UH3-based ferromagnets: New look at an old material

Abstract

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 340pm. The stable form of UH3, known as β-UH3, has rather high TC≈170K. Such high value is rather unusual, considering dU–U=331pm. Properties of metastable α-UH3 with dU–U=360pm 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 TC value remains high and very similar to β-UH3. One can see an increase up to 187K for 15% Zr, followed by a weak decrease. Magnetic moments remain close to 1μB/U atom. An insight is provided by ab-initio calculations, revealing 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 (μ0Hc up to 5.5T) and large spontaneous volume magnetostriction of 3.2⁎10−3. Even higher increase of TC, reaching up to 203K, 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 dU–U but high TC. Although common hydrides are fine powders, some of the new hydrides described as (UH3)(1−x)Tx (T=Zr or Mo) remain monolithic, which allows to study transport and thermodynamic properties.