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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.

We present a detailed low-energy muon spin rotation and x-ray magnetic circular dichroism (XMCD) investigation of the magnetic structure in ultrathin tetragonal (T)-CuO films. The measured muon-spin polarization decay indicates an antiferromagnetic (AFM) order with a transition temperature higher than 200 K. The XMCD signal obtained around the Cu L 2,3 edges indicates the presence of pinned Cu 2+ moments that are parallel to the sample surface, and additionally, isotropic paramagnetic moments. The pinning of some of the Cu moments is caused by an AFM ordering consisting of moments that lie most likely in plane of the film. Moreover, pinned moments show a larger orbital magnetic moment contribution with an approximate ratio of m orb /m spin = 1.5, indicating that these spins are located at sites with reduced symmetry. Some fractions of the pinned moments remain pinned from an AFM background even at 360 K, indicating that T N > 360 K. We propose a simple model that explains these experimental findings, showing that the magnetic order of the T-CuO ultrathin films differs from the theoretical predictions.

Incomplete cancellation of collinear antiparallel spins gives rise to ferrimagnetism. Even if the oppositely polarized spins are owing to the equal number of a single magnetic element having the same valence state, in principle, a ferrimagnetic state can still arise from the crystallographic inequivalence of the host ions. However, experimental identification of such a state as "ferrimagnetic" is not straightforward because of the often tiny magnitude expected for M and the requirement for a sophisticated technique to differentiate similar magnetic sites. We report a synchrotron-based resonant x-ray investigation at the Fe L 2,3 edges on an epitaxial film of CaFe 2 O 4 , which exhibits two magnetic phases with similar energies. We find that while one phase of CaFe 2 O 4 is antiferromagnetic, the other one is ferrimagnetic with an antiparallel arrangement of an equal number of spins between two distinct crystallographic sites with very similar local coordination environments. Our results further indicate two distinct origins of an overall minute M; one is intrinsic, from distinct Fe 3+ sites, and the other one is extrinsic, arising from defective Fe 2+ likely forming weakly coupled ferrimagnetic clusters. These two origins are uncorrelated and have very different coercive fields. Hence, this work provides a direct experimental demonstration of ferrimagnetism solely due to crystallographic inequivalence of the Fe 3+ as the origin of the weak M of CaFe 2 O 4 .

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