Element-resolved ultrafast demagnetization rates in ferrimagnetic CoDy

Abstract

Femtosecond-laser-induced ultrafast magnetization dynamics have been studied in multisublattice CoxDy1-x alloys. By performing element- and time-resolved x-ray spectroscopy, we distinguish the ultrafast quenching of Co 3d and Dy 4f magnetic order when the initial temperatures are below (T=150K) or above (T=270K), the temperature of magnetic compensation (Tcomp). In accordance with former element-resolved investigations and theoretical calculations, we observe different dynamics for Co 3d and Dy 4f spins. In addition, we observe that for a given laser fluence, the demagnetization amplitudes and demagnetization times are not affected by the existence of a temperature of magnetic compensation. However, our experiment reveals a twofold increase of the ultrafast demagnetization rates for the Dy sublattice at low temperature. In parallel, we measure a constant demagnetization rate of the Co 3d sublattice above and below Tcomp. This intriguing difference between the Dy 4f and Co 3d sublattices calls for further theoretical and experimental investigations.