Physics in Quasi-2D Materials for Spintronics Applicationscspin.umn.edu/events/tsd2016/presentations/ChingTzu_Chen_TSD_F… · Physics in Quasi-2D Materials for Spintronics Applications

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May 13, 2016 2016 C-SPIN Topological Spintronics Device Workshop © 2016 IBM Corporation

Physics in Quasi-2D Materials for

Spintronics Applications

Topological Insulators and Graphene

Ching-Tzu Chen

IBM TJ Watson Research Center

IBM Research

Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation

Spintronics

Example: Spin-transfer torque MRAM (magnetoresistive

random access memory)

• Magnetic tunnel junction

• Write – spin-transfer torque

• Read – tunneling magnetoresistance

Electrical

Properties

(e.g., I, V, Q)

Magnetic

Properties

(e.g., M, spin)

2

Building blocks: Spin generation, modulation/control,

detection, transport/conduction, amplification, etc.

IBM Research


Spintronics in Quasi-2D Materials

[1] L. Liu, et al., Phys. Rev. B 91, 235437 (2015).

[2] L. Liu, C.-T. Chen and J. Z. Sun, Nature Phys. 10, 561 (2014).

Luqiao Liu (IBM), Anthony Richardella (PSU), Ion Garate

(Sherbrooke), Nitin Samarth (PSU), Yu Zhu, Jonathan Sun (IBM)

A. Spin-orbit coupling for spin generation

B. Exchange coupling for spin modulation

• Charge-spin conversion in topological insulators and

spin-Hall metals

• Strong interfacial exchange field in graphene/magnetic-

insulator heterostructures

Peng Wei (MIT), Sunwoo Lee (Columbia), Florian Lemaitre, Lucas Pinel,

Davide Cutaia, Yu Zhu (IBM), Wujoon Cha, Jim Hone (Columbia), Don

Heiman (Northeastern), Ferhat Katmis, Jagadeesh Moodera (MIT)

[3] P. Wei, et al., Nature Mat. (2016) , doi:10.1038/nmat4603

IBM Research


Spin-Orbit Coupling for Spin Generation

4

• Boost spin current generation efficiency:

− Isolate spin generation from charge current, bypassing MTJ breakdown limit

− Magnetic moment manipulation for in-plane moment,

assume 𝜃𝑆𝐻 ~ 50%, 𝐼𝑐,𝑀𝑇𝐽−𝑆𝑇𝑇 𝐼𝑐,𝑆𝐻𝐸 ~ 𝑙 𝑡 ~5 (junction size/SH metal thickness)

− Not yet obvious how much benefit for perpendicular moment

Hoffmann IEEE Trans Mag (2013)

SH Metal

FM

IBM Research


Charge-Spin Conversion in TI: Spin-Polarized Tunneling

5

• Topological surface states spin-

momentum locking:

* Quantify charge/spin conversion

electrically

* Energy dependence

* Temperature dependence

* Verify: symmetry

* Verify: surface state vs. bulk state

• Method: 4-terminal spin-polarized

tunneling technique

* Tunneling (Inverse Edelstein effect)

* Potentiometry (Edelstein effect)

* Allow self-consistency check

(Onsager reciprocity relationship)

* Eliminate current shunting

* Isolate TI from FM (CoFeB) influence

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Charge-Spin Conversion in TI: Other Methods

• Potentiometry measurements:* Li, Jonker, et al., Nature Nano (2013)

* Tang, KL Wang et al., Nano Lett (2014)

* JS Lee, Samarth et al., PRB (2015)

* Tian, YP Chen et al., Sci Rep (2015)

• Spin-torque FMR:* Mellnik, Ralph et al., Nature (2014)

* Y. Wang, H. Yang et al., PRL (2015)

• Spin pumping:* Shiomi et al., Saitoh et al., PRL (2014)

* Jamali, JP Wang et al., Nano Lett (2015)

• Spin-torque switching:* Fan, KL Wang et al., Nature Mat (2014),

* Fan, KL Wang et al., Nature Nano (2016)

6

Nature 511, 449 (2014)

Ralph group

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Spin-Polarized Tunneling in Bi2Se3: Zero Bias

7

Tunneling configuration

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Potentiometry Measurement in Bi2Se3: Zero Bias

8

Potentiometry configuration (Edelstein effect)

Onsager relation

θSH ~ 0.8 assuming λsf ~ 1nm

IBM Research

Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation9

Spin-Polarized Tunneling Data: Pt & Ta

𝜃𝑆𝐻 Pt = 0.04 − 0.09

𝜃𝑆𝐻 Ta = 0.05 − 0.11

Liu, Chen, & Sun, Nature Phys. 10, 561 (2014)

5d85d3

IBM Research


Spin-Polarized Tunneling in Bi2Se3: Zero Bias

10

Tunneling configuration

θSH ~ 0.8 assuming λsf ~ 1nm

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Surface State vs. Bulk State Contribution in Bi2Se3

11

Bulk SHE: realistic bandstructure (credit: Flatte, Sahin)

