Pressure variations of the 5f magnetism in UH₃

Pressure variations of the Curie temperature of the 5f ferromagnet β-UH₃ were studied using the Mo-alloyed hydride (UH₃)_0.82Mo_0.18, which is stable in air and has very similar T_C and magnetization per U atom. By means of ac magnetic susceptibility a linear decrease of T_C was observed for pressures up to 3.2 GPa. The coefficient dT_C/dp = −2.05 K/GPa gives dlnT_C/dp = 1/T*dT_C/dp ≈ −0.011 GPa⁻¹. This value is smaller than expected for a 5f-band ferromagnet with relatively short U-U distances and suggests that UH₃ may be more localized than expected. Among AnX compounds, similar dependence was found e.g. for US. Revisiting existing data on lattice elasticity for β-UH₃, bulk modulus B ≈ 100 GPa can be assumed, leading to dlnT_C/dlnV = 1.1. Experimental data are confronted with results of GGA + U electronic structure calculations. Plausible values of direct Coulomb U and Hund's exchange J are deduced. The lattice compression was found to reduce predominantly the orbital moments.