ALS UM 2009

Stochastic Behavior of Magnetic ProcessesStochastic Behavior of Magnetic Processeson a Nanoscaleon a Nanoscale

Mi-Young ImCenter for X-ray Optics, LBNL

Berkeley, CA, USAmim@lbl.gov

ALS UM 2009

Challenge in Nano-magnetism

1 cm 1 mm 1 µm 1 nm

Ultra-smallUltra-small

Ultrathin Films Nanowire Nanoparticles

Novel Manuplating TechniqueNovel Manuplating Technique

B-field Spin current Thermal

Controllability Controllability

Vortex switchingDomain wall motion

Nano-MagnetismNano-Magnetism

1 ms 1 ns 1 ps 1 fs

Ultra-fastUltra-fast

Thermal activation Damping ThermalizationPrecession

ALS UM 2009

Contents

Background Background

Statistical Behavior of Magnetic ProcessesStatistical Behavior of Magnetic Processes

--- Domain Nucleation Process in Ultra Thin Magnetic Film (2D)

--- Domain Wall Depinning Process in Notch Patterned Nanowires (1D)

--- Vortex- State (chirality) Creation Process in Circular Nanodot Arrays (0D)

Summary Summary

ALS UM 2009

Statistical Behavior

Whether the magnetic process is deterministic or stochasticWhether the magnetic process is deterministic or stochastic

Scientific Point of ViewScientific Point of View : century old long-standing question

- Is there any unifying physical mechanism?- Is there any specific law, which governs the complicate magnetic phenomena?

- Which is dominant factor for determination of statistical nature?

Technological Point of ViewTechnological Point of View: substantial issue for application - Is the spin reversal phenomena repeatable?

- Is the domain wall motion controllable? - What is the way to acheive the tunable and

repeatable spin reversal and dynamics?

ALS UM 2009

ReviewIrreversible ReversalReproducible Hysteresis loop

J. M. Deutsche et al., Phys. Rev. Lett.(2004)

M. S. Pierce et al., Phys. Rev. Lett. (2003)

Macroscopic or k-space Contradictory

DWM at Single Time

A. J .Zambano et al., Appl. Phys.Lett. (2004)

Switching Field Distribution

Justin M. Shaw et al., J. Appl. Phys. (2007)

Theoretical approach Single measurement

Simulation for DW Process

E. Martinez et al., Phys. Rev. Lett. (2007)

Reversal Process in Nanodot

V. Novosad, et al., Phys. Rev. B, (2002)

Direct observation in real spaceDirect observation in real space

Statistical measurementStatistical measurement

ALS UM 2009

Our Goal

Observation S. Parkin US Patent 309, 6,834,005 (2004).G. Meier et al. PRL (2007)

Understanding Controlling

S. Parkin US Patent 309, 6,834,005 (2004).

Nanodot (0D)

Ultrathin Films Nanowires

Ultra Thin Film (2D) Nanowire (1D)

Nanodot

possibility for controllable spin process

solution for unsolved-question

ALS UM 2009

Magnetic soft X-ray microscopy at XM-1

Hmax= 5 kOe (perp.)= 2 kOe (long.)

CCD 2048x2048 px2

Mag ~ 2000FOV ~ 10-15 mm

t<70 ps

3rd generationsynchrotron source

E = 250 eV - 1.8 keVl= 0.7 nm - 5 nmE/E=500

element specificity

time resolution

XMCD contrast

polarization

circ. polarization

mm

5

-5

1-1 0intensity

10

0

lateral resolution

r< 25 nm

ALS UM 2009

Domain Nucleation Process in Ultra Thin Magnetic Film

ALS UM 2009

Sample: 50–nm (Co 82Cr18)87 Pt13 / 40-nm Ti / 200-nm Si3N4

Magnetic Domain Evolution Patterns

+H

-H -H-H

-2 -1 0 1 2-600-400-200

0200400600

M

s (em

u/c

c)

Field (kOe)

+400 Oe +600 Oe

2m

+200 Oe

0 Oe

-200 Oe

Nucelation-mediated magnetization reversal behavior that originated from individual switching of grain

M.-Y. Im et al., APL 83, 4589 (2003)

ALS UM 2009

Stochastic Nature

1st cycle

2nd cycle

Stochastic and asymmetric nature of magnetic domain nucleation process

1st cycle (left branch) 2nd cycle (right branch) Both cycles (branches)

Magnetic domain configurations in Magnetic domain configurations in repeated hysteretic repeated hysteretic cycles and different branchescycles and different branches

ALS UM 2009

Degree of Stochastic Nature

))(( 22ijij

ijij

YX

YXr

X and Y : same size matrices 1 : existence 0 : nonexistence of domain nucleation in each pixel r=0 : totally different r=1: completely identical

Average correlation coefficient Average correlation coefficient among domain among domain configurationsconfigurations

-600 -400 -200 0

0.0

0.1

0.2

0.3

0.4

0.5

Applied field (Oe)

Co

rrel

atio

n c

oef

fici

ent

a

0 200 400 6000.0

0.1

0.2

0.3

0.4

0.5

Applied field (Oe)

b

0 200 400 6000.0

0.1

0.2

0.3

0.4

Applied field (Oe)C

orr

elat

ion c

oef

fici

ent

c

Correlation coefficient in both cases increases as magnetization reversal is progressed

M.-Y. Im et al., Adv. Mater 20, 1750 (2008)