Fermi level

𝜎𝐵𝑖2𝑆𝑒3(𝑆𝐻𝐶)~(0.40 − 1.37) × 103(Ω ∙ 𝑐𝑚)−1 assuming 𝜆𝑠𝑓 = 1 − 10 𝑛𝑚

1-2 orders of magnitude larger than theoretical bulk SHE value

Experiment:

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Spin-Polarized Tunneling in Bi2Se3: Finite Bias

12

Energy dependence

TI

VDC

Liu et al., Phys. Rev. B 91, 235437 (2015)

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Optimizing Charge-Spin Conversion via Surface State

13

(Bi0.5Sb0.5)2Te3

𝑑𝑉

𝑑𝐼≈ 𝜂𝑃𝑇𝐼𝑃𝐽𝑅⧠

𝑙

𝑤

Bi2Se3

𝑅𝐻 (Bi2Se3) ~ 60 𝑚Ω vs. 𝑅𝐻 ((Bi0.5Sb0.5)2Te3) ~ 9 Ω

𝜂 (Bi2Se3) ~ 0.01 − 0.1 vs. 𝜂 ((Bi0.5Sb0.5)2Te3) ~ (0.6 ± 0.2)

𝑑𝑉

𝑑𝐼≈

𝜃𝑆𝐻𝑃𝐽𝜌

𝑤∙

𝜆𝑠𝑓

𝑡∙ tanh(𝑡/2𝜆𝑠𝑓)

𝜃𝑆𝐻 (Bi2Se3) ~ 0.8 vs. 𝜃𝑆𝐻 ((Bi0.5Sb0.5)2Te3) ~ 20 ± 5 !

Clearly surface-state spin-

momentum locking effect

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Summary A

Spin-polarized tunneling study on Bi2Se3 and (Bi0.5Sb0.5)2Te3

* Record-high charge-spin conversion observed in TI

* Surface-state origin: spin-momentum locking

* energy dependence information

* RT promising

14

Pt Ta Bi2Se3 (Bi,Sb)2Te3

Liu et al., Phys. Rev. B 91, 235437 (2015)

Liu, Chen, & Sun, Nat. Phys. 10, 561 (2014)

IBM Research


Spin-orbit-torque MRAM and spin logic using TI?

Potential Applications

Q: Is technologically relevant PMA/TI

practical for MRAM?

1. STT in PMA-MTJ:

overcome damping torque

2. SOT in SH-PMA bilayer:

overcome anisotropy torque large!

effBmm

cos

sincossin)(

anieff

anieff

BB

dBdB

Hoffmann IEEE Trans Mag (2013)

SH Metal

FM

αCoFeB ~ 0.4%

15

IBM Research


Graphene Spintronics & Exchange Field

• Spin transport: small spin-orbit coupling, long spin relaxation

length (≥ m)

• Spin generation: spin injection and Zeeman spin-Hall effect

• 2D: classical and quantum effects (e.g. QHE, QSHE, QAHE)

• 2D: spin control by Rashba or Exchange Field (10 – 100 Tesla)

16

Vs Vd

graphene

Vg

Vbg

dielectric

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Graphene/Magnetic-Insulator: Exchange Field

Graphene/EuS as model system: in-situ deposition

• Much better controlled stoichiometry (direct evaporation of target

materials)

• EuS wide band-gap insulator (1.65 eV), no current shunting

• Large exchange splitting in bulk conduction band, ~0.36 eV

(c.f. Busch, Junod, and Wachter, Phys. Lett. 12, 11 (1964))

• Large magnetic moment per Eu ion, 𝑆𝑧 ~ 7𝜇𝐵

• Expect large exchange splitting, Δ ∝ 𝐽 𝑆𝑧

• EuS demonstrated to spin-polarize quasiparticles in Al and Bi2Se3

17

P. Wei, et al., Nature Materials (2016) doi:10.1038/nmat4603

IBM Research


CVD Graphene/EuS Heterostructure: Raman, XRD, TEM

18

• X-ray diffraction (XRD):

high quality single phase growth of EuS

• Transmission electron microscopy:

clean interface

• Raman spectra:

graphene structure intact

• Hall-bar devices: fabrication simple

• EuS deposition at final step,

capped with AlOx:

EuS properties preserved

IBM Research


Electrical Detection of Interfacial Exchange Field:

Abanin et al. Science (2011)

Abanin et al. PRL (2011)

19

Applied field (µ0H) + exchange field (𝐵𝑒𝑥𝑐) = total Zeeman field (𝐵𝑍)

spin splitting (𝐸𝑍)

spin-polarized electron vs. hole-like carriers near Dirac point

µ0H (┴) couples to orbital motion:

transverse spin current (Zeeman spin Hall effect)

inverse spin-Hall-like effect nonlocal voltage/resistance (𝑅𝑛𝑙,𝐷 ≡ 𝑉𝑛𝑙,𝐷/𝐼)

𝑅𝑛𝑙,𝐷 ∝1

𝜌𝑥𝑥 𝐸𝑍

𝜕𝜌𝑥𝑦

𝜕𝜇

2

𝜇𝐷

Zeeman spin-Hall effect & nonlocal transport

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Zeeman Spin-Hall Effect: EuS induced enhancement

