an overview of spintronics in 2d...
TRANSCRIPT
© 2014 ICQM
An Overview of Spintronics in 2D
Materials
Wei Han (韩伟)
1
2
Outline
I. Introduction to spintronics (Lecture I)
II. Spin injection and detection in 2D (Lecture I)
III. Putting magnetic moment in 2D (Lecture II)
IV. Spin Hall effect and spin orbit torque in 2D (Lecture II)
V. Acknowledgement
3
Summary of Lecture I
Electron
Spin
Magnetic structure
1988 年GMRHistory
~1997 年Hard drive
1991-2004TMR2001- present
MRAMLow R
High R
4
Summary of Lecture I
Graphene is a very good candidate material for spin channels
Large spin signal (with tunnel barrier)
Long spin lifetime (6.2 ns in BLG)
Long spin diffusion length (> 10 micro meters at RT)
Easy to manipulate (Gate)
Electrical detection of spin --momentum locking in TI
Graphene “wins” the match for tunnel barrier
5
Outline
I. Introduction to spintronics (Lecture I)
II. Spin injection and detection in 2D (Lecture I)
III. Putting magnetic moment in 2D (Lecture II)
IV. Spin Hall effect and spin orbit torque in 2D (Lecture II)
V. Acknowledgement
6
Outline
I. Introduction to spintronics (Lecture I)
II. Spin injection and detection in 2D (Lecture I)
III. Putting magnetic moment in 2D (Lecture II)
Graphene
Topological insulator
7
Why make graphene magnetic
8
How to make graphene magnetic
Ferromagnetic oxides/grapheneAdatom and moleculedoping of graphene
Doping effect
Examples:Mn doped GaAsZnO
Proximity effect
Examples:Co-Pt (Magnetism in Pt)
9
Making graphene magnetic
Yazyev and Helm, PRB (2007)
H vacancy
10
Making graphene magnetic
Ferromagnetic ??
Cervenka, et al, Nature Physics (2009)
11
Making graphene magnetic
Paramagnetic ??
F doped
Nair, et al, Nature Physics (2012)
Defects
12
Making graphene magnetic
Question
Ferromagnetic ??
Paramagnetic ??
vacancy
13
Nonlocal spin transport geometry
IINJ VNL
spininjector
spindetector
chargecurrent
pure spincurrent
• Localized measurement
• Direct coupling of spin to magnetic moment
With magneticmoment Magnetic moment could
scatter pure spin currentthrough exchange interaction:
pure spincurrent
Hex = Aex Se SM
graphene
Making graphene magnetic
14
Making graphene magnetic
Si
SiO2
(backgate)
Graphene spinvalve device
I V+ -
Atomic hydrogen source
Hydrogen
• All measurements done in ultrahigh vacuum (UHV)
• Compare immediately before and after hydrogen doping
15
Aha! A dip in the nonlocal spin signal.
This may be the magnetic moment!
Making graphene magnetic
Orange arrows = amount of spinat the spin detector
pure spincurrent
At zero field At high field
Due to exchange coupling, pure spin current is scattered by magnetic moment
Hex = Aex Se SM
Fewer spins at detector
Scattering by exchange coupling is suppressed
Se and SM
More spins at detector
decouple!
16
Making graphene magnetic
17
Making graphene magnetic
H vacancy
Paramagnetic !!
McCreary, et. al, PRL (2012)
Doping effect
18
Making graphene magnetic
Proximity effect with EuO
T. Santos, et. al., PRL 101, 147201 (2008)
TC = 69 K
0.1 nm rms 0.2 nm rms 0.5 nm rms
19
Making graphene magnetic
A. Swartz, et. al, ACS Nano (2012)
Proximity effect with EuO
Graphene on SiO2 5 nm EuO on Graphene on SiO2
• No success of observing magnetic graphene/EuO in our group
20
Making graphene magnetic
Wang, et al, PRL (2015)
The first observation of Proximity effect in graphene with YIG
21
Making graphene magnetic
What if we try harder on EuO/Graphene?
