Speaker
Description
Uranium metal as well as other light actinides are weak Pauli paramagnets, i.e. do not exhibit magnetic moments. The reason is a large overlap of the 5f wave functions centered on nearest U neighbours. The 5f states form consequently a broad 5f band. Increasing the U-U spacing in compounds makes the 5f band narrower and eventually U moments are formed and order (ferro- or antiferromagnetic ordering) unless the hybridization with ligand states, which can similarly broaden the band, is too strong. The critical U-U distance, d(U-U) necessary (but not sufficient) to form U magnetic moments is 340-360 pm. From handful of exceptions with lower d(U-U), the most conspicuous is uranium hydride, UH3. Despite low d(U-U) = 331 pm in the common beta-UH3 form, ferromagnetism with moments exceeding 1 magneton Bohr appears below relatively very high Curie temperature Tc = 165 K. We found that hydrogenating U alloys with early d-metals (Mo, Ti, Zr) Tc can even increase, exceeding 200 K. A hint why it is like that was provided by ab-initio calculations indicating a charge transfer from the U-7s and 6d states towards the H-1s states. The charge transfer surprisingly does not involve the 5f states, which remain populated to 2.7-2.8 electrons per U atom. Such situation reduces the 5f-6d hybridization as one of the delocalizing mechanisms, and a narrow 5f band remains at the Fermi level. Magnetism is much more pronounced than in case the U metal is hypothetically blown up to the volume of UH3. A thorough study of UH3 by XPS/UPS indicates indeed fingerprints for the charge transfer. This work opens new suggestions where to seek for more magnetic uranium compounds. Similar charge transfer has to take place e.g. in binary and ternary pnictides. If the transfer is too large, as e.g in oxides, the product becomes non-metallic, and ordering temperatures have to drop down. The highest Tc = 216 occurs in UCu2P2. Much larger d(U-U) in this case is perhaps contra-productive, the exchange interaction at a long distance is getting weaker. Therefore we studied a lattice compression in external hydrostatic pressure. Indeed, a dramatic increase of Tc has been traced so far to the pressure of 4 GPa, yielding Tc = 255 K. As the pressure derivative is still high, there is a good chance that a room-temperature 5f ferromagnet, with all its specific features originating from the strong spin-orbit interaction, as giant magnetic anisotropy, will soon become reality.
