UH3-based ferromagnets: New look at an old material

Abstract

UH 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 3 , known as β-UH 3 , has rather high T C ≈ 170 K. Such high value is rather unusual, considering d U-U = 331 pm. Properties of metastable α-UH 3 with d U-U = 360 pm could be never well established. Using the fact that α-UH 3 is in fact bcc U with interstitials filled by H, we attempted to synthesize α-UH 3 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-UH 3 was obtained, 20% Zr yielded single phase α-UH 3 . The T C value remains high and very similar to β-UH 3 . One can see an increase up to 187 K for 15% Zr, followed by a weak decrease. Magnetic moments remain close to 1 μ 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 (μ 0 H 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 U 6 Fe and U 6 Co, 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 (UH 3 ) (1-x) T x (T = Zr or Mo) remain monolithic, which allows to study transport and thermodynamic properties.

Hydrogen in actinides: electronic and lattice properties

“…In the case of α-UH 3 , this is a result of orbital moment of 3.0 µ B /atom and spin moment in the opposite direction, i.e. −2.0 µ B /atom, being in a very good agreement with measurements [10,21,86]. In the case of β-UH 3 , the uranium atoms occupy two Wyckoff positions, 2a as in the former hydrides, and 6c.…”

“…The size of the increase does, however, depend on the alloying element (see figure 2). The largest was found for Mo, where T C exceeds 200 K [21,22]. The influence of Mo, which is unlikely to contribute by sizeable own magnetic moments, is arguably only indirect, modulating the H/U ratio, when one U atom of the U 4 tetrahedra is replaced by Mo.…”

“…More insight into the structure details was obtained by the PDF analysis [20]. So far, only the Zr alloying gives the α-UH 3 structure, which may link to the larger atomic radius of Zr compared to U, while other transition metals used have all the atomic diameters smaller [21].…”

Crystal Structure and Magnetic Properties of Uranium Hydride UH2 Stabilized as a Thin Film

“…Not only that it is not suppressed by alloying of late d-metals as Fe or Co, which tend to be magnetic (although often destabilize 5f magnetism by strong 5f-3d hybridization), but also by doping of early dmetals as Mo or Zr, which carry very seldom their own magnetic moments. 17 The fact that (UH 3 )-Mo compounds have T C values exceeding 200 K is truly a unique situation, underlined by the fact that the highest T C among U ferromagnets is only marginally higher (216 K at UCu 2 P 2 ). 18 In such a situation it is very important to synthesize other possible U hydrides and explore their properties.…”

Hydrogen absorption in uranium-based alloys with cubic γ -U structure

“…Such nanocrystalline hydrides with typical grain size of 2 nm are formed for all Mo concentration x ⩾ 0.12, i.e. for the precursor alloys with bcc structure [17].…”