Influence of laser-excited electron distributions on the X-ray magnetic circular dichroism spectra: Implications for femtosecond demagnetization in NiK. Carva, D. Legut and P. M. Oppeneer
Department of Physics and Materials Science, Uppsala University, Box 530 - S-75121 Uppsala, Sweden, EU
received 19 February 2009; accepted in final form 7 May 2009; published June 2009
published online 9 June 2009
In pump-probe experiments an intensive laser pulse creates non-equilibrium excited-electron distributions in the first few hundred femtoseconds after the pulse. The influence of non-equilibrium electron distributions caused by a pump laser on the apparent X-ray magnetic circular dichroism (XMCD) signal of Ni is investigated theoretically here for the first time, considering electron distributions immediately after the pulse as well as thermalized ones, that are not in equilibrium with the lattice or spin systems. The XMCD signal is shown not to be simply proportional to the spin momentum in these situations. The computed spectra are compared to recent pump-probe XMCD experiments on Ni. We find that the majority of experimentally observed features considered to be a proof of ultrafast spin momentum transfer to the lattice can alternatively be attributed to non-equilibrium electron distributions. Furthermore, we find the XMCD sum rules for the atomic spin and orbital magnetic moment to remain valid, even for the laser-induced non-equilibrium electron distributions.
78.47.J- - Ultrafast pump/probe spectroscopy (< 1 psec).
78.20.Ls - Magnetooptical effects.
78.70.Dm - X-ray absorption spectra .
© EPLA 2009