Ultrafast probe of magnetization dynamics in multiferroic CoCr2O4 and Co0.975Ge0.025Cr2O4

Abstract

We report on element-resolved ultrafast magnetization dynamics in multiferroic ${\mathrm{CoCr}}_{2}{\mathrm{O}}_{4}$ and ${\mathrm{Co}}_{0.975}{\mathrm{Ge}}_{0.025}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ after optical excitation above the electronic band gap. We observe demagnetization dynamics in the range of several picoseconds, up to two orders of magnitude faster than previously reported demagnetization in other ferrimagnetic insulators. Moreover, we find that the dynamics of the two magnetic ions differ significantly just below the Curie point. The dynamics of the low-temperature multiferroic phase are almost two times slower than those in the ferrimagnetic phase. This suggests that the additional magnetic cycloidal component, which is coupled to electric polarization at low temperatures, might influence the ultrafast magnetization dynamics.