Computational analysis of transient XMCD sum rules for laser pumped systems: When do they fail?

Abstract

In highly non-equilibrium conditions of laser induced spin dynamics, magnetic moments can only be obtained from the spectral information, most commonly from spectroscopy of core or semi-core states using the so-called x-ray magnetic circular dichroism (XMCD) sum rules. The validity of these sum rules in tracking femtosecond spin dynamics remains, however, an open question. Employing the time dependent extension of the density functional theory (TD-DFT), we compare spectroscopically obtained spin moments with those directly calculated from TD-DFT densities. We find that for pump pulses with low fluence, these two very distinct routes to the spin moment are, for Co and Ni, in excellent agreement, validating the experimental approach. However, for short and/or intense pulses with high fluence, the XMCD sum rules fail with errors exceeding 50%. This failure is most pronounced during the pulse and occurs when the pump pulse excites charges out of the d-band and into sp-character bands, invalidating the core or semi-core to d-state transitions assumed by the XMCD sum rules.