Magnetism and X-Rays: Past, Present, and A Vision of the Future

Joachim Stöhr Stanford Synchrotron Radiation Laboratory

Stanford University

1993 2003

http://www-ssrl.slac.stanford.edu/stohr/index.htm

200X

Femtosecond single shot image

100 picoseconds dynamics

Static image

Press release by the Royal Swedish Academy of Sciences, Nobel Prize in Physics: B. N. Brockhouse and C. G. Shull

1994``Neutrons are small magnets…… (that) can be used to study the relative orientations of the small atomic magnets. ….. the X-ray method has been powerless and in this field of application neutron diffraction has since assumed an entirely dominant position. It is hard to imagine modern research into magnetism without this aid."

2004:It is hard to imagine modern research into magnetism without the aid of x-rays!

Present:

Past:

Present: Size > 100 nm, Speed > 1 nsecFuture: Size < 100 nm, Speed < 1 nsec Ultrafast Nanoscale Dynamics

Some Magnetic Devices in Computers

Experimental X-Ray Methods

Non-resonant magnetic x-ray scattering is weak

Relative intensity of

spin scattering: 10 -

4

Relative intensity of charge scattering: 1

First experiment:F. de Bergevin, M. Brunel: Phys. Lett. A 39, 141 (1972)

Theory:J.L. Erskine, E.A. Stern: Phys. Rev. B 12, 5016 (1975)M. Blume: J. Appl. Phys. 57, 3615 (1985)B.T. Thole, G. van der Laan, G.A. Sawatzky: Phys. Rev. Lett. 55, 2086 (1985)

Development of X-Ray Techniques for Magnetism

Experiments:X-Ray Magnetic Resonant Scattering:K. Namikawa, M. Ando, T. Nakajima, H. Kawata: J. Phys. Soc. Jpn 54, 4099 (1985)

X-Ray Magnetic Linear Dichroism:G. van der Laan, B.T. Thole, G.A. Sawatzky, J.B. Goedkoop, J.C. Fuggle, J.M. Esteva, R. Karnatak, J.P. Remeika, H.A. Dabkowska: Phys. Rev. B 34, 6529 (1986)

X-Ray Magnetic Circular Dichroism:G. Schütz, W. Wagner, W. Wilhelm, P. Kienle, R. Zeller, R. Frahm, G. Materlik: Phys. Rev. Lett. 58, 737 (1987)

X-Ray Magnetic Imaging:J. Stöhr, Y. Wu, B. D. Hermsmeier, M. G. Samant, G. R. Harp, S. Koranda, D. Dunham, B. P. Tonner:Science 259, 658 (1993)

Valence Shell Properties and X-Ray Magnetic Circular Dichroism (XMCD)

Thole et al., PRL 68, 1943 (1992); Carra, et al., PRL 70, 694 (1993); Stöhr and König, PRL 75, 3748 (1995)

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

Fe metal – L edge

Magnetic Spectroscopy and Microscopy

Soft X-Rays are best for magnetism!

x-ray "spin"

bulk

surface

• Full Field Imaging

• Electrostatic (30 kV)

• 20 - 50 nm Resolution

• Linear and circular polarization

PEEM-2 at ALS

P o l a r i z e d

X - r a y s

Element Specific Magnetic Imaging:

Ferromagnetic Domains in Magnetite – Magnetic Fe and Oxygen

528 530 532

1.0

1.1

1.2

1.3

Ele

ctro

n Y

ield

Photon Energy [eV]

Oxygen

700 710 720

3

6

9

Ele

ctro

n Y

ield

Photon Energy (eV)

Fe

Magnetite Fe3O4

12 m

I+

I-

Spectro-Microscopy of Ferromagnets on Antiferromagnets

Tune to Ni edge – use linear polarization – antiferromagnetic domains

Tune to Co edge – use circular polarization – ferromagnetic domains

H. Ohldag et al., PRL 86, 2878 (2001).

[010]

2m870 8740

5

10

15

Ele

ctro

n Y

ield

Photon Energy(eV)

NiOXMLD

776 778

0

4

8

Ele

ctro

n Y

ield

Photon Energy (eV)

CoXMCD

780

Experimental Results:

• Exchange bias

• Time resolved imaging of magnetic structures

Exchange bias – a 50 year puzzle

Blue layer: direction is fixed by exchange bias

Red layer: direction determines resistance

-1500 -1000 -500 0 500 1000 1500

-1.0

-0.5

0.0

0.5

1.0

Mag

netiz

atio

n(a.

u.)

Applied Field (Oe)

FM 1

FM 2

AFM

Conventional techniques cannot study the magnetic FM-AFM interface

The spin-valve sensor

A ferromagnet has a preference direction when in contact with an antiferromagnet

The Basic Model – Meiklejohn (~ 1960)

Bulk FM spins: S1

Bulk AFM spins:S2 = S2

Uncompensated spins: S2

40+ years of theoretical models - reduce bias by:

• new effective number of spins S2

• twist of AFM spins – domain wall with energy

Observed loop shift (bias) is 100 times smaller than expected from model !

