Intersite Charge Transfer through Eu 4f Band Engineering in Eu_1–xSr_xVO₂H Oxyhydride Films

Abstract

Perovskite oxyhydride EuVO 2 H exhibits a Mott insulating state derived from V III O 2 layers and a ferromagnetic state derived from Eu II H layers, with compressive strain inducing intersite charge transfer (ICT) in thin films, accompanied by an increased ferromagnetic transition temperature (T C ) and the emergence of perpendicular magnetic anisotropy (PMA). While ICT has previously been adjusted by film thickness (or substrate strain), this study demonstrates its control by Eu-site substitution with Sr. Utilizing topochemical hydride reduction, we fabricated Eu 1−x Sr x VO 2 H solid solution thin films on SrTiO 3 substrates, providing −0.6% compressive strain. With an increasing Sr content, the Eu valence remains almost unchanged, indicating a reduced level of electron transfer from the Eu 4f band to the V 3d xy band. This observation is ascribed to the reduced dispersion of the Eu 4f band, in contrast with our previous study on EuVO 2 H in which the empty V 3d xy bandwidth was manipulated by film thickness, manifesting the possibility of independent control over the d band and f band, an achievement unexplored in previous oxide research. Sr substitution not only decreases T C but also suppresses PMA. In addition, the application of high pressure to Eu 0.9 Sr 0.1 VO 2 H powder induces a continuous valence evolution of Eu, as opposed to that of pristine EuVO 2 H, which undergoes a first-order valence transition. This study offers possibilities for independent control of the electronic structure of systems containing both transition metals and rare earth elements.