UH₃-based ferromagnets: new look at an old material

UH₃ is the first discovered material with ferromagnetism based purely on the 5f electronic states, known for more than half century. Although the U metal is Pauli paramagnet, the reduced 5f-5f overlap in compounds allows for moment formation and ordering, typically if the U-U spacing exceeds the Hill limit, i.e. about 340 pm. The stable form of UH₃, known as β-UH₃, has rather high T_C ≈ 170 K. Such high value is rather unusual, considering d_U-U = 331 pm. Properties of metastable α-UH₃ with d_U-U = 360 pm could be never well established. Using the fact that α-UH₃ is in fact bcc U with interstitials filled by H, we attempted to synthesize α-UH₃ starting from the γ-U alloys, with the bcc structure retained to room temperature by doping combined with ultrafast cooling. While up to 15% Zr a contamination by β-UH₃ was obtained, 20% Zr yielded single phase α-UH₃. The T_C value remains high and very similar to β-UH₃. One can see an increase up to 187 K for 15% Zr, followed by a weak decrease. Magnetic moments remain close to 1 micro_B/U atom. An insight is provided by ab-initio calculations, revealing a a charge transfer towards H-1s states, depopulating the U-6d and 7s states, leaving almost pure 5f character around the Fermi level. The 5f magnetism exhibits a high coercivity (micro₀H_c up to 5.5 T) and large spontaneous volume magnetostriction of 3.2*10^-3. Even higher increase of T_C, reaching up to 203 K, can be achieved in analogous Mo stabilized hydrides, which yield an amorphous structure. The compounds represent, together with known hydrides of U₆Fe and U₆Co, a new group of robust 5f ferromagnets with small d_U-U but high T_C. Although common hydrides are fine powders, some of the new hydrides described as (UH₃)_(1-x)T _x (T = Zr or Mo) remain monolithic, which allows to study transport and thermodynamic properties.