Pressure Dependence of Ferromagnetism in Uranium Hydride UH$_3$ up to 20 GPa Revealed by NV Centers Magnetometry

Abstract: The origin of ferromagnetism in the stable $β$-UH$_3$ phase is still debated. With a high Curie temperature and a short uranium-uranium interatomic distance, $β$-UH$_3$ is placed outside the known systematics of uranium compounds. Pressure provides a powerful means of tuning magnetism toward its suppression, thereby offering valuable insights into its underlying mechanisms, yet the high-pressure behavior of $β$-UH$_3$ has remained largely unexplored. Here, we combine in the Diamond Anvil Cell (DAC) the development of pure $β$-UH$_3$ synthesis and widefield nitrogen-vacancy (NV) center magnetometry to measure the pressure dependence of the Curie temperature in $β$-UH$_3$ up to about 20~GPa. We present two data analysis methods, vector magnetic field reconstruction and statistical analysis of the optically detected magnetic resonance (ODMR) response, which enable us to directly image the magnetic dipole of $β$-UH$_3$ during isobaric warming. We observe a linear decrease in the Curie temperature with pressure, yielding a slope of $\text{d}T_C/\text{d}P=-3.86\,(8)$~K/GPa. Ferromagnetism in $β$-UH$_3$ is predicted to reach 0~K at approximately 44~GPa, where a quantum critical point may emerge.