E12 = J12 S1 S2

E22 = J22 S2 S2

&

anisotropy of AFM

EK

Exchangecoupling:

E22 Ek

50 years of models…need experimental tests…

SAFM: uncompensated spins near AFM surface

Origin ? Number ?Size ?Parallel or perpendicular ?

Domain wall formation ?

Mauri-Siegmann model

Malozemoff model Koon model

Reduce bias through effective SAFM

Reduce bias through domain wall

E22 Ek

: Domain wall energy

Co on NiO(001)

[010]

Bare NiO(001)

2nm Co on NiO(001)

NiO after deposition

Co causes Ni spins at NiO surface to rotate into plane

AFM and FM spins couple parallel

2m

X-Rays-in / Electrons-out - A way to study Interfaces

FM Co – tune to Co edge – circular polarization

AFM NiO – tune to Ni edge – linear polarization

FM Ni(O) – tune to Ni edge – circular polarization

NiO

Co

NiO

CoNi–rich NiO

Interface Microscopy

Chemically induced interfacial Ni spins provide the magnetic link

AFM: NiO FM: CoFM:

Ni-rich NiO

Linear pol. Ni edge

Interfacial spins

Circular pol. Ni edge Circular pol. Co edge

-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

X-Ray Picture of Exchange Bias

Co

NiO

Co/IrMn

AFM axis is rotated at interface

The interface is not sharp - SAFM

SAFM || SFM

Imaging: Element specific FM loops: Free spins: 96% of ML – coercivity

Pinned spins SAFM : 4% of ML Small number determines bias size

The role of interfacial spins: SAFM

pinned spins

Nanaoscale Magnetization Dynamics - Smaller and Faster

Time resolved x-ray microscopy

Laser pump – x-ray probesynchronization

< 1 ps

< 100 ps

328 nst

excitationlaser pulse

observationx-ray pulse

50 nm / 100 ps resolution

PEEM2

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

Magnetic Patterns in 20 nm Co90Fe10 films on waveguide

3mM

x-ray"spin"

Fieldpulse

Field response

Field response

Opposite rotation is caused by direction of vortex core magnetization, i.e. chirality

Two pattern with same static structure, but …..

H

slow

fast (<1ns)

Response to a fast field pulse

Instanteneous precession determined by torque: T = H x m

H

m

T

Tiny vortex core determines fast dynamics of the whole domain structure!

"precession"

"damping"

A Vision of the Future……..

• Improved microscopes – toward atomic resolution

• X-ray lasers - ultrafast single shot imaging

……..

Lenses

High voltage feedthroughs

Deflector

CCD -alignment

Separator

Manipulator

Lenses

Electronmirror

Tomorrow: 5 nm spatial resolution with PEEM3

CCD

Spatial Resolution of PEEM3

4-5 nm

In 2007: The first x-ray laser - LINAC COHERENT LIGHT SOURCE (LCLS)In 2007: The first x-ray laser - LINAC COHERENT LIGHT SOURCE (LCLS)

2 Km

0 Km

3 Km

• SASE gives 106 intensity gain over spontaneous emission

• FELs can produce ultrafast pulses (of order 100 fs)

Growth of X-Ray Brightness and Magnetic Storage Density

Free electron lasers

We are here

each pulse:1012 photons< 100 fs coherent

Lensless Imaging by Coherent X-Ray Scattering

Challenge: Inversion from reciprocal to real space image

Eisebitt et al. (BESSY)

A Glimpse of the Future……..

• Ultrafast magnetic processes

Experimental Principle of Ultrafast Field Pulses

• Relativity allows 1010 electrons in short bunch of < 1 ps length

• High field pulses up to 5 T = 50,000 Oe

C. H. Back, R. Allenspach, W. Weber, S. S. P. Parkin, D. Weller, E. L. Garwin, H. C. Siegmann, Science 285, 864 (1999)

100 fs – 10 ps

Torques on Magnetization by Beam Field

Maximum torque

Minimum Torque

The Ultimate Speed of Magnetic Switching

tpulse= 3 ps tpulse= 100 fs

Deterministic switching Chaotic switching

Under ultrafast excitation the magnetization fractures !

90 m90 m

Magnetization fracture under ultrafast field pulse excitation

Uniform precession chaotic excitation

Squaw Valley, April 2003

The magnetism "team" – Stanford (SSRL) - Berkeley (ALS)Funded by: DOE-BES and NSF

Missing: Hans Christoph Siegmann

Conclusions• X-rays have become an important probe of magnetic materials and phenomena

• X-rays offer elemental, chemical and magnetic specificity with nanoscale spatial

resolution

• Transmission experiments probe bulk, electron yield experiments probe surfaces and

interfaces

• X-rays allow time-dependent studies, paving the way for picosecond nanoscale

technology

• Future x-ray sources, new techniques and instrumentation will allow the

complete exploration of magnetic phenomena in space and time

For more, see: http://www-ssrl.slac.stanford.edu/stohr/index.htm H. C. Siegmann and J. Stöhr Magnetism: From Fundamentals to Nanoscale Dynamics Springer 2004 (to be published)

The end