ULTRAFAST HIGH-DENSITY SOT-MRAM:ENERGY-EFFICIENT COMPUTING
AND SOT SWITCHING USING TOPOLOGICAL EFFECTS
X. Li, C. Bi, and S. X. Wang
Center for Magnetic NanotechnologyDept. of Materials Science and Eng.
Dept. of Electrical EngineeringStanford University
Sept. 20, 2018
Memories for Energy Efficient Computing
2
• No existing technology combines high write efficiency, speed and density• STT-RAM in various stages of production; SOT-RAM n MeRAM in R&D
Xiang Li PhD, 2018
nsSOT-RAM
Magnetoresistive random access memory (MRAM): 2 vs 3 terminals
3
Read/Write line
MTJRL
FL
Tunnel barrier
Read line
RL
FL
Write lineSOT layer
STT SOT
• Tunnel magnetoresistance (TMR) ratio:
100×−
=P
PAP
RRRTMR
P: Parallel state, AP: Anti-parallel state
Spin Transfer Torque (STT) MRAM
4
Electron flow
𝑚𝑚r
𝑚𝑚f
𝐼𝐼c0 =2𝑒𝑒ℏ
1𝜂𝜂𝛼𝛼𝑀𝑀𝑠𝑠𝑉𝑉𝐻𝐻keff
perp.
Gilbert damping Energy barrier
Critical current:
𝜂𝜂: spin-transfer efficiency, 𝑀𝑀𝑠𝑠: Saturation magnetization, V: Volume of free layer[1] B. Dieny, Introduction to Magnetic Random-Access Memory. Wiley-IEEE Press, 2017.
Trade-off between 𝛼𝛼 and 𝐻𝐻keffperp.[1]:
Co/Pt, Co/Pd NiFe, FeB𝐻𝐻keffperp. Strong Weak
𝛼𝛼 High Low
• Challenging to achieve low critical current and high retention
ref.
free
Spin-Hall Effect & SOT-MRAM
HM (Heavy metal)FM (Ferromagnetic metal)
Electron flow
z
xy𝑚𝑚f
𝜏𝜏SH ∝ 𝐽𝐽in𝑚𝑚f × 𝑚𝑚f × �𝜎𝜎 �𝜎𝜎: direction of spin (∥ �𝑦𝑦)
𝐼𝐼c0 =2𝑒𝑒ℏ
1𝜃𝜃SH
𝑀𝑀𝑠𝑠𝑉𝑉𝐻𝐻keffperp.
2−𝐻𝐻x
2
Critical current:
• Strong 𝐻𝐻keffperp. material can be selected without increasing 𝐼𝐼c0
Spin-Hall angle
5
𝐻𝐻𝑥𝑥: In-plane external field, 𝑀𝑀𝑠𝑠: Saturation magnetization, V: Volume of free layer
Three-Terminal SOT-MRAM
Q. Hao, Phys. Rev. Appl., vol. 3, pp.034009, March 2015
Conventional structure
Memory Cell area F2
DRAM 6
STT-MRAM 6
3-T Spin-Hall MRAM 12 - 18
• Memory cell area too large for 3-terminal devices6
Two-Terminal SOT-MRAM
7
𝐽𝐽in max. =𝜋𝜋𝑑𝑑MTJ2
4𝑤𝑤Ta(𝑡𝑡Ta+𝑡𝑡CoFeB)𝐽𝐽out
𝐽𝐽in 𝐽𝐽out
𝑤𝑤Ta
𝑡𝑡Ta𝑡𝑡CoFeB
Spin-Hall effect ∝ In-plane current density 𝑑𝑑MTJ
• Key to realizing high 𝐽𝐽in is the alignment of MTJ on Ta wire• No trade-off between critical current and retention• Cell area F2 = 6
IBM patent app US20140264511A1 (2014)
Two-Terminal SOT Device Images
8• 110 nm MTJ pillar on 220 nm Ta wire 𝐽𝐽in~14𝐽𝐽out
Bottom electrode lead 1
Top electrode lead 1
Bottom electrode lead 2
Top electrode lead 2
Al2O3 Passivation layer
Full-Stack Structure of MTJ
MgO(1.2)
CoFeB(1.3)
Ru (0.85)
Ta(0.4) Co (0.4)
Ta (3.8)
Pd (0.6)
Pd (0.6)Co (0.3)Ru (1.5)
x3
CoFeB(1.0)
Sub.
