Pressure-induced thermal expansion anomalies in dhcp iron hydride associated with magnetoelastic coupling.

Iron hydride with a double hexagonal close-packed structure (dhcp-FeHx) undergoes a ferromagnetic-paramagnetic transition without changing its crystal structure. Despite its relevance to metal-hydrogen interactions and magnetically driven elasticity, the extensive investigation of this phase is almost limited to room temperature. Here, we performed x-ray diffraction measurements at high pressure and high temperature, identifying the singularity in the temperature-volume relationship as the Curie temperature (TC). Pressurization lowered the TC of dhcp-FeHx and produced pronounced volume anomalies, indicating that pressure enhanced magnetoelastic coupling. Density functional theory combined with dynamical mean-field theory (DFT + DMFT) reproduced the spontaneous magnetization and its negative pressure dependence of TC, consistent with our experimental results. This establishes a methodology for determining magnetic transition temperatures and magnetoelastic coupling effects and highlights dhcp-FeHx as a unique model system for providing new insights into itinerant-electron magnetism.