creating and manipulating magnetic skyrmions at room...

Creating and manipulating magnetic

skyrmions at room temperature

Suzanne G.E. te Velthuis

Materials Science Division

Argonne National Laboratory

Supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division.

Acknowledgements

Axel Hoffmann Wanjun Jiang

Wei Zhang, M. Benjamin Jungfleisch, Frank Y. Fradin, John E. Pearson, Olle Heinonen

Argonne National Laboratory

Xichao Zhang, Yan Zhou

University of Hong Kong

Xiao Wang, Xuemei Cheng

Bryn Mawr College

Pramey Upadhyaya, Guoqiang Yu, Yaroslav Tserkovnyak, and Kang L. Wang

University of California Los Angeles

Discovery of Magnetic Skyrmions

S. Mühlbauer et al., Science 323, 915 (2009)

Magnetic Skyrmions

Non-Trivial Topology

Topological Charge

Q = ±1

Stabilized by Chiral Dzyaloshinkii-Moriya Interaction

Dij ×(Si ´Sj )

Efficient Manipulation by Electric Charge Currents

Emergent Magnetic Field

2

2

0

2

0 )(J

D

aRh

h ~ 100 T

Motion due to Spin Hall Effect

A. Fert et al., Nature Nano. 8, 152 (2013)

Stabilizing Skyrmions

Broken inversion symmetry leads to Dzyaloshinskii-Moriya

Interaction (DMI) Hdmi

= -Dij×(S

i´S

j)

Bulk DMI

Vortex-like

Interfacial DMI

Hedgeho

g

B20 compound MnSi, FeGe, FeCoSi Ta/CoFeB/MgO, Pt/Co/MgO, Ir/Fe/Pd


Encoding Information in Spin Textures

Racetrack Memory

S. S. P. Parkin, M. Hayashi, and L. Thomas, Science 320, 190 (2008)

Skyrmions


Generating Individual Skyrmions

N. Romming, et al., Science 341, 636 (2013)

Using spin-polarized scanning tunneling microscope

Spin-transfer torque switches skyrmion core reversibly

How to experimentally

create and/or manipulate room-temperature mobile skyrmions

in common material systems.

Topics

Skyrmion Hall Effect

Blowing Magnetic Skyrmion Bubbles

Ferromagnetic Films with Perpendicular Anisotropy

3 nm TaOx

5 nm Ta

1.1 nm CoFeB

Increasing Magnetic Field

20 um

-20 -10 0 10 20

-500

-250

0

250

500

M (

em

u/c

c)

H (mT)

Polar MagnetoOptic Kerr Effect (MOKE) Imaging

W. Jiang et al., Science 349, 283 (2015)

Domain Evolution with Magnetic Field

20 μm

+1 mT

+0.83 mT

+0.57 mT

+0.40 mT

-0.29mT

-0.11mT

+0.06 mT

+0.24mT

-0.46mT

-0.66mT

-0.83 mT

-1 mT


H


H = - 0.5 mT

Jc = + 0.5 MA/cm2



H = +0.5 mT

Jc = 0.5 x MA/cm2


Skyrmion Generation Phase Diagram

-

Jc

+Jc H = +0.5 mT +Jc H = -0.5 mT

Before current pulse

After current pulse

J

-0.3 0.0 0.3

-6

-3

0

3

6

J C (

105 A

/cm

2 )

H (mT)


Low Current Transformation Dynamics

B = +0.46 mT, DC current Je = + 0.068 MA/cm2


Spin Hall Effect

Spin Hall

Charge Current

Transverse Spin Imbalance

Recent Review: Axel Hoffmann, IEEE Trans. Magn. 49, 5172 (2013)

Ta

CoFeB

Spin Orbit Torque

On Adjacent Ferromagnet

Chiral Domain Wall Torques

K.-S. Ryu, et al., Nat. Nanotechn. 8, 527 (2013)

S. Emori, et al., Nat. Mater. 12, 611 (2013)

DMI + Spin Hall

= Efficient Torque

Inhomogeneous Chiral Spin Orbit Torques

Stripe Domain with Homogeneous Current

Stripe Domain with Inhomogeneous Current


Different Geometries

Width Shape C

D

Skyrmion generation is robust Spatially divergent currents are the key


Micromagnetic Simulation of Transformation

H = 5 Oe Ms = 650 emu/cm3

Ha = 8868 Oe A = 3 erg/cm DMI = 0.5 erg/cm2 a = 0.02

sTa = 0.83 MS qsh = 10%

Apply 0.5 V for 30 ns Relax at 0 V for 40 ns

O. Heinonen et al., Phys. Rev. B 93, 094407 (2016)

Micromagnetic Simulation of Transformation

Apply 5 V for 40 ns Relax at 0 V for 80 ns

H = 5 Oe Ms = 650 emu/cm3

Ha = 8868 Oe A = 3 erg/cm DMI = 0.5 erg/cm2 a = 0.02

sTa = 0.83 MS qsh = 10%

Two distinct mechanisms for

skyrmion generation!

