Femtosecond X-ray Magnetic Circular Dichroism (fs XMCD)

The historic Einstein - de-Haas experiment demonstrated the equivalence of magnetism and angular momentum in 1915. Today it is known that in solids most of the magnetic moment is carried by the electron spin, and a smaller part by the orbital momentum of the electrons.

When a fs laser pulse is absorbed by a magnetic sample, a fast demagnetization is observed. We use Synchrotron radiation to investigate this process on the microscopic level, by means of XMCD. This gives us direct access to the spin and orbital angular momentum of the electrons. As the encountered processes are ultrafast, we rely on the FEMTOSPEX source at the BESSY II storage ring to deliver ultrashort 100 fs x-ray pulses.

The figure illustrates a fs pump-probe experiment on a 15 nm thin nickel film: the dichroic signals at the L3 and L2 absorption edges decrease within just a few 100 fs after laser excitation. Using the XMCD sum rules to evaluate spin and orbital angular momentum, we find that both decrease on the ultrafast time scale. As angular momentum is conserved, and analogous to Einstein – de-Haas, the angular momentum from spin and orbit is transferred to the crystal lattice on the same fs time scale.

References:

• C. Stamm, T. Kachel, N. Pontius, R. Mitzner, T. Quast, K. Holldack, S. Khan, C. Lupulescu, E.F. Aziz, M. Wietstruk, H. A. Dürr, W. Eberhardt
  Femtosecond modification of electron localization and transfer of angular momentum in Ni
  Nature Materials 6, 740 (2007). http://dx.doi.org/10.1038/nmat1985
• C. Stamm, N. Pontius, T. Kachel, M. Wietstruk, H. A. Dürr
  Femtosecond x-ray absorption spectroscopy of spin and orbital angular momentum in photoexcited Ni films during ultrafast demagnetization
  Phys. Rev. B 81, 104425 (2010), http://dx.doi.org/10.1103/PhysRevB.81.104425