Element-resolved ultrafast demagnetization rates in ferrimagnetic CoDy

Femtosecond-laser-induced ultrafast magnetization dynamics have been studied in multisublattice $\mathrm{C}{\mathrm{o}}_x\mathrm{D}{\mathrm{y}}_{1\ensuremath{-}x}$ 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 ($T=150\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) or above ($T=270\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), the temperature of magnetic compensation (${T}_{\mathrm{comp}}$). 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 ${T}_{\mathrm{comp}}$. This intriguing difference between the Dy $4f$ and Co $3d$ sublattices calls for further theoretical and experimental investigations.