A M Adamska

Magnetic properties of the hydrogenated unconventional superconductor UCoGe–H

Abstract The effect of hydrogenation on magnetic properties of the weakly ferromagnetic superconductor UCoGe was studied. UCoGe hydrides were synthesized at T = 773 K at different pressure of H2 gas (8, 5, 2 bar). The hydrogenation at 8 bar and 5 bar results in a change of the crystal structure from orthorhombic (TiNiSi structure type) to hexagonal (ZrBeSi type) with H concentrations just below 2 H/f. u. For one of such β-hydrides, UCoGeH1.7, the Curie temperature increases from 3 K up to 50 K and the spontaneous magnetic moment from 0.03 μB/f. u. to 0.05 μB/f. u. An α-hydride of UCoGe (ΔV/V = 0.25 %) with a hydrogen concentration 0.1 H/f. u. was obtained at 2 bar. It has the same crystal structure as a pure compound and surprisingly exhibits no magnetic order.

Magnetism in hydrogenated UTGe compounds

UTGe compounds form α-hydrides with a small H concentration and β-hydrides accommodating up to 2 H atoms/f.u. Hydrogen absorption has a significant impact on the 5f magnetism. The Curie temperature-TC of suspected ferromagnetic superconductor UCoGe increases to 50 K. Magnetic susceptibility of spin fluctuator UFeGe increases, but long-range magnetic order is not achieved. For ferromagnetic URhGe only α-hydride could be obtained, which moderate increase of TC and spontaneous moment. The α-hydride of UCoGe is surprisingly paramagnetic and non-superconducting despite of a small volume expansion (0.25 %).

Magnetism of UTGe compounds tuned by hydrogen absorption

UTGe compounds absorb hydrogen up to 2 H atoms/f.u., which has a dramatic impact on the 5f magnetism. The TC value of UCoGe increases to 50 K. Magnetic susceptibility of spin fluctuator UFeGe markedly increases. For ferromagnetic URhGe only α-hydride could be obtained, which moderate increase of TC and spontaneous moment. The α-hydride of UCoGe is surprisingly paramagnetic despite of a small volume expansion (0.25 %).

Characterization of U-based thin films: the UFe2+x case

We have characterized UFe2+x films prepared by sputter deposition onto fused silica (SiO2) and Si(111) substrates with the film thickness ranging from 75 nm to 900 nm. The X-ray diffraction results showed an amorphous character of the deposited material. Some of the films showed in addition a pattern of highly textured cubic Laves phase. Rutherford Backscattering Spectroscopy with 2 MeV He+ ions has been used to determine the composition, thickness and concentration depth profile of the films. A large ageing affect was observed within 1 month after that the films were exposed to air. Magnetic measurements revealed TC increasing with relative Fe concentration and reaching approx. 450 K in UFe3.0.

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.

Electronic Properties of ζ-U-Pu Alloys

The ζ-phase, existing between 35 and 70% U in Pu, belongs to the high density phases seen from the point of view of systematics of allotropic modifications of Pu metal. Despite the volume per actinide atom only slightly higher than for α-Pu, it magnetic susceptibility is much higher than for α-Pu and exceeds even the δ-Pu value. Similarly, the Sommerfeld coefficient γ > 40 mJ/mol Pu K 2 exceeds the experimental δ-Pu value. The data confirm that the volume is not the primary control parameter affecting the situation around the Fermi level of common Pu phases and they point against the traditional belief that they are essentially narrow 5 f band systems.

Uranium compounds prepared by sputter deposition: UFe2+x

UFe2 is one of rare examples of combination of 3d and 5f magnetism in a compound with relatively high TC = 162 K. U-excess is known to reduce the TC value markedly. To prepare the Laves phase with an Fe excess is much more difficult. It can be achieved by sputter deposition, but the structure becomes amorphous. TC can increase at least to 240 K (for UFe4), but the segregation of α-Fe cannot be avoided for very high Fe concentrations.