physics in quasi-2d materials for spintronics...
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IBM Research
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
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
Spin-Polarized Tunneling in Bi2Se3: Zero Bias
7
Tunneling configuration
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
Spin-Polarized Tunneling in Bi2Se3: Zero Bias
10
Tunneling configuration
θSH ~ 0.8 assuming λsf ~ 1nm
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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:
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
Spin-Polarized Tunneling in Bi2Se3: Finite Bias
12
Energy dependence
TI
VDC
Liu et al., Phys. Rev. B 91, 235437 (2015)
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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)
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
Graphene/EuS: Quantum Hall regime
22
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
.)
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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
IBM Research
Ching-Tzu Chen May 13, 2016 2016 C-SPIN TSD Workshop © 2016 IBM Corporation
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