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Luqiao Liu 12/05/17 p1
Magnetic Switching with Compensated Ferrimagnet
and Topological Insulator
J. Finley1, J.H. Han1, S. Sidiquii1, A. Richadella2, N. Samarth2, Luqiao Liu1
1. Department of Electrical Engineering and Computer Science, MIT
2. Department of Physics, Penn State University
Dec 5th, 2017
Luqiao Liu 12/05/17 p2
charge based devices
$10 million/year
𝑃~𝑓𝐶𝑉2
Spintronics for beyond CMOS applications
Luqiao Liu 12/05/17 p3
spin based device
𝐸~𝑁𝜇𝐵𝐵
charge based devices
𝑃~𝑓𝐶𝑉2
Spintronics for beyond CMOS applications
Luqiao Liu 12/05/17 p4
How does spin based device perform currently?
A. Power consumption for logic device
transistor switching vs magnetic switching
0.01
0.1
1
10
100
1000
101001000
sw
itch
ing
en
erg
y (
fJ/b
it)
Technology node (nm)
Pentium
Pentium Pro
PowerPC 750
Pentium III
Atholon
Pentium 4VIA C7
Pentium
Pentium Pro
PowerPC 750
Pentium III
Atholon
Pentium 4VIA C7
Athlon FX-57
PS3 Cell BEPA6T-1682M
Athlon FX-69
QX6700i7 920
atom
i7 875K
i7 2600K
FX 8150
Magnetic switching
1994 2000 2006 2012
Victor Zhirnov, et al, INTECHOPEN.com
Luqiao Liu 12/05/17 p5
How does spin based device perform currently?
B. Speed performance of magnetic device
D. Nikonov and I. Young, Proc. IEEE, 101, 2498 (2013)
Candidate spin devices for logic
operation
Preferred corner
Luqiao Liu 12/05/17 p6
Fundamental Limit on energy consumption
Minimum energy for thermal stability
(10 years’ retention)
~ 60 kBT (10-19 J << pJ)E
𝜇𝐵𝐵 𝑁𝜇𝐵𝐵(10-23 J)
N ~ 104 electrons
(assuming a B field ~ 1T)
Luqiao Liu 12/05/17 p7
Fundamental limit on operating speed
Minimum operating speed for magnetic device
Red and green lines: traces of
magnetic moment during switching
Ferromagnetic resonance frequency: 𝑓 ≈ 𝛾
2𝜋𝐻𝑎𝑛
Han: Anisotropy field ~0.1 Tesla
f ~ GHz
Luqiao Liu 12/05/17 p8
Content
Magnetic reading and writing for zero moment magnet
• from ferromagnet to compensated ferrimagnet
Magnetic switching of low Ms magnet with topological insulator
• room temperature switching of high Tc magnet with topological insulators
Luqiao Liu 12/05/17 p9
Magnetic Switching: Ferromagnet vs Anti-ferromagnet
𝑓 ~ 𝛾2𝜋𝐻𝑎 𝑓 ~ 𝛾
2𝜋𝐻𝑒𝑥𝐻𝑎
Hex: exchange field between two spin sub-lattices > 100 Tesla
10 GHz ~1THz
Luqiao Liu 12/05/17 p10
Pros and Cons for AFM Spintronics
Fast dynamics
No stray field: higher density
better security
AFM How to read and write information into AFM?
No response to external field (spin torque?)
AFM
B
No efficient reading mechanism
R
Luqiao Liu 12/05/17 p11
Recent Progresses of Magnetic Reading and Writing in AFM
Spin Orbit Torque Induced Magnetic
Switching in CuMnAs
P. Wadley et al., Science 351, 587-590, (2016).
Spin Torque Induced Exchange Bias
Change in F/AF Bilayer
Z. Wei, et al, Phys. Rev. Lett. 98, 116603 (2007)
Núñez, A. S., et al, Phys. Rev. B 73, 214426 (2006)
Luqiao Liu 12/05/17 p12
Recent Progress of Magnetic Reading and Writing in AFM
Anisotropy Tunneling Magnetoresistance
Marti, X.Nature Mater. 13, 367-374, (2014)
