due to Rashba spin-
orbit coupling
ky
kx
Fermi contours depicting spin-momentum locking
• Hybridization of π orbitals ofgraphene.
• An effective electric field normalto the interface.
• Translates into an effectivemagnetic field in carrier rest offrame.
Ez
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Summary
❖ Large spin-charge interconversion observed in inverted Pt/graphene vdWsystem.
❖ Spin Hall effect in Pt can not fully explain our large spin-chargeconversion signals.
❖ Points to Rashba spin-splitting at Pt/graphene interfaces.
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
SOT switching in vdW systems
Heavy Metal (SOC)
Magnetic system
Alghamdi et. al. (2019)
3D-heavy system / vdW magnetic system
Wang et. al. (2019)
Shi et. al. (2019)
vdW heavy System / 3D magnetic system vdW heavy System / vdW magnetic system
?
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
SOT switching in WTe2/FGT systems
Shi et. al. (2019)
Wang et. al. (2018)
MacNeill et. al. (2016)
Source of spin current
for SOT: WTe2
vdW magnet: FGT
Deng et. al. (2018)
5 µm 5 µm
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
SOT switching in WTe2/FGT systems
Temperature dependent AHE of FGT
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
SOT switching in WTe2/FGT systems
Temperature dependent AHE of FGT
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
SOT switching in WTe2/FGT systems
SOT switching of FGT @ 190 K
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
SOT switching in WTe2/FGT systems
Threshold charge current density for SOT switching
~5 × 1010𝐴
𝑚2 in FGT(12.6 nm)/Pt at 120 K
Wang et al., Sci. Adv. 5, eaaw 8904 (2019)
~2 × 1011𝐴
𝑚2 in FGT(15 nm)/Pt at 180 K
Alghamdi et al., Nano Lett. 19, 4400 (2019)
Summary
❖ Realization of magnetization switching of vdW magnet using SOToriginating from a vdW material.
❖ The switching efficiency looks better (or at least same) as compared withconventional heavy metal.
❖ Towards all-2D component memory devices.
Simran Singh 2D van der Waals Spin Systems Workshop, 2020
I-Hsuan Kao Ryan Muzzio Jyoti Katoch
Collaborators
-Josh Goldberger (The Ohio State University)
-Jiaqiang Yan (Oak Ridge National Laboratory )
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Spin precession of SH injected spins in graphene
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