Energy and angular momentum transfer in ferromagnetic solids
Laser pulses have probed for the first time the spontaneous magnetization on the 10 -100 femtosecond time scale via the magneto-optic Kerr-effect or x-ray induced circular magnetic dichroism (XMCD). The experiments reveal a surprising phenomenon: Ultrafast loss of spin polarization of the valence electrons within several 100 fs with Ni and Co, and even more surprisingly, ultrafast all-optical magnetic switching. It seems to contradict earlier experience on the stability of the electron spin polarization in the electric crystal field. However, recent experiments with XMCD indicate that electron correlation in the magnetic transition metals is strong enough to invalidate the rigid band model when about 1% of the 3d-electrons are excited form the Fermi-level to higher lying states. This level of excitation is required when demagnetization is to be induced with short laser pulses. The breakdown of the rigid band model then makes it impossible to fully understand the magneto-optic experiments whose interpretation is based on the constancy of the band states.
In order to study ultrafast magnetization dynamics which is at the root of understanding magnetism in the transition metals with its many applications, we plan a different experiment that has a time resolution given by the duration of the laser pulse as well, but can determine the magnetization without relying on the rigid band model. We intend to measure the spin polarization of electrons photoemitted from the magnetic sample hit by an intense x-ray laser pulse from LCLS.