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Magnetism and X-Rays: Past, Present, and A Vision of the Future. Joachim Stöhr Stanford Synchrotron Radiation Laboratory Stanford University. Static image. Femtosecond single shot image. 100 picoseconds dynamics. 1993. 2003. 200X. http://www-ssrl.slac.stanford.edu/stohr/index.htm.

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Magnetism and X-Rays:

Past, Present, and A Vision of the Future

Joachim Stöhr

Stanford Synchrotron Radiation Laboratory

Stanford University

Static image

Femtosecond single shot image

100 picoseconds dynamics

1993

2003

200X

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


Past:

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

1994

``Neutronsare 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."

Present:

2004:

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


Some Magnetic Devices in Computers

Present: Size > 100 nm, Speed > 1 nsec

Future: Size < 100 nm, Speed < 1 nsec

Ultrafast Nanoscale Dynamics



Non-resonant magnetic x-ray scattering is weak

Relative intensity of

charge scattering: 1

Relative intensity of

spin scattering: 10-4

First experiment:

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


Development of X-Ray Techniques for Magnetism

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)

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. B34, 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)


Fe metal – L edge

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


Magnetic Spectroscopy and Microscopy

x-ray "spin"

Soft X-Rays are best for magnetism!


bulk

surface


PEEM-2 at ALS

  • Full Field Imaging

  • Electrostatic (30 kV)

  • 20 - 50 nm Resolution

  • Linear and circular polarization


I+

I-

Oxygen

Fe

9

1.3

1.2

6

Electron Yield

Electron Yield

1.1

3

1.0

528

530

532

700

710

720

Photon Energy [eV]

Photon Energy (eV)

Element Specific Magnetic Imaging:

Ferromagnetic Domains in Magnetite – Magnetic Fe and Oxygen

Magnetite

Fe3O4

12 mm


Co

XMCD

8

Electron Yield

4

s

s

[010]

0

2mm

780

776

778

Photon Energy (eV)

15

NiO

XMLD

10

Electron Yield

5

0

870

874

Photon Energy(eV)

Spectro-Microscopy of Ferromagnets on Antiferromagnets

Tune toCo edge – use circular polarization – ferromagnetic domains

Tune toNiedge – use linear polarization – antiferromagnetic domains

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



W

Exchange bias – a 50 year puzzle

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

The spin-valve sensor

FM 1

FM 2

AFM

Blue layer: direction is fixed by exchange bias

Red layer: direction determines resistance

Conventional techniques cannot study the magnetic FM-AFM interface


E22Ek

The Basic Model – Meiklejohn (~ 1960)

Exchange

coupling:

Bulk FM spins: S1

E12 = J12 S1 S2

Uncompensated spins: S2

E22 = J22S2S2

&

anisotropy of AFM

EK

Bulk AFM spins:

S2=S2

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

  • 40+ years of theoretical models - reduce bias by:

  • new effective number of spins S2

  • twist of AFM spins – domain wall with energy


50 years of models…need experimental tests…

E22Ek

Reduce bias through effective SAFM

SAFM: uncompensated spins

near AFM surface

Origin ?

Number ?

Size ?

Parallel or perpendicular ?

Malozemoff model

Koon model

Reduce bias through domain wall

: Domain wall energy

Mauri-Siegmann model

Domain wall formation ?


Co on NiO(001)

s

s

s

[010]

2mm

NiO after deposition

2nm Co on NiO(001)

Bare NiO(001)

Co causes Ni spins at NiO surface to rotate into plane

AFM and FM spins couple parallel


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


Interface Microscopy

Co

Co

Interfacial spins

Ni–rich NiO

NiO

NiO

FM:

Ni-rich NiO

AFM: NiO

FM: Co

Circular pol. Ni edge

Circular pol. Co edge

Linear pol. Ni edge

Chemically induced interfacial Ni spins provide the magnetic link


X-Ray Picture of Exchange Bias

The role of interfacial spins: SAFM

Co/IrMn

Co/NiO

Co

NiO

pinned

spins

Imaging:

Element specific FM loops:

  • AFM axis is rotated at interface

  • The interface is not sharp -SAFM

  • SAFM || SFM

  • Free spins: 96% of ML – coercivity

  • Pinned spins SAFM: 4% of ML

  • Small number determines bias size



Time resolved x-ray microscopy

PEEM2

50 nm / 100 ps

resolution

Laser pump – x-ray probe

synchronization

< 1 ps

excitation

laser pulse

< 100 ps

observation

x-ray pulse

t

328 ns


100 m

100 m

10 m

2 m

2 m

Production of Magnetic Field Pulses

Photoconductive switch

H ~ 200 Oe

Conducting wire

50  =>

I = 200 mA, 10 V bias

Magnetic Cells

Current


Magnetic Patterns in 20 nm Co90Fe10 films on waveguide

M

3mm

Field

pulse

x-ray

"spin"


Two pattern with same static structure, but …..

Field

response

Field

response

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


Response to a fast field pulse

Instanteneous precession determined by torque:

T = H x m

H

slow

"damping"

fast (<1ns)

T

"precession"

m

H

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



Tomorrow: 5 nm spatial resolution with PEEM3

Lenses

CCD

Deflector

Separator

Lenses

Manipulator

High voltage feedthroughs

CCD -alignment

Electron

mirror




  • SASE gives 10 SOURCE (LCLS)6 intensity gain

  • over spontaneous emission

  • FELs can produce ultrafast

  • pulses (of order 100 fs)

l


Free electron lasers SOURCE (LCLS)

We are here

Growth of X-Ray Brightness and Magnetic Storage Density

each pulse:

1012 photons

< 100 fs

coherent


Lensless Imaging by Coherent X-Ray Scattering SOURCE (LCLS)

Eisebitt et al. (BESSY)

Challenge: Inversion from reciprocal to real space image



Experimental Principle of Ultrafast Field Pulses SOURCE (LCLS)

100 fs – 10 ps

  • Relativity allows 1010electrons 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, Science285, 864 (1999)


Torques on Magnetization by Beam Field SOURCE (LCLS)

Maximum torque

Minimum Torque


The Ultimate Speed of Magnetic Switching SOURCE (LCLS)

tpulse= 3 ps

tpulse= 100 fs

90 mm

90 mm

Deterministic switching

Chaotic switching

Under ultrafast excitation the magnetization fractures !


Magnetization fracture under ultrafast field pulse excitation
Magnetization fracture under ultrafast field pulse excitation

Uniform precession

chaotic excitation


The magnetism "team" – Stanford (SSRL) - Berkeley (ALS) excitation

Funded by: DOE-BES and NSF

Squaw Valley, April 2003

Missing: Hans Christoph Siegmann


Conclusions
Conclusions excitation

  • 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 excitation


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