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We report a soft x-ray resonant magnetic scattering study of the spin configuration in multiferroic thin films of Co 0.975 Ge 0.025 Cr 2 O 4 (Ge-CCO) and CoCr 2 O 4 (CCO) under low and high magnetic fields from 0.2 to 6.5 T. A characterization of Ge-CCO at a low magnetic field was performed, and the results were compared with those of pure CCO. The ferrimagnetic phase transition temperature T C ≈ 95 K and the multiferroic transition temperature T S ≈ 27 K in Ge-CCO are comparable with those observed in CCO. In Ge-CCO, the ordering wave vector (qq0) observed below T S is slightly larger compared with that of CCO, and unlike CCO, the diffraction intensity consists of two contributions that show a dissimilar x-ray polarization dependence. In Ge-CCO, the coercive field observed at low temperatures was larger than the one reported for CCO. In both compounds, an unexpected reversal of the spiral helicity, and therefore the electric polarization, was observed on simply magnetic field cooling. In addition, we find a change in the helicity as a function of momentum transfer in the magnetic diffraction peak of Ge-CCO, indicative of the presence of multiple magnetic spirals.

We report on element-resolved ultrafast magnetization dynamics in multiferroic CoCr 2 O 4 and Co 0.975 Ge 0.025 Cr 2 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.