Ultrafast demagnetization in Ni: theory of magneto-optics for non-equilibrium electron distributions

The sensitivity of the magneto-optical Kerr response to electronic thermalization processes in ultrafast pump–probe experiments is studied by evaluating the complex conductivity tensor of Ni for non-equilibrium electron distributions. The electronic structure and optical matrix elements are calculated within density functional theory. To account for the electronic redistributions generated by the intense pump-laser pulse during the initial stages of electronic thermalization, two kinds of model electron distributions are considered which mimic the so-called dichroic bleaching or state-blocking effect. Thus, certain optical transitions which are allowed under equilibrium conditions are not accessible to the probe laser. It is shown that the conductivity tensor and the complex Kerr angle can be modified substantially by the nonequilibrium electron distributions. Moreover, in striking contrast to the case for ordinary equilibrium conditions, the Kerr rotation and ellipticity are no longer proportional to the magnetization of the sample. The Kerr response at ultrashort times can therefore not be taken as a measure of demagnetization. (Some figures in this article are in colour only in the electronic version)