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.
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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)