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Magnetization Switching Using Spin Orbit Torques from Sputtered Conductive WTex
Xiang (Shaun) Li,
Mahendra DC, Chong Bi, and S. X. WangMaterials Science and Engineering, Electrical Engineering
Peng Li, Yuri SuzukiApplied Physics
Stanford University
2019 E3S Annual RetreatUniversity of California, Berkeley, September 19-20, 2019
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9/19/2019 Page 2
0.1
1
10
100
1000
10000
Read time (ns)
Write time (ns)
Cell size (F2)Read energy (fJ)
Write energy (fJ)
SRAMeDRAMDRAMSTT-MRAMFeFETReRAMPCM
Xiang Li, Kang L. Wang, et al., MRS Bulletin, vol. 43, pp. 970-977, 2018.
S. Salahuddin, K. Ni, and S. Datta, Nature Electronics, vol. 1, pp. 442-450, 2018.
Standby Power
Limited Endurance
Comparison of Existing and Emerging Memories
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SOT-MRAM Towards SRAM Performance
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Bit switching energy estimates:
~250 fJ @ 110 nm
Scaling down possibility:
~10 fJ @ 22 nm ~0.5 fJ @ 5 nm
Sato, Wang, et al., Nature Electronics, 1(9), 508, 2018
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Leverage New Physics for More Efficient Write
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http://www.cpfs.mpg.de/2520563/Dirac-semimetal_-Transport-by-massless-fermionsJ. Phys. Soc. Jpn. 87, 041002 (2018)https://www.eecs.mit.edu/node/6509
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Topological Insulator and Weyl Semimetals
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https://www.eecs.mit.edu/node/6509
K. Kuroda, et al., Physical Review B, vol. 94, Nov 18 2016.
• High resistivity of topological insulators lowers overall on/off ratio (over 4000 𝜇Ω cm for 5 nm films)
• Weyl semimetals are promising candidates, for example WTe2
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Sputtered Weyl Semimetal WTex
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100 150 200 250
0
100
200
300
400
500
600 A10
1 A4
276.9 nm
76.9 nm
36.4 nm
36.4 nm
27.6 nm
A5
2A
6
1
A9
1 A2
1
Inte
nsity (
arb
itra
ry u
nit)
Raman shift (cm-1)
A5
1
27.6 nm
Y. C. Jiang, J. Gao, and L. Wang, Scientific
Reports, vol. 6, p. 19624, 01/22/online 2016.
• Sputter deposition desired for industry adoption• Signature Raman peaks of sputtered WTe2
Xiang Li, Shan Wang, et al., in preparation
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High Resolution TEM Analysis
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• TEM shows W:Te ratio of 1:1.34 with visible WTex grains/clusters
Xiang Li, Shan Wang, et al., in preparation
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Low Frequency Current Induced Resistance Change
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• Charge-to-spin conversion efficiency characterized using harmonic current induced magnetization oscillation
• HDL is the damping-like effective field that drives switching
CoFeB
HDL
Mo
damping-like effective field
H
V
𝑰𝒔𝒊𝒏(𝝎𝒕)𝝓
x
y Raw data
Fitted data
-6000 -3000 0 3000 6000-15
-10
-5
0
5
10
15
= 45o
R x
y
2 (
m
)H (Oe)
I = 0.5 mA
Q Shao, Kang Wang et al., 2018 IEDM
𝑅2𝜔 = 𝑅𝑃𝐻𝐹𝐿𝐻𝑒𝑥𝑡
𝑐𝑜𝑠2𝜙𝑐𝑜𝑠𝜙 +𝑅𝐴2
𝐻𝐷𝐿𝐻𝑒𝑥𝑡 − 𝐻𝐾
+ 𝑅𝑡ℎ 𝑐𝑜𝑠𝜙
Xiang Li, Shan Wang, et al., in preparation
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Charge-to-Spin Conversion Efficiency
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• Mo between WTe2 and CoFeB partially absorb spin-polarized electrons• Real 𝜉𝑆𝑇 value of WTe2 should be larger 𝜉𝑆𝑇 =
2𝑒𝑀𝑆𝑡𝐶𝑜𝐹𝑒𝐵ℏ
𝐻𝐷𝐿𝐽𝑊𝑇𝑒2Xiang Li, Shan Wang, et al., in preparation
0.0 0.5 1.0 1.5 2.00
2
4
6
8
10
WTeX(5)/Mo(1)/CoFeB(1)
Mo(6)/CoFeB(1)
Linear Fit
Linear FitHD
L (
Oe)
I (mA)
ST= 0.50 0.03
WTeX
ST = 0.039 0.002Mo
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Unidirectional Spin Hall Magnetoresistance (USMR)
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• Spin currents interaction with magnetization affects interface scattering, thus changes longitudinal resistance
-15 -10 -5 0 5 10 15
-10
-5
0
5
-5000 0 5000-4
-2
I = 0.7 mA
R x
x
2
(m
)
Hy (Oe)
C. O. Avci et al., Nature Physics, vol. 11, p. 570, 2015.
Xiang Li, Shan Wang, et al., in preparation
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Current-Induced Field-Free Switching
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• Switching current density in WTex 0.77-1.41 MA/cm2
-15 -10 -5 0 5 10 15
-10
-5
0
5
-5000 0 5000-4
-2
I = 0.7 mA
R x
x
2
(m
)
Hy (Oe)
-8 -4 0 4 8
-10
-5
0
5
Ipulse
(mA)
R x
x
2
(m
)
- My initialization
My initialization
H = 0
tpulse
= 1 ms
Xiang Li, Shan Wang, et al., in preparation
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Resistivity (𝜇Ω ⋅ 𝑐𝑚)
𝜉 𝑆𝑇
0.1
1
10
100
100 1000 10000
Benchmark: Power Efficiency of SOT-MTJ Cell
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Physical Review Applied, vol. 10, Sep 6 2018.
Assumes 6 nm SOT write line under 1 nm in-plane CoFeB.
Resistivity (𝜇Ω 𝑐𝑚)
Pow
er
1
10
100
100 1000 10000
435.2
300
713
12000
Sputtered WTex
Sputtered WExfoliated WTe2*Sputtered BiSex**
Xiang Li, Shan Wang, et al., in preparation
*Yang et al., Nature
Nano, 2019.
**Wang et al.,
Nature Materials,
2018.
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High Frequency Current Induced Magnetic Resonance
2018 NSF Site Visit Page 139/19/2019
• Fitted resonance peak with symmetric and asymmetric Lorentzian line shapes
𝜃 =
Luqiao Liu, et al., PRL 106, 036601 (2011)
Raw data
Fitted Data
Symmetric Lorentzian
Asymmetric Lorentzian
200 300 400
0.0
1.0µ
2.0µ
3.0µ
WTeX
Vm
ix (
V)
H (Oe)
f = 5 GHz
t = 58.2 nm
ST = 0.82
Xiang Li, Shan Wang, et al., in preparation
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SOT-MTJ Integration and Test
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Conclusions
• MRAM holds great promise to replace or complement SRAM for data-centric applications as high-density on-chip memory
• Unique topological band structure gives rise to highly spin polarized electrons in Weyl semimetal WTe2
• Sputtered 5 nm WTex at room temperature shows attractive charge-to-spin conversion efficiency (0.5), low switching current density (1 MA/cm2), and low thin film resistivity (570 𝝁𝛀⋅𝒄𝒎)
• Greatly improved energy/delay performance compared with other topological materials such as BiSe or exfoliated WTe2
• Even larger charge-to-spin conversion efficiency up to 0.8
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Acknowledgements
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