UH sub(3)-based ferromagnets: New look at an old material

• Published in: Journal of magnetism and magnetic materials Vol. 400; pp. 130 - 136
• Main Authors: Havela, L, Paukov, M, Tkach, I, Matej, Z, Kriegner, D, Maskova, S, Vondrackova, B, Pracharova, M, Turek, I, Divis, M, Cieslar, M, Drozdenko, D, Kim-Ngan, N-TH, Andreev, A V
• Format: Journal Article
• Language: English
• Published: 01-02-2016
• Subjects: Alloys; Body centered cubic lattice; Coercive force; Ferromagnetism; Hydrides; Interstitials; Magnetism; Order disorder
• ISSN: 0304-8853
• DOI: 10.1016/j.jmmm.2015.07.053

UH sub(3) 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 UH sub(3), known as beta -UH sub(3), has rather high T sub(C) approximately 170 K. Such high value is rather unusual, considering d sub(U-U)=331 pm. Properties of metastable alpha -UH sub(3) with d sub(U-U)=360 pm could be never well established. Using the fact that alpha -UH sub(3) is in fact bcc U with interstitials filled by H, we attempted to synthesize alpha -UH sub(3) starting from the gamma -U alloys, with the bcc structure retained to room temperature by doping combined with ultrafast cooling. While up to 15% Zr a contamination by beta -UH sub(3) was obtained, 20% Zr yielded single phase alpha -UH sub(3). The T sub(C) value remains high and very similar to beta -UH sub(3). One can see an increase up to 187 K for 15% Zr, followed by a weak decrease. Magnetic moments remain close to 1 mu sub(B)/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 ( mu sub(0)H sub(c) up to 5.5 T) and large spontaneous volume magnetostriction of 3.2*10 super(-3). Even higher increase of T sub(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 U sub(6)Fe and U sub(6)Co, a new group of robust 5f ferromagnets with small d sub(U-U) but high T sub(C). Although common hydrides are fine powders, some of the new hydrides described as (UH sub(3)) sub((1-x))T sub(x) (T=Zr or Mo) remain monolithic, which allows to study transport and thermodynamic properties.