5 nm EuO on Graphene
However, there is NO if.
??
Note: Sometimes, you are so close to the peak. Just try harder and move one more step!!
22
Making graphene magnetic
Note: Sometimes, you are so close to the peak. Just try harder and move one more step!
23
Making graphene magnetic
Moving forward
Wei Yuan, Yuelei Zhao, Chi Tang, Tang Su, Qi Song, Jing Shi*, and Wei Han*, Appl. Phys. Lett. 107, 022404 (2015).
24
Why make TI magnetic
Quantum Anomalous Hall effect
25
How to make TI magnetic
Doping effect by Cr/V
Chang, et al, Science (2013)
More information, please see the results of Q. Xue group (Tsinghua University), K. Wang group (UCLA), J. Moodera (MIT), etc
Observation of the QAH
26
How to make TI magnetic
Proximity effect
Wei, et al, PRL (2013)
27
Outline
I. Introduction to spintronics (Lecture I)
II. Spin injection and detection in 2D (Lecture I)
III. Putting magnetic moment in 2D (Lecture II)
IV. Spin Hall effect and spin orbit torque in 2D (Lecture II)
V. Acknowledgement
28
Introduction to spin-orbit torque
Spin transfer torque
Brataas, et al. Nature Mater. 11, 372-381 (2012)
tST =ℏ
2�� × (�� × ��)
29
Experimental observation of the spin transfer torque
Ralph, D. C. & Stiles, JMMM. 320, 1190-1216 (2008).
Charge Current + Spin Current
Introduction to spin-orbit torque
30
Ralph, D. C. & Stiles, JMMM. 320, 1190-1216 (2008).
Charge Current
Introduction to spin-orbit torque
Disadvantage
Heat!!!
31
D'yakonov, M. I. & Perel', J. Exp. Theor. Phys. Lett. 13, 467-469, (1971).Hirsch, J. E. Phys. Rev. Lett. 83, 1834-1837, (1999).Zhang, S. Phys. Rev. Lett. 85, 393-396, (2000).
How about pure spin current?Spin orbit coupling (spin Hall effect)
Introduction to spin-orbit torque
32Kato et al., Science 306 (5703) 2004
The first observation of spin Hall effect
Introduction to spin-orbit torque
33
Materials with large spin orbit couplings – high Z materials
Low
Large
Introduction to spin-orbit torque
34
Materials with large spin orbit couplings – high Z materials
Spin Hall current torque to FM
tSTM
JC
tST =ℏ
2�� × (�� × ��)
NM
FM
Introduction to spin-orbit torque
35Liu et al., PRL 109, 096602 (2012)
Spin Hall current torque to FM
Introduction to spin-orbit torque
36
The main Challenge
How to get larger spin orbit torque?
FM
PtJC
Increase the Efficiencyof the SOT
Search for larger SOT in quantum materials
37
FM
PtJC
Question: 100%?