O. Heinonen et al., Phys. Rev. B 93, 094407 (2016)

Motion of Skyrmions

1. Use large current to generate skyrmions 2. Subsequently use smaller currents to move skyrmions


Motion of Skyrmions

t=0 s

t=3.5 s

t=5.4 s

t=6.8 s

S = 1 ×

+Jc


Topologically Trivial Bubbles

(S = 0) bubble stabilized by in-plane magnetic fields

Expansion (S = 0)

(S = 0)

Shrinking

J. Choi, et al., PRL (2007) O. Lee et al., PRB (2014) S. Emori, et al., Nat. Mater. (2013) K.-S. Ryu, et al., Nat. Nano. (2013) S. Emori, et al., PRB. (2014)


Response to Currents with In-Plane Magnetic Fields

Je=+0.05 MA/cm2

Je=+0.25 MA/cm2

Je=-0.05 MA/cm2

Je=-0.25 MA/cm2

B// =10 mT


No Bubble Formation Without Uniform Chirality

B// = 5 mT Je = + 0.5 MA/cm2


Towards Investigating Individual Skyrmions

20 μm

I1-

I2- Single Skyrmion line

gen

eratio

n lin

e

I1+

I2+

Vxy1

Vxy1

Vxy2

Vxy2

W. Jiang et al., AIP Advances 6, 055602 (2016)

Observing Motion of Individual Skyrmions

10 μs

V

Je = + 0.5 MA/cm2

Vsk ≈ 1 m/s

“Spin current can efficiently move skyrmions at the

speed of 10 m/s at current density level of MA/cm2”

J. Sampaio and V. Cros et al.,

Nat. Nano. (2013)

Pulse 1

Pulse 2

Pulse 4

Pulse 6

Pulse 5

Pulse 3

0 μm

30 60 90

0

8 4

Pulse 7

W. Jiang et al., AIP Advances 6, 055602 (2016)

Topics




Electric charge qe

Lorentz force qe(v×B)

Classic Hall effect Skyrmion Hall effect Topological charge qt

Magnus force 4qt(v×ez)

K. Everschor-Sitte and M. Sitte, J. Appl. Phys. 115, 172602 (2014)

Motion of Rotating Objects

Famous Brazilian Expert: Roberto Carlos

Q = -1 Q = +1

Electrons (-1) / holes (+1)

Courtesy of Shizeng Lin

A Hall effect of topological charge?

Electric charge qe

Lorentz force

Classic Hall effect Skyrmion Hall effect topological charge Q

Magnus force

R. Tomasello,et al., Scientific Reports 4, 6784 (2014)

Skyrmion Motion

Small J creep motion Bigger J steady motion (inaccessible)

arXiv:1510.07353 (current density, defects)

Complication from inhomogeneous currents



Thiele Equation:

G= (0,0,-4pQ)

vy

vx

=1

aD

Skyrmion Motion with Homogeneous Current

W. Jiang et al., arXiv:1603.07393

10

0 μm

Current Dependence of Motion

-1 +1

5 μm


(stochastic) hopping

Drive-dependent Skyrmion Hall angle

Skyrmion is pinned

Evolution of Velocity/Hall angle on currents


rather than predicted constant value

Numerical Simulations

arXiv:1605.01427

Driving force FD

R =

vy/

v x

Dissipative force term Dissipative force term – isolated skyrmions


0

q

radialdistance

p

z

m q

d γdw D vy/vx Φsk

10 nm 10 nm 1.2 40 89°

100 nm 10 nm 12 4 76°

1000 nm 10 nm 123 0.4 22°

Skyrmion Motion: Edge vs. Interior

Edge of the sample

SiO2 Substrate

Oscillatory skyrmion edge transport: Competition between edge repulsive force and magnus force

Spin Hall

Magn

us

Interior of the sample

Edge repulsive force


Conclusions

• Magnetic Skyrmions

• Use interfacial interactions to

stabilize them at room-temperature

• Skyrmions in Metals: Ta/CoFeB/TaOx

• Use inhomogeneous current for

generating skyrmions

• New model system for studying

instabilities in complex flows

• Skyrmion Hall Effect

• Observed current dependent

Hall angle

J

-0.3 0.0 0.3

-6

-3

0

3

6

J C (

105 A

/cm

2 )

H (mT)

creating and manipulating magnetic skyrmions at room...

Documents