X-ray Imaging of Magnetic Nanostructures and their Dynamics

Joachim Stöhr Stanford Synchrotron Radiation Laboratory

1895 1993

X-Rays have come a long way……

2003

1 m

Sug-Bong Choe1 Yves Acremann2

Andreas Bauer1,2 Andreas Scholl1

Andrew Doran1 Aaron Lindenberg3

Howard A. Padmore1

1 Advanced Light Source2 Stanford Synchrotron Radiation Laboratory 3 UC Berkeley

Hendrik Ohldag2

Squaw Valley, April 2003

Jan Lüning2

Kortright and Kim, Phys. Rev. B 62, 12216 (2000)

Fe metal – L edge

Soft X-Rays are best for magnetism!

TransmissionX-ray

Microscope

Reconstructionfrom

Speckle Intensities

5 m(different areas)

Imaging by Coherent X-Ray Scattering

Phase problem can be solved by “oversampling” speckle image

S. Eisebitt, M. Lörgen, J. Lüning, J. Stöhr, W. Eberhardt, E. Fullerton (unpublished)

Magnetic Spectroscopy and Microscopy

868 870 872 874

0.00

0.05

0.10

0.15

TE

Y (

a.u.

)

Photon Energy(eV)

777 778 779

0

4

8

TE

Y (

a.u.

)

Photon Energy (eV)

m

[010]

NiOXMLD

CoXMCD

Spectromicroscopy of Ferromagnets and Antiferromagnets

AFM domainstructure at surface of NiOsubstrate

FM domainstructure inthin Co film onNiO substrate

H. Ohldag, A. Scholl et al., Phys. Rev. Lett. 86(13), 2878 (2001).

-3k -2k -1k 0 1k 2k 3k

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

Applied Field (Oe)

-15

-10

-5

0

5

10

15

Mn

Co

XM

CD A

sym

met

ry (%

)

Co/NiO

Magnetic characterization of interfacial spins

loop of interfacial spins -only 4% are pinned

Co

NiO

Stöhr et al., Phys. Rev. Lett. 83, 1862 (1999)Thomas et al., Phys. Rev. Lett. 84, 3462 (2000) Scholl et al., Science 287, 1014 (2000)Nolting et al., Nature 405, 707 (2000)Regan et al. Phys. Rev. B 64, 214422 (2001)Ohldag et al., Phys. Rev. Lett. 86 2878 (2001)Ohldag et al., Phys. Rev. Lett. 87, 247201 (2001)Ohldag et al., Phys. Rev. Lett. 91, 017203 (2003)

Publications:

Co/IrMn

Exchange Bias Model from X-Rays

ideal AFM poly AFM

Present limitations of magnetic recording

• Present method of magnetic switching is unfavorable: – present recording time ~1 ns

– unfavorable torque and dependent on thermal activation

dt

MdM

M

HM

dt

Md

1

- 1

Fast Magnetization Dynamics is governed by Landau-Lifschitz-Gilbert equation:

Precession torque Gilbert damping torqueAngular momentum change

Typically 100 ps

We want to understand on atomic level controls switching time, ~1 optimal

1 Tesla field: 90o rotation in 10 ps

Time Resolved X-Ray Microscopy

Laser pump – x-ray probesynchronization

< 1 ps

< 100 ps

328 nst

excitationlaser pulse

observationx-ray pulse

Production of Magnetic Field Pulses

100 m

100 m

2 m 2 m

Photoconductive switch

Current

Conducting wire

Magnetic Cells

10 m

H ~ 200 Oe

50 => I = 200 mA, 10 V bias

Current

magnetic field

0 2000 4000 6000 8000

1.00

1.02

1.04

1.06

De

flect

ion

ra

tio

Delay (ps)

0

50

100

150

200

Ma

gn

etic

fie

ld (

Oe)

Sample and Magnetic Field Pulse

MMagnetic Field Pulse ~ 150 Oe at Maximum < 50 ps rising time > 300 ps decaying time with some reflection

20 nm Co90Fe10 films with in-plane anisotropy (1 m) x (1-3 m) rectangles

Observation of Vortex Motion

-50 0 50 100

-50

0

50

100

y di

spla

cem

ent (

nm)

x displacement (nm)

0 200 400 600

-200

0

200

400

y di

spla

cem

ent (

nm)

x displacement (nm)

1 m x 1 m 2 m x 1 m

H

Vortex speed ~ 100 m/s

1.5 m x 1 m

Vortices rotate oppositely - vortex cores point in opposite directions

ConclusionsThe challenge of the future is to control the magnetization on the nanometer length scale

and picosecond/femtosecond time scale

Our current capabilities are:

• image the magnetization with 50 nm spatial resolution,

• image the response of the magnetization with 100 ps time- and 100 nm spatial resolution

Outlook into the future:

• 5 nm spatial resolution – PEEM3, under construction

• 100 fs time resolution: pump-probe excitations

single snapshots of equilibrium dynamics

Modern x-ray sources offer unique opportunities for studies of the ultrafast magnetic nanoworld

The End

Vortex Structure And Vortex Motion

HPlane view

Elevation view

Motion antiparallel to field!

torque

Landau-Lifshitz equation:(neglect damping)

The field acts like a screw driver.

Depending on the orientation of the thread pitch,

the screw (vortex) will move either forward or backward

Magnetostatic field is always

perpendicular to the vortex deviation

Vortex Precession

Under a field pulse,the vortex moves from the center.

Happlied

Hmagnetostatic

M

After the field pulse,the vortex continues to move radiallydue to the magnetostatic energy.

Induced magnetostatic field is always perpendicular to the vortex motion.

Vortex will precess forever if there is no damping.

H dH

xdx

Small Angle ScatteringCoherence length larger than domains,but smaller than illuminated area

SpeckleCoherence lengthlarger than illuminated area

Incoherent vs. Coherent X-Ray Scattering

log (intensity)40

-20

-40

0

20

-40 -20 0 20 40

-40

-20

0

20

40

scattering vector q (m-1)

scat

teri

ng v

ecto

r q

(m

-1)

log (intensity)40

-20

-40

0

20

-40 -20 0 20 40

-40

-20

0

20

40

scattering vector q (m-1)

scat

teri

ng v

ecto

r q

(m

-1)

informationabout

domainstatistics

trueinformation

about domainstructure

Pulse Structure

Possible solutions: - gated detector, pulse picker

- pump at 500 MHz