Interface transparency of the spin current
Introduction to spin-orbit torque
38
Efficient spin orbit torque in Pt/Co and Pt/Py
Py
PtJC
Co
PtJC
eg: Different spin density states
Book: Magnetic Multilayers and Giant Magnetoresistance(editor: Uwe Hartmann & R. Coehoorn )
EF (eV)
Our approach
39
x
yz
V
HexttH
tSTM
JC
Au
Pt/FM
9GHz 10GHz 11GHz 12GHz 13GHz
10000 30002000Hext (Oe)
-10
0
10
-20
20
30
Vm
ix(µ
V)
Liu et al., PRL 106, 036601 (2011)
Photolithography Ion Beam etching/deposition
ST-FMR
Spin Torque FerroMagnetic Resonance
400 1000 2000 3000
-20
-10
0
10
20
30
Hext
(Oe)
Data fit S A
Vm
ix (V
)
tSTtH
���
��= −��� × ����
+��� ���
��+�
ℏ
������ × ��,��(�� × �� × ��)
− ��� × ���
spin torque tST
torque from RF field tH
damping
external effective field
Symmetric Component: Spin Hall torque
Antisymmetric Component: Torque from RF field
Landau-Lifshitz-Gilbert equation
Spin torque ferromagnetic resonance
41
0 1000 2000 3000
-20
-10
0
10
20
30
Vm
ix (V
)
Hext
(Oe)
tSTM
JC
Spin torque
��� =�
�
�������
ℏ[1 + (4�����/����)]
�/�
0 1000 2000 3000
-20
-10
0
10
20
30
Hext
(Oe)
Vm
ix (V
)
tH
M
JC
Symmetric Component Antisymmetric Component
Field torque
Efficient spin orbit torque in Pt/Co and Pt/Py
42
109 1211
Freq (GHz)
0.0013
0.04
0.08
0.12
109 1211
Freq (GHz)
0.0013
0.04
0.08
0.12
ƟS
H
ƟS
H
Py
PtJC
Co
PtJC d
t
d
t
6 nm Pt/6 nm Py
6 nm Pt/6 nm Co
Efficient spin orbit torque in Pt/Co and Pt/Py
43
Interface transparency
��� � = −
�
2����� − ���,����
FM
PtJC
In Pt layer:
�� � = �⃗��� �⁄ +��� �⁄
��� 0 = ��
� �� = ��(�� × �� × �� ) ∗ e h⁄
In FM layer:
d
t
z = 0x y
z
44
Interface transparency
The continuity of the spin current density at the interface(z = 0, and z = -d)
�� � = −��(2� � �⁄ )���,�� tanh��2�
sinh2� + ���
2�
sinh���2�
− ��� 0
cosh� + ���2�
sinh���2�
��� = ���,��(�� × �� × �� )�
��
�� coth����
+��ℎ2��
FM
PtJC d
t
z = 0x y
z
45
Interface transparency model
�����
���
Interface transparency of the spin current
� =�����
���=
�↑↓tanh(���2�)
�↑↓coth����
+��ℎ2��
�↑↓:intrinsic spin mixing conductanceλ: spin diffusion length of Pt σ: conductivity of Pt
FM
PtJC d
t
46
Interface transparency
���(�)
��� ∞= 1 − sech
�
�
Data
=1.2 nm =1.4 nm =1.6 nm
400 10060
0.00
0.02
0.04
0.06
d (Å)
0.08
20 80
� = 1.4 nm
d Pt/6 nm Py
Spin diffusion length
47
Interface transparency
Damping parameters for FM and Pt/FM obtained from conventional FMR
Pt/Py Pt/Co
Geff (1019m-2) 1.52±0.34 3.96±0.39
6 Pt/t Py, Co
Intrinsic mixing conductance
48
Interface transparency
Pt/Py Pt/Co
Geff (1019m-2) 1.52 ± 0.34 3.96 ± 0.39
σPt (µΩ*cm) 15 ± 1 15 ± 1
� (nm) 1.4 ± 0.2 1.4 ± 0.2
�↑↓ (1019m-2) 2.4 ± 0.4 11.1 ± 3.1
T 0.25 ± 0.05 0.65 ± 0.06
1
����=
1
�↑↓+
1
����
ℎ2��
SHA of Pt ~ 0.19
� =�����
���=
�↑↓tanh(���2�
)
�↑↓coth����
+��ℎ2��
Intrinsic mixing conductance
49
Interface transparency
0.0 0.2 0.4 0.6 0.8 1.00.00
0.05
0.10
0.15
x
higher G larger T higher SHA
� =�����
���=
�↑↓tanh(���2�
)
�↑↓coth����
+��ℎ2��
Co Ni
SHA of Pt in 6 nm Pt- 6 nm Co1-xNix
50
Interface transparency plays an important role for spin orbit torque
Py
PtJC
Co
PtJC
W. Zhang*, Wei Han*, Xin Jiang, See-Hun Yang, Stuart Parkin, Nat Phys. (2015)
Interface transparency
51
The main Challenge
How to get larger spin orbit torque?