P. Wadley et al., Science 351, 587-590, (2016).
B. G. Park, Nature Mater. 10, 347-351, (2011).
Anisotropy Magnetoresistance/Planar Hall Effect
High R when J//M
low R when J┴M
Luqiao Liu 12/05/17 p13
Electrical Manipulation and Detection of AFM Dynamics
• Difficult to switch and probe
AFM with the same magnetic element Ferrimagnet with unequal magnetic
atoms
• Antiparallel alignment of different atoms
• Zero net moment achievable
• Possible to switch and detect (track the
orientation of only one element)
Atom 1 Atom 2
Luqiao Liu 12/05/17 p14
Reading: Fermi Sea vs Fermi Surface Properties
Magnetic moment: Fermi Sea Property
Transport Property: Fermi Surface Property
𝑀 ∝ 𝐸𝐹[𝐷↑(𝐸) − 𝐷↓(𝐸)]𝑑𝐸
D(E) density of states
𝑇𝑀𝑅 ∝2𝑃1𝑃2
1−𝑃1𝑃2𝑃1,2 =
𝐷↑(𝐸)−𝐷↓(𝐸)
𝐷↑ 𝐸 +𝐷↓(𝐸)|𝐸=𝐸𝐹
M = 0
Spin Polarization: 0
M = 0
Spin Polarization ≠ 0Finite Magnetoresistance
(Hall effect, …)
Luqiao Liu 12/05/17 p15
Writing: Effect of Spin Torque on Two Sublattices
Spins Perpendicular to Magnetic Moment Equilibrium
Effective field
AFM
JS
𝜏𝑆𝑇 ∝ 𝑚 × (𝜎 × 𝑚)
Effective Field
Effective field on two sub-lattices add constructively
This works for spin orbit torque switching on all colinear AFM
Luqiao Liu 12/05/17 p16
Chemically tunable Magnetism in Co1-xTbx Alloys
Co Tb
H (Oe)
M vs Tb concentration
Luqiao Liu 12/05/17 p17
Magnetic Orientation Detectable at Compensation
Co Tb
Anomalous Hall effect remains finite when M 0
Magnetic moment
Anomalous Hall
effect
Luqiao Liu 12/05/17 p18
Spin Hall Effect Induced Switching at Compensated Sample
Switching is observed for samples with almost zero moment!
Luqiao Liu 12/05/17 p19
Quantitative Determination of Spin Orbit Torque
J. Finley, L. Liu, Physical Review Applied, 6, 054001 (2016)
𝐻𝑒𝑓𝑓 = −ℏ𝐽𝑠
2𝑒𝜇0𝑀𝑆𝑡ෝ𝒎 × ෝ𝝈 × ෝ𝒎
0∞
Heff
Conservation of Angular moment!
1. Both SOT effective field and coercive field scales as 1/Ms:
Switching of compensated ferrimagnet as easy as FM!
2. Thermal barrier: 𝐸 = 𝐻𝐾𝑀𝑆, thermal barrier remains constant as FM compensated
ferrimagnet
1/Ms trend
Luqiao Liu 12/05/17 p20
Parallel Efforts on Similar Systems from Other Groups
S. Ham et al, Arxiv: 1703.00995 (2017) N. Roschewsky, et al APL,109, 112403 (2016)
Arxiv: 1703.00995 (2017)
K. Ueda, et al, APL,109, 232403 (2016)R. Mishra, et al PRL 118, 167201 (2017)
Spin torque efficiency diverges faster than 1/Ms !?
Non-conservation of angular momentum?
Luqiao Liu 12/05/17 p21
Ferrimagnetic Materials: Beyond Rare Earth – Transition Metal
Issues with Rare Earth – Transition Metal Alloy
Large damping: 𝛼 > 0.1
temperature dependence of compensation point
Difficult for fabrication: rare earth ultra-active with oxygen
Fast magnetic dynamics need to be demonstrated!
Luqiao Liu 12/05/17 p22
Content
Magnetic reading and writing for zero moment magnet
• from ferromagnet to compensated ferrimagnet
Magnetic switching of low Ms magnet with topological insulator
• room temperature switching of high Tc magnet with topological insulators
Luqiao Liu 12/05/17 p23
Charge to Spin Conversion using Spin Orbit Interaction
Transverse spin current Js is generated from a longitudinal current Jc.
Ԧ𝐽𝑠 ∝ Ԧ𝐽𝑐 × Ԧ𝜎
++
++
+
--
--
-
B
Jc
M. Dyakonov and V. Perel, Phys. Lett. A 35, 459460 (1971)
Spin Hall Effect
Luqiao Liu 12/05/17 p24
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
60
80
100 B
ext = -3.5 mT
dV
/dI (k)
IDC
(mA)
Spin Orbit Torque induced Magnetic Switching
Current in Ta causes switching in MTJ bottom electrode
Switching is monitored with tunneling magnetoresistance
R
Ta
Tunneling Magnetoresistance
L. Q. Liu, et al, Science, 336, 555 (2012)
Tunnel Junction
Fixed layer
Free layer
Tunnel barrier
Luqiao Liu 12/05/17 p25
The Quantum Extreme of Spin Hall Effect
B
From Hall effect to Quantum Hall effect
From spin Hall effect to Quantum spin Hall effect
e-
h+
3D version: topological insulator
Luqiao Liu 12/05/17 p26
Accumulated spin at normal metal/oxide barrier interface
Spin dependent chemical potential matching at the interface
Spin signal Measurable charge signal
Quantify the SHE using tunnel junctions
Spin accumulation at the surface
FM
TIμ↑
oxide
Spin Potentiometer
∆𝑉
TI
Luqiao Liu 12/05/17 p27
Quantify Charge to Spin Conversion in TI using TI/MgO/CoFeB Junction
5nm
Bi2Se3
MgO
CoFeB
substrate
Effective spin Hall angle
Bi2Se3: ~ 0.9
(Bi,Sb)2Te3: ~20
orders of magnitude larger than heavy metals
L. Liu et al, Nat. Phys. 10, 561; PRB 91, 235437
C. H. Li, et al, Nat. Nano, 9, 218;
J. H. Tang, et al, Nano Lett, 9, 5423
(Bi,Sb)2Te3/MgO/CoFeB
Luqiao Liu 12/05/17 p28
Charge Spin Conversion Efficiency from Spin Orbit Interaction
Six orders of magnitude improvement over the past decade in spin
generation efficiency!
