Static image

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

Joachim Stöhr Stanford Synchrotron Radiation Laboratory Stanford University Femtosecond single shot image 100 picoseconds dynamics

1993 2003

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

200X

Past:

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.

"

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

Experimental X-Ray Methods

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. B

34

, 6529 (1986)

X-Ray Magnetic Circular Dichroism:

G. Sch ü tz, W. Wagner, W. Wilhelm, P. Kienle, R. Zeller, R. Frahm, G. Materlik: Phys. Rev. Lett.

58

, 737 (1987)

X-Ray Magnetic Imaging:

J. St ö hr, 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 M agnetic C ircular D ichroism ( 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

P o la riz ed X -ra ys

• Full Field Imaging • Electrostatic (30 kV) • 20 - 50 nm Resolution • Linear and circular polarization

Element Specific Magnetic Imaging: Ferromagnetic Domains in Magnetite – Magnetic Fe and Oxygen

Magnetite Fe 3 O 4 9 6 3 Fe 700 710 720 Photon Energy (eV)

I

+ I 1.3

1.2

1.1

1.0

Oxygen 528 530 532 Photon Energy [eV] 12 m m

Spectro-Microscopy of Ferromagnets on Antiferromagnets

Tune to

Co

edge – use

circular

polarization – ferromagnetic domains 8 Co XMCD H. Ohldag

et al.,

PRL

86

, 2878 (2001).

4 s 0 776 778 Photon Energy (eV) 780 Tune to

Ni

edge – use

linear

polarization – antiferromagnetic domains 15 NiO XMLD 10 5 [010] s 0 2 m m 870 874 Photon Energy(eV)

Experimental Results:

• Exchange bias • Time resolved imaging of magnetic structures

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 W

Blue layer:

direction is fixed by exchange bias

Red layer:

direction determines resistance

1.0

0.5

0.0

-0.5

-1.0

-1500 -1000 -500 0 500 1000

Applied Field (Oe)

1500 Conventional techniques cannot study the magnetic FM-AFM interface

The Basic Model – Meiklejohn (~ 1960)

Bulk FM spins:

S

1 Exchange coupling: E 12 = J 1 2

S

1

S 2

Uncompensated spins:

S 2

Bulk AFM spins:

S

2 =

S

2 E 22 = J 2 2

S

2

S

2 & anisotropy of AFM E K

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

S 2

• t wist of AFM spins – domain wall with energy  E 22 E k

50 years of models…need experimental tests… Reduce bias through effective

S AFM S AFM

: uncompensated spins near AFM surface

Origin ? Number ?

Size ?

Parallel or perpendicular ?

Malozemoff model Koon model  E 22 E k : Domain wall energy

Domain wall formation ?

Reduce bias through domain wall Mauri-Siegmann model

Co on NiO(001)

[010] s s

2

m

m Bare NiO(001) NiO after deposition 2nm Co on NiO(001) Co causes Ni spins at NiO surface to rotate into plane AFM and FM spins couple parallel

s

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 NiO Co Ni –rich NiO NiO

Interfacial spins

AFM: NiO

Linear pol. Ni edge

FM: Ni-rich NiO

Circular pol. Ni edge

FM: Co

Circular pol. Co edge

Chemically induced interfacial Ni spins provide the magnetic link

Co NiO

X-Ray Picture of Exchange Bias

The role of interfacial spins:

S

AFM

Co/NiO

-0.1

-0.2

-0.3

0.2

0.1

0.0

-5 -10 -15 0.3

15 10 5 0 Co M n -3k -2k -1k 0 1k Applied Field (Oe) 2k 3k pinned spins Imaging: Element specific FM loops:  AFM axis is rotated at interface  The interface is not sharp -

S

AFM 

S

AFM ||

S

FM  Free spins: 96% of ML – coercivity  Pinned spins

S

AFM :

4% of ML

 Small number determines bias size

Nanaoscale Magnetization Dynamics - Smaller and Faster

Time resolved x-ray microscopy

PEEM2 50 nm / 100 ps resolution Laser pump – x-ray probe synchronization 

t

excitation laser pulse observation x-ray pulse

328 ns < 1 ps < 100 ps

Production of Magnetic Field Pulses Photoconductive switch 100 m m 2 m m 2 m m 50 W

Conducting wire

=> I = 200 mA, 10 V bias

Current

10 m m

Magnetic Cells H

~ 200 Oe

Magnetic Patterns in 20 nm Co 90 Fe 10 films on waveguide

M

3 m m

x-ray "spin"

Field pulse

Field response

Two pattern with same static structure, but …..

Field response

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

H

Response to a fast field pulse

Instanteneous precession determined by torque:

T

=

H

x

m

slow

"damping"

fast (<1ns)

"precession" m T H Tiny vortex core determines fast dynamics of the whole domain structure!

A Vision of the Future……..

•

Improved microscopes

– toward atomic resolution •

X-ray lasers

- ultrafast single shot imaging ……..

Tomorrow

: 5 nm spatial resolution with PEEM3

Lenses Manipulator Separator Deflector Lenses CCD High voltage feedthroughs CCD -alignment Electron mirror

Spatial Resolution of PEEM3

4-5 nm

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

0 Km 2 Km 3 Km

l •

SASE gives 10 6 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

each pulse :

10 12 photons < 100 fs coherent

We are here

Lensless Imaging by Coherent X-Ray Scattering

Eisebitt et al. (BESSY) Challenge: Inversion from reciprocal to real space image

A Glimpse of the Future……..

• Ultrafast magnetic processes

Experimental Principle of Ultrafast Field Pulses

100 fs – 10 ps

• Relativity allows 10 10

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)

Torques on Magnetization by Beam Field

Maximum torque Minimum Torque

The Ultimate Speed of Magnetic Switching t pulse =

3 ps

t pulse =

100 fs 90

m

m Deterministic switching 90

m

m Chaotic switching Under ultrafast excitation the magnetization fractures !

Magnetization fracture under ultrafast field pulse excitation Uniform precession chaotic excitation

The magnetism "team" – Stanford (SSRL) - Berkeley (ALS) Funded by: DOE-BES and NSF Squaw Valley, April 2003 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