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Imaging Ultrafast Demagnetization Dynamics after a Spatially Localized Optical Excitation

When an ultrashort optical laser pulse excites a magnetic material it responds by an almost instantaneous reduction of its magnetization. Almost 20 years after its first observation, this phenomenon still fascinates and puzzles physicists because magnetization is directly connected to the conserved quantity of angular momentum, i.e. the ultrafast loss of magnetization must be accompanied by a concomitant transfer of angular momentum.  Therefore, the challenge that any explanation of ultrafast demagnetization dynamics faces refers to the complex transfer of energy and angular momentum between non-equilibrium electronic, spin and lattice degrees of freedom occurring on a femtosecond time scale. A large number of experimental and theoretical efforts suggest that both local (electron-phonon or impurity-mediated) spin-flip scattering events and non-local superdiffusive spin transport play a dominant role during the optically induced magnetization dynamics. More specifically, in the case of superdiffusive spin transport, energy- and spin-dependent electron lifetimes and velocities induce spin-polarized currents, leading to a significant ultrafast spatial rearrangement of magnetic order.

Figure 1.  Reconstructions of the magnetic domains for unpumped  (a)  and  pumped  samples  for  selected  time delays (b) – (d). The black and white areas within the elliptical field of view represent  domains  with  magnetization M aligned  either parallel or antiparallel to the incident X-ray beam direction. In the upper area of the elliptical object hole, the magnetic contrast is reduced.

Recent progress on all-optical control of ultrafast demagnetization and all-optical switching promises technological advances in magnetic data manipulation. However, both techniques will rely on nano-metre localization of light (i.e. far beyond its diffraction limit) as well as detailed knowledge on how non-local damping leads to a spatial transfer of magnetic information.  The unique properties of the free-electron laser FERMI at the scattering endstation DIPROI allowed for the first time to directly visualize the spatial and temporal evolution of magnetic order of a Cobalt based ferromagnet after a deliberately localized optical excitation. The ultrashort, coherent and circular polarized X-ray pulses at the Cobalt M-edge (58.9 eV) met all requirements for implementation of X-ray magnetic circular dichroism (XMCD) based Fourier-Transform-Holography to image the ultrafast changes of a magnetic domain pattern.

Figure 1 shows the reconstructed domain patterns for unpumped and pumped samples for selected delay times between optical excitation and holographic probing. The black and white areas within the elliptical field of view represent domains with magnetization M aligned either parallel or antiparallel to the incident X-ray beam direction. The magnetic domains of a width of ≈70 nm are clearly resolved. The XMCD contrast shows a significantly reduced magnetic contrast between oppositely magnetized domains in the upper part of the elliptical object hole, indicating a pronounced demagnetization by the locally enhanced optical excitation. 
Figure 2 shows the summed and normalized magnetic contrast of a 200 x 200 nm² large area centred at the maximum of the localized excitation. One clearly observes an initial ultrafast drop of magnetization followed by its recovery on a longer time scale, in very good agreement with typical magnetization transients.
Detailed analysis revealed that during the process of demagnetization (time <350 fs) the lateral profile increased in position and width by approximately 60 nm and 40 nm, respectively, which we interpret as superdiffusive spin currents altering the nano-scale domain network. This implies that engineering size and shape of magnetic domains will give further control on the time scale and spatial confinement of non-local demagnetization. Furthermore, using optical antennas, e.g. wedge-shaped metallic plates, for plasmonic driven light enhancement will allow nanoscale spatial control of the magnetic order, which will be particularly intriguing for localized all-optical switching applications in nanoscale devices.

Figure 2.  Summed and normalized magnetic contrast of a 200 x 200 nm² large area centred at the maximum demagnetization. One observes an ultrafast loss of magnetization followed by its recovery on a longer time scale.

This research was conducted by the following research team:

C. von Korff Schmising,1 B. Pfau,1 M. Schneider,1 C. M. Günther,1 M. Giovannella,1,2 J. Perron,3,4 B. Vodungbo,3,4 L. Müller,5 F. Capotondi,6 E. Pedersoli,6 N. Mahne,6J. Lüning,3,4and S. Eisebitt1,7,8

1   Institut für Optik und Atomare Physik, Technische Universität Berlin,  Berlin, Germany.
2   Dipartimento di Fisica, Università di Pisa,  Pisa, Italy.
3   Sorbonne Universités, UPMC Université Paris 06, Paris, France
4   CNRS, Paris, France
5   Deutsches Elektronen-Synchrotron DESY,  Hamburg, Germany
6   Elettra-Sincrotrone Trieste, Italy
7   Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin, Germany
8   Division of Synchrotron Radiation Research, Department of Physics, Lund University,  Lund, Sweden

Contact person:
Clemens von Korff Schmising: c.vonkorffschmising@tu-berlin.de
Stefan Eisebitt: eisebitt@physik.tu-berlin.de
 


Reference

 C. von Korff Schmising, B. Pfau, M. Schneider, C. M. Günther, M. Giovannella, J. Perron, B. Vodungbo, L. Müller, F. Capotondi, E. Pedersoli, N. Mahne, J. Lüning, and S. Eisebitt “Imaging Ultrafast Demagnetization Dynamics after a Spatially Localized Optical Excitation” Phys. Rev. Lett. 112, 217203 (2014); DOI:10.1103/PhysRevLett.112.217203

Last Updated on Wednesday, 02 July 2014 14:38