Electron magnetic circular dichroism

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Electron magnetic circular dichroism
Pavel Novák Institute of Physics ASCR, Prague,
Czech Republic
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Scope

• Motivation
• Short history
• XMCD X-ray magnetic circular dichroism
• EMCD electron magnetic circular dichroism
• Modelling of experiment
• Results
• Outlook
• Conclusions

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Motivation
Characterization of very smal magnetic objects (
10 nm)
Necessity of very short wavelengths
X-ray magnetooptics XMCD X-ray Magnetic
Circular Dichroismus predicted 1975 experim
ental verification 1987 first possibility to
determine separately spin and orbital magnetic
moment Disadvantage necessity of synchrotron
Is it possible to obtain analogous information
using electron microscope?
Positive answer in principle study of
subnanometric objects possible
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Short history
2003 Peter Schattschneider et al. (TU Vienna)
basic idea of EMCD
EU projektu CHIRALTEM submited Chiral
Dichroism in the Transmission Electron
Microscope invitation to our group to
participate as theoretical support
2004 project approved within program NEST 6
Adventure
Our group Ján Rusz, Pavel Novák, Jan Kune,
Vladimír Kamberský
2005 experimental verification, microscopic
theory, first workshop
2006 paper in Nature, second workshop
2007 sensitivity increased by order of
magnitude planned third workshop, closing the
project
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Circular magnetic dichroism

• Circular dichroism
• absorption spectrum of polarized light is
  different
• for left and right helicity

?
Symmetry with respect to time inversion must be
broken magnetic field magnetically ordered
systems
Microscopic mechanism inelastic diffraction of
light, electric dipol transitions coupling of
light and magnetism spin-orbit interaction
X-ray circular dichroism circular dichroism in
the X-ray region
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XANES and XMCD
XANES X-ray near edge spectroscopy
Transition of an electron from the core level of
an atom to an empty state
Crosssection of XANES
polarization vector
XMCD X-ray magnetic circular dichroism differen
ce of XANES spectra for left and right helicity
,
Selection rules Orbital moment L -gt
L1 ?ML 0, 1
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L-edge iron spectrum
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Comparison Energy Loss Near Edge Spectroscopy
(ELNES) and X-ray Absorption Near Edge
Spectroscopy (XANES)
ELNES inelastic scattering of the fast electrons
transition from the core state of an atom to an
empty state
Diferential cross section
ELNES
XANES
polarization vector
(ELNES)
(XANES) is equivalent to
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Comparison ELNES and XANES
XANES ELNES
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EMCD
Problem of EMCD how to obtain in the position of
an atom the circularly polarized electric
field
Solution (Schattschneider et al. 2003) it is
necessary to use two coherent, mutually
perpendicular, phase shifted electron beams
(preferably the phase shift p/2)
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EMCD
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EMCD
Differential cross section
Mixed dynamical form factor
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Mixed dynamic form factor (MDFF)
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Coherent electron beams first way (Dresden)
External beam splitter possibility to study
arbitrary object
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Coherent electron beams second way (Vienna)
crystal as a beam splitter limitation single
crystals
Electron source
incoming electron beam-plane wave wave vector k
in crystal S(Bloch state), in k, kG, k2G .
in crystal S(Bloch state), out
outcoming electron beam-plane waves k, kG, k2G
..
detector
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Coherent electron beams second way
Two positions A, B of detector in the
diffraction plane
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Modelling the experiment crystal as a beam
splitter
1/ Microscopic calculation of MDFF

• Program package based on WIEN2k
• calculation of the band structure
• matrix elements
• Brillouin zone integration, summation

2/ Electron optics originally program package
IL5 (M. Nelhiebel, 1999) new program package
DYNDIF
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Modelling the experiment crystal as a beam
splitter
Electron optics

• more general (eg. it includes higher order Laue
  zones )
• more precise potentials, possibility to use
  ab-initio potentials
• can be used for all type of ELNES

DYNDIF

• DYNDIF includes experimental conditions
• angle of incident electron beam
• detector position, thickness of the sample
• results depend on the structure and
• composition of the system

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Results
First result EMCD L edge of iron
XMCD EMCD Calculation
P.Schattschneider, S.Rubino, C.Hébert, J. Rusz,
J.Kune, P.Novák, E.Carlino, M.Fabrizioli,
G.Panaccione, G.Rossi, Nature 441, 486 (2006)
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Results of simulation dichroic maps
Dependence of the amplitude of dichroism on
detector position
fcc Ni qx, qy, ?x, ?y determine the angle
of incoming electron beam
qy
qx
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Results dependence on the thickness of the sample
bcc Fe
ELNES(1)
ELNES(2)
EMCD ELNES(1)-ELNES(2)
hcp Co
EMCD
Exp. EMCD
fcc Ni
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New way of EMCD measurement with order of
magnitude increased signal/noise ratio
hcp Co, thickness 18 nm
Dichroic signal as a function of the diffraction
angle (in units of G)
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Outlook

• strongly correlated electron systems
• band model is inadequate for electron structure
  determination
• necessity to use effective hamiltonian for MDFF
  calculation
• electron optics (DYNDIF) unchanged

• program DYNDIF after user friendly
  modification part of the
• WIEN2k package

• sum rules for EMCD (determination of spin and
  orbital moment)

• Using the princip of EMCD for electron holography

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Conclusion
EMCD new spectroscopic method with potentially
large impact in nanomagnetism
Computer modelling increasingly important part
of the solid state physics
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Thanks to the CHIRALTEM project
and to all present for their attention