Magnetism and X-Rays:
This presentation is the property of its rightful owner.
Sponsored Links
1 / 48

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


  • 50 Views
  • Uploaded on
  • Presentation posted in: General

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.

Download Presentation

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

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Magnetism and x rays past present and a vision of the future

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


Magnetism and x rays past present and a vision of the future

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!


Magnetism and x rays past present and a vision of the future

Some Magnetic Devices in Computers

Present: Size > 100 nm, Speed > 1 nsec

Future: Size < 100 nm, Speed < 1 nsec

Ultrafast Nanoscale Dynamics


Magnetism and x rays past present and a vision of the future

Experimental X-Ray Methods


Magnetism and x rays past present and a vision of the future

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)


Magnetism and x rays past present and a vision of the future

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)


Magnetism and x rays past present and a vision of the future

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)


Magnetism and x rays past present and a vision of the future

Fe metal – L edge

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


Magnetism and x rays past present and a vision of the future

Magnetic Spectroscopy and Microscopy

x-ray "spin"

Soft X-Rays are best for magnetism!


Magnetism and x rays past present and a vision of the future

bulk

surface


Magnetism and x rays past present and a vision of the future

PEEM-2 at ALS

  • Full Field Imaging

  • Electrostatic (30 kV)

  • 20 - 50 nm Resolution

  • Linear and circular polarization


Magnetism and x rays past present and a vision of the future

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


Magnetism and x rays past present and a vision of the future

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).


Magnetism and x rays past present and a vision of the future

  • Experimental Results:

  • Exchange bias

  • Time resolved imaging of magnetic structures


Magnetism and x rays past present and a vision of the future

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


Magnetism and x rays past present and a vision of the future

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


Magnetism and x rays past present and a vision of the future

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 ?


Magnetism and x rays past present and a vision of the future

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


Magnetism and x rays past present and a vision of the future

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


Magnetism and x rays past present and a vision of the future

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


Magnetism and x rays past present and a vision of the future

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


Magnetism and x rays past present and a vision of the future

Nanaoscale Magnetization Dynamics - Smaller and Faster


Magnetism and x rays past present and a vision of the future

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


Magnetism and x rays past present and a vision of the future

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


Magnetism and x rays past present and a vision of the future

Magnetic Patterns in 20 nm Co90Fe10 films on waveguide

M

3mm

Field

pulse

x-ray

"spin"


Magnetism and x rays past present and a vision of the future

Two pattern with same static structure, but …..

Field

response

Field

response

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


Magnetism and x rays past present and a vision of the future

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!


Magnetism and x rays past present and a vision of the future

  • A Vision of the Future……..

  • Improved microscopes – toward atomic resolution

  • X-ray lasers - ultrafast single shot imaging

  • ……..


Magnetism and x rays past present and a vision of the future

Tomorrow: 5 nm spatial resolution with PEEM3

Lenses

CCD

Deflector

Separator

Lenses

Manipulator

High voltage feedthroughs

CCD -alignment

Electron

mirror


Magnetism and x rays past present and a vision of the future

Spatial Resolution of PEEM3

4-5 nm


Magnetism and x rays past present and a vision of the future

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

0 Km

2 Km

3 Km


Magnetism and x rays past present and a vision of the future

  • SASE gives 106 intensity gain

  • over spontaneous emission

  • FELs can produce ultrafast

  • pulses (of order 100 fs)

l


Magnetism and x rays past present and a vision of the future

Free electron lasers

We are here

Growth of X-Ray Brightness and Magnetic Storage Density

each pulse:

1012 photons

< 100 fs

coherent


Magnetism and x rays past present and a vision of the future

Lensless Imaging by Coherent X-Ray Scattering

Eisebitt et al. (BESSY)

Challenge: Inversion from reciprocal to real space image


Magnetism and x rays past present and a vision of the future

  • A Glimpse of the Future……..

  • Ultrafast magnetic processes


Magnetism and x rays past present and a vision of the future

Experimental Principle of Ultrafast Field Pulses

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)


Magnetism and x rays past present and a vision of the future

Torques on Magnetization by Beam Field

Maximum torque

Minimum Torque


Magnetism and x rays past present and a vision of the future

The Ultimate Speed of Magnetic Switching

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


Magnetism and x rays past present and a vision of the future

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

Funded by: DOE-BES and NSF

Squaw Valley, April 2003

Missing: Hans Christoph Siegmann


Conclusions

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)


Magnetism and x rays past present and a vision of the future

The end


  • Login