ALS UM 2009

eff fluct eff fluct

d MM H h M M H h

dt M

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

20

2

1B

s

k TVar

M V

gyromagnetic ratio dimensionless damping

coefficient parameter hfluc fluctuating magnetic field

LLG equation incl. thermal term

Micromagnetic simulation of magnetization reversal Micromagnetic simulation of magnetization reversal patterns in patterns in repeated hysteretic cycles at 300 Krepeated hysteretic cycles at 300 K

Thermal flucutation effect play a role on stochastic nature in domain nucleation process

Thermal Fluctuation Effect

-100 0 100 200 300 400 500

0.2

0.3

0.4

0.5

Co

rrel

atio

n C

oef

fici

ent

Applied Field (Oe)

ALS UM 2009

Domain Wall Depinning Process in Notch Patterned Nanowires

ALS UM 2009

Permalloy (Ni80Fe20)

SEM imagesSEM images

Wire width (w): 150, 250, 450 nm Notch depth (Nd): 30, 50 % Film thickness (t): 50, 70 nm

Notch Patterned Permalloy Nanowire

+H

-H-H

-H

MTXM imageMTXM image

ALS UM 2009

-47 Oe

-383 Oe

-413 Oe

-430 Oe

-489 Oe

-141 Oe

-189 Oe

-236 Oe

-259 Oe

-371 Oe

-24 Oe

-106 Oe

-124 Oe

-129 Oe

-319 Oe

w= 150 nm w= 250 nm w= 450 nm

Domain walls are stopped at precise position

Domain Wall Evolution Patterns

ALS UM 2009

Depinning field of domain wall in Depinning field of domain wall in repeated hysteretic repeated hysteretic cyclescycles

-100 Oe H -530 Oe

DW depinning process shows stochastic behavior in repeated measurements

Stochastic Nature

DW depinning process is not completely governed byDW pinning mechanism

ALS UM 2009

Multiplicity of Domain-wall Types

The multiplicity of domain-wall type generated in the vicinity of a notch is responsible for the observed stochastic nature

Vortex wall

Transverse wall

courtesy S. Parkin

-440 Oe

-450 Oe

-485 Oe

-490 Oe

ALS UM 2009

Degree of Stochastic Nature

Standard deviationStandard deviation of DW depinning fieldof DW depinning field

200 300 400

10

20

30

40

t=30 nm, Nd=30 %

t=50 nm, Nd=50 %

S

tan

da

rd D

ev

iati

on

(O

e)

Wire Width (nm)

30 40 50

20

40

60

t=30 nm, w=250 nmt=50 nm, w=250 nm

Notch Depth (%)

M.-Y. Im et al, Phys. Rev. Lett. 102, 147204 (2009)

Standard deviation of the depinning field is minimized to below 7 Oe

The DW depinning process can be controllable in properly designed nanowire

ALS UM 2009

Vortex State (chirality) Creation Process in Nanodot Arrays

ALS UM 2009

Permalloy Nanodot Arrays

MTXM ImageMTXM Image

Dot Size (r): 200, 400, 600, 800, 1000 nm Film Thickness (t): 40, 70, 100 nm

800 nm• Chirality

in-plane circular domain structure• Polarity

out-of-plane component of magnetization

Vortex StateVortex State

Ni80Fe20 :t=100 nm, r=800 nm

Normalized ImagesNormalized Images

In-plane Out-of-plane

1000 nm

800 nm

600 nm

400 nm

200 nm

ALS UM 2009

Statistical Behavior of Vortex State (chirality) Creation Process

Ni80Fe20 (t=40 nm, r=1000 nm, d=200 nm)

2nd1st

+x saturation

Overlapped images

Overlapped imagesSwitched Dots

Switched Dots

In-plane domain state in In-plane domain state in repeated measurements and repeated measurements and changing the field directionchanging the field direction

Stochastic nature of creation process of chirality in repeated (different saturation field direction) measurements

M.-Y. Im, Peter Fischer, et al., in preparation

+xH

-x saturation

ALS UM 2009

Summary

Statistical Behavior of Magnetic Processes on a NanoscaleStatistical Behavior of Magnetic Processes on a Nanoscale

Direct observation of stochastic behavior - Domain nucleation process in ultra thin ferromagnetic system - Domain wall depinning process in nanowire system - Vortex state creation process in nanodot system

Investigation of the origin (thermal fluctuation, multiplicity, aspect ratio, etc.) of stochastic behavior

Answering for long-standing fundamental question on nanomagnetism Providing of controllable magnetic process

ALS UM 2009

Thanks to…

• Peter Fischer, B. Mesler, A.E. Sakdinawat, W. Chao, R. Oort, B. Gunion, S.B. Rekawa, P. Denham, E.H. Anderson, D.T. Attwood (CXRO Berkeley USA)

• S.-C. Shin (KAIST, Taejeon), S.-K. Kim (SNU, Seoul), S.B. Choe (SNU, Seoul), D.-H. Kim (Chungbuk U)

• L. Bocklage, Judith Moser, A. Vogel, R. Eiselt, M. Bolte, G. Meier, B. Krüger (U Hamburg, Germany)

• S. Kasai (NIMS in Jap.), K. Yamada, K. Kobayashi, T. Ono (U Kyoto), A. Thiaville (U Paris-Sud)

• ALS and CXRO staff

Thank you for attention!