FM
PtJC
Increase the Efficiency of the SOT
Search for larger SOT in 2D Quantum materials
52
1) Large SHE in 2D Ir-Mn
Motivated by a recent theoretical work of AHE in IrMn3
Large spin orbit coupling of Ir transfer to Mn.
Non-collinear antiferromagnetism
53
PML10
AFM
PM
Ir1-xMnx
c/a=1
L12
AFM
c/a>1
Ir47Mn53Ir
Mn
Ir25Mn75
Ir14Mn86
1) Large SHE in 2D Ir-Mn
54
Ir1-xMnx Grown on SiO2/Si substrates
Related work on Ir20Mn80 and IrMn have also been seen by other groups.IrMn: Zhang, W. et al. Phys. Rev. Lett. 113, 196602, (2014).Ir20Mn80: Mendes, J. B. S. et al. Phys. Rev. B 89, 140406, (2014)
1) Large SHE in 2D Ir-Mn
55
Ir1-xMnx Grown on SiO2/Si substrates
Related work on Ir20Mn80 and IrMn have also been seen by other groups.IrMn: Zhang, W. et al. Phys. Rev. Lett. 113, 196602, (2014).Ir20Mn80: Mendes, J. B. S. et al. Phys. Rev. B 89, 140406, (2014)
1) Large SHE in 2D Ir-Mn
56
Facet dependent SHE
1) Large SHE in 2D Ir-Mn
57
Facet dependent SHE
20 40 60 80 100 1200.00
0.05
0.10
0.15
0.20
(100) IrMn3
(111) IrMn3
p-IrMn3
S
HA
d (A)
1) Large SHE in 2D Ir-Mn
W. Zhang*, Wei Han*, Xin Jiang, See-Hun Yang, Stuart Parkin, under review
58
2) Spin orbit Torque in Topological insulators
59
2) Spin orbit Torque in Topological insulators
Mellink, et al. Nature (2014)
Spin Hall angle: 2.0-3.5
Bi2Se3
60
Spin orbit Torque in Topological insulators
Fan, et al, Nature Mater. (2014)
T=1.9 KSHA > 100
(Bi0.5Sb0.5)2Te3
61
Current Status of Spin orbit Torque
Materials Effective SHA Research group
Semiconductor GaAs 0.0005-0.005 UCSB (Awschalom)
Metal Pt 0.19 PKU (Han) & IBM (Parkin)
Cornell (Ralph & Burhman)
β-Ta 0.15 Cornell (Ralph & Burhman)
β-W 0.3 Cornell (Ralph & Burhman)
Bi doped Cu 0.24 Japan (Otani) & France (Fert)
Quantum
Materials
TI 2.0-3.5 (~500?)
>100 (1.9 K)
Cornell (Ralph & Burhman)
UCLA (Wang)
IrMn3 ~0.3 PKU (Han) & IBM (Parkin)
62
Summary of Lecture II
Doped graphene, Paramagnetic moment
H vacancy
FM in graphene by proximity effect
63
Summary of Lecture II
Doped TI, QAH
FM in TI by proximity effect
64
Interface Transparency for SOT
Facet dependent SOT in IrMn3
Py
PtJC
Co
PtJC
20 40 60 80 100 1200.00
0.05
0.10
0.15
0.20
(100) IrMn3
(111) IrMn3
p-IrMn3
S
HA
d (A)
Summary of Lecture II
TI has very large SOT
65
Summary of Lecture II
Questions still to be answered:
FM in graphene by doping
QAH in Graphene heterostructures
Robust QAH at higher temperature
More effective SOT in quantum materials
66
Acknowledgement
To the mentors for my research life
67
Roland K. Kawakami
Stuart. S. P. Parkin
Acknowledgement
To the mentors for my research life
68
Acknowledgement
The good team@PKU
69
Summary
71