UH-based ferromagnets: new look at an old material.

UH is the first discovered material with ferromagnetism based purely on the 5 electronic states, known for more than half century. Although the U metal is Pauli paramagnet, the reduced 5-5 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 UH, known as β-UH, has rather high ≈ 170 K. Such high value is rather unusual, considering = 331 pm. Properties of metastable α-UH with = 360 pm could be never well established. Using the fact that α-UH is in fact U with interstitials filled by H, we attempted to synthesize α-UH starting from the γ-U alloys, with the structure retained to room temperature by doping combined with ultrafast cooling. While up to 15% Zr a contamination by β-UH was obtained, 20% Zr yielded single phase α-UH. The value remains high and very similar to β-UH. One can see an increase up to 187 K for 15% Zr, followed by a weak decrease. Magnetic moments remain close to 1 μ/U atom. An insight is provided by calculations, revealing a a charge transfer towards H-1 states, depopulating the U-6 and 7 states, leaving almost pure 5 character around the Fermi level. The 5 magnetism exhibits a high coercivity (μ up to 5.5 T) and large spontaneous volume magnetostriction of 3.2*10. Even higher increase of , 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 UFe and UCo, a new group of robust 5 ferromagnets with small but high . Although common hydrides are fine powders, some of the new hydrides described as (UH)T (T = Zr or Mo) remain monolithic, which allows to study transport and thermodynamic properties.