Magnetic Switching with Compensated Ferrimagnet and … › sites › default › files › 2dcc ›...

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


charge based devices

$10 million/year

𝑃~𝑓𝐶𝑉2

Spintronics for beyond CMOS applications


spin based device

𝐸~𝑁𝜇𝐵𝐵

charge based devices

𝑃~𝑓𝐶𝑉2

Spintronics for beyond CMOS applications


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


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


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)


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


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


Magnetic Switching: Ferromagnet vs Anti-ferromagnet

𝑓 ~ 𝛾2𝜋𝐻𝑎 𝑓 ~ 𝛾

2𝜋𝐻𝑒𝑥𝐻𝑎

Hex: exchange field between two spin sub-lattices > 100 Tesla

10 GHz ~1THz


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


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)


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


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


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, …)


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


Chemically tunable Magnetism in Co1-xTbx Alloys

Co Tb

H (Oe)

M vs Tb concentration


Magnetic Orientation Detectable at Compensation

Co Tb

Anomalous Hall effect remains finite when M 0

Magnetic moment

Anomalous Hall

effect


Spin Hall Effect Induced Switching at Compensated Sample

Switching is observed for samples with almost zero moment!


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


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?


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!


Content


• from ferromagnet to compensated ferrimagnet


• room temperature switching of high Tc magnet with topological insulators


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


-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


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


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


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


Charge Spin Conversion Efficiency from Spin Orbit Interaction

Six orders of magnitude improvement over the past decade in spin

generation efficiency!


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)


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)


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


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


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.


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


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)


Summary


• 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


• Room temperature switching of magnet with topological insulators

• TI has current/energy advantage for magnetic switching compared with other spin Hall metals

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