Co (0.4)
Co (0.4)
9
unit: nm
Full-Stack Structure of MTJ
MgO(1.2)
CoFeB(1.3)
Ru (0.85)
Ta(0.4) Co (0.4)
Ta (3.8)
Pd (0.6)
Pd (0.6)Co (0.3)Ru (1.5)
x3
CoFeB(1.0)
Sub.
Co (0.4)
Co (0.4)
Sub.
orMgO
Ru
Ru
MgO
Source of spin-Hall effectFree layerTunnel barrier
Pinned layer
Capping layer
Reference layer
• Reduces stray field on free layer Increases stability
Antiferromagnetic coupling spacer
10
unit: nm
0 1 2 3 4 50
200
400
600
800
1/wTa [1/um]
Crit
iacl
cur
rent
[uA]
Underlayer Width Dependence of Critical Current
11
o MTJ diameter: 110 ± 5 nmo Pulse current duration: 10 mso External field: 100 Oeo Reference layer magnetization:
STT assists spin-Hall
• Critical current is x3 smaller than pure STT device under external field
Spin-Hall torque ∝ 𝐽𝐽in ∝I
𝑤𝑤Ta
Purely STT
Nature Electronics, 2018
Critical current density vs pulse width
12
Bit switching energy estimates:
~250 fJ @ 110 nm
Scaling down possibility:
~10 fJ @ 22 nm ~0.5 fJ @ 5 nm
Nature Electronics, 2018
Energy-Efficient Computing Example
13
Nature Electronics 1 (7), 398 (2018).
Topological Insulators and SOT Switching
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NATURE PHYSICS, 5 (6), 438, 2009
NATURE, 511(7), 449, 2014
Topological Insulators and SOT Switching
15Nature Materials, 17(9), 800–807, 2018
Topological Insulators and SOT Switching
16
Materials Resistivity ReferencesTopologicalinsulator SmB6 300 µΩ·cm
J. Yong et al., Appl. Phys. Lett. 105,222403 (2014)
Topologicalsemimetal TaAs 200 µΩ·cm
C. Zhang et al., Phys. Rev. B 95,085202 (2017)
Topologicalinsulators BiSex 1792 µΩ·cm
A. R. Mellnik et al., Nature 511, 449(2014)
BiTex 2000 µΩ·cm D. Qu et al., Science 329, 821 (2010)Heavymetals W 260 µΩ·cm
C. F Pai et al., Appl. Phys. Lett. 101,122404 (2012)
Ta 190 µΩ·cm L. Liu et al. Science 336, 555 (2012)
Resistivity of representative SOT materials at room temp.
Conclusions
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• Fabricated and demonstrated the first two-terminal spin-orbit torque (SOT) MRAM cells. SOT devices can switch much faster (~1 ns) than pure STT devices.
• Critical switching current of SOT device is x3 smaller than pure STT device of the same geometry under external field
• This device is free from the trade-off between 𝛼𝛼 and 𝐻𝐻keffperp. (trade-
off between critical current and retention)
• Topological materials promise to deliver even more efficient switching.
• SOT-MRAM is well suited for energy-efficient ultrafast computing.
Acknowledgements
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Conclusion
Microfluidics allow reaction limited flow regime for protein analyte, and enables larger particles that improve assay performance by an order of magnitude.
The design and operation of magneto-nanosensors are intricate: sensor-particle interaction, and location of MNP must be carefully considered.
Magneto-nanosensors enable a number of killer applications, including ultrasensitive protein interaction assays, cancer early detection, and mobile health.
