S. Mašková

UH 3 -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 TC ≈ 170 K. Such high value is rather unusual, considering dU-U = 331 pm. Properties of metastable α-UH3 with dU-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 TC 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 μB/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 (μ0Hc up to 5.5 T) and large spontaneous volume magnetostriction of 3.2*10-3. Even higher increase of TC, 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 dU-U but high TC. 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.

Magnetic Properties of Tb2Pd2In; Single Crystal Study

A pilot polycrystalline magnetization study of Tb2Pd2In in comparison with new single-crystal data reveal interesting magnetocrystalline anisotropy with field induced spin-flip transitions both in the c- and a-axis of the tetragonal structure.

Lattice and Magnetic Anisotropies in Uranium Intermetallic Compounds

Examples of UNiAlD2.1 and UCoGe illustrate that the soft crystallographic direction coincides quite generally with the shortest U-U links in U intermetallics. It leads to a simple rule, that the easy magnetization direction and the soft crystallographic direction (in the sense of highest compressibility under hydrostatic pressure) must be mutually orthogonal.

5f Magnetism---Specific Features And Boundaries

Magnetism of light actinides exhibits fascinating and potentially useful features. One of them is a giant anisotropy of the two‐ion type, apparent mostly in U‐based systems. Here we demonstrate on the example of U2Ni2Sn and its hydride the anisotropy switches over the direction of U moments on a very fine scale. The study of amorphous sputter deposited UFex films reveals how the Curie temperature can be tuned so as to exceed the room temperature.

Hydrogenation affecting the 5f magnetism

In the case of 5f magnetism, the net increase of Tc due to the interstitial doping by hydrogen is generally understood as due to the increase of the density of states at the Fermi level, resulting from 5/band narrowing. The record value of Tc among UTX compound can be achieved in hydrides of U(Co,Fe)Sn series (Tc = 108 K for 15% of Fe). This concept, based on the 5f band formed by periodic array of U sites, does not work for Th doped compounds. In the case of U sublattice diluted by a non-magnetic element the expansion cannot have dominant effect, partly because already doping by non-magnetic Th leads to an expansion.

Effect of hydrogen doping in UTX compounds

Magnetic measurements on the hydrogenated systems U(Co,Fe)Sn and (U,Th)CoSn were performed as part of our investigation of UTX compounds (T—transitional metal and X—p-metal). Both series studied crystallize in the hexagonal structure of ZrNiAl type, space group P6¯2m symmetry. The crystal-structure type is not changed upon the hydrogenation. In all the cases it leads to a unit-cell expansion (ranging between 3.0% and 3.6%). The Curie temperature TC increases in the hydrides. This increment is reduced with increasing Fe concentration. In the compounds with the Th substitution, the Th concentration at which the ferromagnetism vanishes (∼60%) remains practically unchanged in the hydrides.

Hydrogen as a probe into 5f-magnetism

Abstract We performed magnetic measurements on the hydrogenated system (U, Th)CoSn as part of an investigation of UTX compounds (T – transitional metal, X – p-metal). The U1–xThxCoSn compounds crystallize in the hexagonal structure (ZrNiAl type, space group P 2m). Hydrogen absorption of approximately 1 H/f. u. could be achieved. The hydrogenation does not change the crystal structure. In all cases it leads to a unit-cell expansion by 3.0 % to 3.6 %. Studies of the temperature dependence of the magnetic susceptibility indicate a certain increase in the Curie temperature TC in the hydrides, but the Th concentration at which ferromagnetism vanishes remains practically unchanged (U0.2Th0.8CoSn has a paramagnetic ground state).