20

Magnetic origin of 𝑹𝒏𝒍,𝑫

• Enhancement of Rnl signal by EuS deposition

• Reduction of Rnl signal upon EuS oxidation

• Rnl correlates with M(T)

Tc5 10 15 20 25 30

0.6

0.7

0.8

0.9

1.0

T (K)

Rnl,D/R

nl,D (

5 K

)

0.5

0.6

0.7

0.8

0.9

1.0M

/M

(5 K

)

5 10 15 20 25 30

0.6

0.7

0.8

0.9

1.0

T (K)

Rnl/R

nl (

5 K

)

graphene/AlOx

𝑅𝑛𝑙,𝐷 ∝1

𝜌𝑥𝑥 𝐸𝑍

𝜕𝜌𝑥𝑦

𝜕𝜇

2

𝜇𝐷

T = 4.2 K

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EuS Induced Interfacial Exchange Field

21

Quantifying the EuS induced exchange field

0 1 2 3 410

-3

10-2

10-1

100 T = 4.2 K graphene/AlO

X

graphene/EuS/AlOX

0H (T)

Rn

l,D / R

nl,D (

3.8

T)

Onset of exchange

at 𝜇0𝐻0~1Tesla

Graphene/AlOx: x10

Graphene/EuS: x103

0 1 2 3 40.0

0.5

1.0

1.5

2.0

0.0

4.3

8.6

13.0

17.3

BZ (T

)

0H (T)

EZ (

me

V)

graphene/EuS

𝑅𝑛𝑙,𝐷 = 𝑅0 + 𝛽 𝜇0𝐻 ∙ 𝐸𝑧2

When 𝜇0𝐻 = 3.5T, 𝐵𝑍 >15 T

𝐸𝑍 and 𝐵𝑍 are lower-bound estimates because:

• We assume 𝐸𝑍 contributed only by 𝜇0𝐻 at onset

•𝛽(𝜇0𝐻)

𝛽(𝜇0𝐻0)depends on mobility and is smaller in graphene/EuS (<10% correction)

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Graphene/EuS: Quantum Hall regime

22

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Quantum Effect: Landau Level (LL) Splitting

23

-6 -4 -2 0 2 4 60

2

4

6

8

10

-5.5 -3.7 -1.8 0.0 1.8 3.7 5.5

n=+1

n=0 data

Gaussian fit

Gaussian s-

Gaussian s+

Rxx (

k

)

nC (10

11 cm

-2)

n=-1

0

3

6

9

0

2

4

-0.5 0.0 0.5 1.0

0

2

4

n = 0

n = 0

Rx

x (

k

)

n = 0

I ~ 0.1A

Rn

l (k

)

I ~ 1A

I ~ 0.1A

Vg-V

D (volt)

Rn

l (k

)

-0.5

0.0

0.5

-1 0 1-0.5

0.0

0.5

0H = 3.5 T

T = 16 K

Rx

y (

e2/h

)

0H = 3.5 T

T = 4.7 K

Vg - V

D (volt)

• = 0 LL splitting in Rxx :

• New Rxy plateau at n = 0 :

• = 0 LL splitting in Rnl :

-1 0 1

0

dR

xy/d

Vg (

a.u

.)

-1 0 1

0

Vg-V

D (volt)

dR

xy/d

Vg (

a.u

.)

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Quantum Effect: Spin-Polarized Chiral Edge States

24

0 1 2 3 40.3

0.4

0.5

0.6

0.7

0H (T)

Rx

x,

D (

h/e

2)

T = 4.2K

0 2 43

6

9

Rxx,

D (

k

)

0H (T)

Graphene/AlOx

• Unique regime: Zeeman >> orbital field (µ0H)

• Chiral spin edge modes gapless modes

Graphene/AlOxGraphene/EuS

D. Abanin et al., PRL (2006)

Graphene/EuS

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Summary B: Graphene/EuS Exchange Field

Electrical detection via Zeeman SHE

* Orders of magnitude enhancement in ZSHE 𝑅𝑛𝑙

* 𝑅𝑛𝑙(𝑇) correlates with 𝑀(𝑇): magnetic origin

* Giant 𝐵𝑍 (> 15T) when EuS nearly polarized (spin control)

* Observed unusual chiral spin edge modes in low field

25

0 1 2 3 410

-3

10-2

10-1

100 T = 4.2 K graphene/AlO

X

graphene/EuS/AlOX

0H (T)

Rnl,D / R

nl,D (

3.8

T)

0 1 2 3 40.3

0.4

0.5

0.6

0.7

0H (T)

Rx

x,

D (

h/e

2)

T = 4.2K

0 2 43

6

9

Rxx,

D (

k

)

0H (T)

Graphene/AlOx

Graphene/EuS

P. Wei, et al., Nature Mat. (2016), doi:10.1038/nmat4603. arXiv: 1510.05920

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Physics in Quasi-2D Materials for Spintronics Applicationscspin.umn.edu/events/tsd2016/presentations/ChingTzu_Chen_TSD_F… · Physics in Quasi-2D Materials for Spintronics Applications