Luqiao Liu 12/05/17 p29
Spin Orbit Torque Induced Magnetic Dynamics by Topological Insulator
Room temperature switching remains to be demonstrated.
Spin Torque FMR
Mellnik et al, Nature 511, 449 (2014)
Switching of magnetic TI at low Temperature
Yasuda et al, arXiv:
1612.06862 (2016)Fan et al, Nat. Mater. 13,
699 (2014)
Luqiao Liu 12/05/17 p30
Rare Earth-Transition Metal Alloy as Magnetic Electrode
Previous SOT switching of PMA film: Ta/CoFeB/MgO, Pt/Co/Al
Interfacial Anisotropy, sensitive to seeding layer texture
Rare Earth-Transition Metal alloy:
Bulk PMA
CoTb exhibits PMA when grown on Bi2Se3 for a very wide thickness range
-600 -300 0 300 600
-400
-200
0
200
400
M (
em
u/c
m3)
Hz (Oe)
Luqiao Liu 12/05/17 p31
TI Induced Magnetic Switching at Room Temperature
-400 -200 0 200 400
-2
-1
0
1
2
RH (
)
Hz (Oe)
-4 -2 0 2 4
-1.2
-0.6
0.0
0.6
1.2 Hx = - 1000 Oe
RH (
)
Je (10
6 A/cm
2)
-4 -2 0 2 4
-1.2
-0.6
0.0
0.6
1.2 Hx= + 1000 Oe
RH (
)
Je (10
6 A/cm
2)
R vs I
R vs H MBE Bi2Se3/Co1-xTbx/SiNx
Luqiao Liu 12/05/17 p32
Quantitative Determination of Spin Torque Efficiency at TI/CoTb interface
-120 -60 0 60 120
-2
-1
0
1
2
H effz
- 1x106 A/cm
2
+ 1x106 A/cm
2
Hx = + 800 Oe
RH (
)
Hz (Oe)
-800 -400 0 400 800
-8
-4
0
4
8
sat
H satx
(
10
-6 O
e A
-1 c
m2)
Hx (Oe)
Shifting of MH curve under applied current Effective field normalized with applied current
SOT is effective as a perpendicular H field in
domain wall
Luqiao Liu 12/05/17 p33
Surface vs Bulk Contributions
Bi2Se3 vs (Bi,Sb)2Te3
Bi2Se3 Nat. Phys. 6, 584 (2010) (Bi,Sb)2Te3 Nat. Comm. 2, 574 (2011)
ARPES at RT
Material Bi2Se3 (Bi,Sb)2Te3
Resistivity (μΩ cm) 1060 4020Transport at RT
• Bi2Se3 has more contributions from bulk while (Bi,Sb)2Te3 has more contributions
from surface states.
Luqiao Liu 12/05/17 p34
Surface vs Bulk Contributions
Topological surface states make significant contributions to the efficient SOT.
Material Bi2Se3 (Bi,Sb)2Te3
𝛼SH 0.16 0.40
Bi2Se3 vs (Bi,Sb)2Te3
Luqiao Liu 12/05/17 p35
Comparison with Other Materials for Spin Orbit Torque
Effective Spin Hall angle of Bi2Se3: 0.16, (Bi,Sb)2Te3: 0.4
Surface effect might be the dominant factor as Bi2Se3 is more bulk conductive.
Power consumption for magnetic switching: 𝑃 ∝ 𝐼2𝑅 ∝𝑅
𝛼𝑆𝐻2
Bi2Se3 will lower down power consumption of magnetic switching despite of the
higher resistivity
Effective Spin Hall Angle Power Consumption
J. Han, L. Liu, et al, PRL 119,077702 (2017)
Luqiao Liu 12/05/17 p36
Summary
Magnetic reading and writing for zero moment magnet
• Compensated Ferrimagnet has the advantage of AFM
zero Ms, immunity to external field, fast speed
• Compensated Ferrimagnet allows easy reading and writing
Non zero spin polarization at Fermi surface
• Magnetic switching and detection very close to the compensation point is demonstrated in RE-TM system
Magnetic switching of low Ms magnet with topological insulator
• Room temperature switching of magnet with topological insulators
• TI has current/energy advantage for magnetic switching compared with other spin Hall metals
Luqiao Liu 12/05/17 p37