Spin Galvanic Effects and Magnetization Manipulation in Layered … · 2020. 9. 7. · SOT...

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)


❑ 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


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.


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


-400 -200 0 200 400

-1.0

-0.5

0.0

0.5R

NL (

)

By (Oe)

-5 -4 -3 -2 -1 0 1 2 3 4 5

0.00

0.25

0.50

RN

L (

)

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



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



-4 -2 0 2 4

-200

-100

0

100

200

RREE (

m

)

Bx (kOe)

-1

0

1

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


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



IC

V

Experimental demonstration of spin to charge conversion

-4 -2 0 2 4

-2

0

2

=180

VIR

EE (

V

)

B (kOe)

=0

VIREE

-4 -2 0 2 4

-2

-1

0

1

2 =90

VIR

EE (

V

)

B (kOe)


φ

0 20 40 60 80 100 120 140 160 180

-4

-2

0

2

4

V

IRE

E (

V

)

(degrees)


Experimental demonstration of spin to charge conversion and vice-versa

Detailed angualr dependance of spin to charge signals


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

1

2

3

4

5

6

T = 200 K ; IC = 10 A

V

RE

E (

V

)

VG (Volts)

0 100 200 300 400 5000

100

200

300

400

V

RE

E (

V

)

Ic (A)

-4 -2 0 2 4

-150

-100

-50

0

50

100

150

VR

EE (

V

)

Bx (kOe)

VREE

0 50 100 150 200 2500

1

2

3

4 Vg= -40 V

Vg= -20 V

Vg= 0 V

Vg= +20 V

Vg= +40 V

V

RE

E (

V

)

Temperature (K)

-45 -30 -15 0 15 30 450

500

1000

1500

Re

sis

tan

ce

(

)

Gate Voltage (V)

Charge current dependence Gate voltage dependence Temperature dependence



Spin-charge interconversion at Pt/Gra

1D- drift diffusion model of spin transport in Pt/Graphene/FM non-local spin valve

Tiancong Zhu


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)


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


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


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


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


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)


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


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.



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

?


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



Temperature dependent AHE of FGT



SOT switching of FGT @ 190 K



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.


I-Hsuan Kao Ryan Muzzio Jyoti Katoch

Collaborators

-Josh Goldberger (The Ohio State University)

-Jiaqiang Yan (Oak Ridge National Laboratory )

IC

B

y B

xBz

V

-1.0 -0.5 0.0 0.5 1.0

-200

0

200M

y

M-y

RIS

HE (

m

)

Bz (kOe)

ISHE

30

-2 -1 0 1 2

-200

0

200

400

600 M

y

M-y

RS

H (

m

)

Bz (kOe)

IC

Pt Inj.

B

y B

xBz

V

Spin precession of SH injected spins in graphene

31

Spin Galvanic Effects and Magnetization Manipulation in Layered … · 2020. 9. 7. · SOT...

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