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We report on element-resolved ultrafast magnetization dynamics in multiferroic CoCr2O4 and Co0.975Ge0.025Cr2O4 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. ISSN:1098-0121 ISSN:0163-1829 ISSN:1550-235X ISSN:0556-2805 ISSN:2469-9969 ISSN:1095-3795 ISSN:2469-9950

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 T up to 6.5 T. A characterization of Ge-CCO at a low magnetic field is performed and the results are compared to those of pure CCO. The ferrimagnetic phase transition temperature $T_C \approx 95$ K and the multiferroic transition temperature $T_S \approx 27$ K in Ge-CCO are comparable to those observed in CCO. In Ge-CCO, the ordering wave vector $\textit{(qq0)}$ observed below $T_S$ is slightly larger compared to 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., Comment: 20 pages, 12 figures

Ultrafast electron delocalization induced by a femtosecond laser pulse is a well-known process in which electrons are ejected from the ions within the laser pulse duration. However, very little is known about the speed of electron localization out of an electron gas in correlated metals, i.e., the capture of an electron by an ion. Here, we demonstrate by means of pump-probe x-ray techniques across the Eu L-3 absorption edge that an electron localization process in the EuNi2(Si0.27Ge0.79)(2) intermetallic material occurs within a few hundred femtoseconds after the optical excitation. Spectroscopy and diffraction data collected simultaneously at low temperature and for various laser fluences show that the localization dynamics process is much faster than the thermal expansion of the unit cell along the c direction which occurs within picoseconds. Nevertheless, this latter process is still much slower than pure electronic effects, such as screening, and the subpicosecond timescale indicates an optical phonon driven origin. In addition, comparing the laser fluence dependence of the electronic response with that found in other intermediate 4f valence materials, we suggest that the electron localization process observed in this Eu-based correlated metal is mainly related to changes in the 4f hybridization. The observed ultrafast electron localization process sparks fundamental questions for our understanding of electron correlations and their coupling to the lattice., Physical Review Research, 3 (3), ISSN:2643-1564

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 L2,3 edges on an epitaxial film of CaFe2O4, which exhibits two magnetic phases with similar energies. We find that while one phase of CaFe2O4 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 Fe3+ sites, and the other one is extrinsic, arising from defective Fe2+ 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 Fe3+ as the origin of the weak M of CaFe2O4., The resonant x-ray diffraction experiments were performed at the X11MA beamline in the Swiss Light Source under Proposal No. 20191307 and at the BL17SU in the SPring-8 under Proposal No. 20200012. The x-ray magnetic circular dichroism measurements were performed at the XTreme beamline in the Swiss Light Source during in-house access. H.U. acknowledges the National Centers of Competence in Research in Molecular Ultrafast Science and Technology (NCCR Grant MUST-No. 51NF40-183615) from the Swiss National Science Foundation and from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 801459–FP-RESOMUS. E.S. is supported by the NCCR Materials’ Revolution: Computational Design and Discovery of Novel Materials (NCCR MARVEL Grant No. 182892) from Swiss National Foundation and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 884104 (PSI-FELLOW-III-3i). N.O.H. acknowledges financial support of the Swiss National Science Foundation, Grant No. 200021_169017. M.B. is supported by the Swiss National Science Foundation through Projects No. 200021-196964 and 200021_169698, respectively. Financial support by the Groningen Cognitive Systems and Materials Center (CogniGron) and the Ubbo Emmius Funds of the University of Groningen is also gratefully acknowledged, With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).