| AVS 65th International Symposium & Exhibition | |
| Actinides and Rare Earths Focus Topic | Wednesday Sessions |
| Session AC+MI+SA-WeM |
| Session: | Magnetism, Complexity, and Superconductivity in the Actinides and Rare Earths |
| Presenter: | Ladislav Havela, Charles University, Prague, Czech Republic |
| Authors: | L. Havela, Charles University, Prague, Czech Republic M. Paukov, Charles University, Prague, Czech Republic M. Dopita, Charles University, Prague, Czech Republic L. Horak, Charles University, Prague, Czech Republic P. Minarik, Charles University, Prague, Czech Republic M. Divis, Charles University, Prague, Czech Republic I. Turek, Charles University, Prague, Czech Republic D. Legut, VSB-Technical University of Ostrava, Czech Republic T. Gouder, European Commission - Joint Research Centre A. Seibert, European Commission - Joint Research Centre F. Huber, European Commission - Joint Research Centre E.A. Tereshina-Chitrova, Institute of Physics, Academy of Sciences of the Czech Republic |
| Correspondent: | Click to Email |
Most of f-elements form with hydrogen both di- and trihydrides. Actinide and rare-earth dihydrides occur, as a rule, in the CaF2 structure type. Uranium represents an exception, only UH3 is present in the binary phase diagram. It exists in two different structure types. The metastable form α-UH3 forms in the Cr3Si structure type, which is in fact bcc U lattice filled with hydrogen. The stable form β-UH3 has a larger cubic cell with two different U sites. Both forms are ferromagnets with the total U moment of ≈ 1 μB/U and the Curie temperature TC ≈ 165 K. We have recently synthesized UH3 thin films using a reactive sputter deposition. XRD analysis indicated the β-UH3 structure, modified by a pronounced (00l) texture and compressive residual strains imposed by the deposition dynamics. Magnetization measurements proved TC = 165 K.
The sputter deposition on a cooled substrate (T = 170 K) using Si wafer the crystal structure turned different. The deposited material is undoubtedly cubic, of the fcc type, and the lattice parameter a = 5.3598 ± 0.0014 Å is very close to that of PuH2 (a = 5.359 Å) and NpH2+x (a = 5.343-5.355 Å). Hence we can assume that UH2 in the fluorite structure has been formed. The key role in stabilization plays likely the effect of substrate (Si has a = 5.431 Å) in combination with low temperature deposition. The UH2 film was subsequently subjected to magnetization measurements, which indicated a ferromagnetic ground state with TC ≈ 125 K. This is lower than in the UH3 phases, although the U-U spacing in UH2 should be higher, 3.78 Å, than in both UH3 phases (3.31 and 3.60 Å for β- and α-UH3, respectively). This fact points to the U-U interaction being more important than the U-U spacing. The ferromagnetic state is also the ground state obtained from ab-initio calculations. Scalar relativistic calculations (LDA) for experimental lattice parameter yield the spin moment μS = 2.0 μB/U. LDA+U (U = 2.25 eV) gives the equilibrium lattice parameter a = 539.9 Å, i.e. 0.7% larger than the experimental one, the ferromagnetic ground state with (111) easy-magnetization direction and the magnetic anisotropy energy Ea = 9 meV. The total moment 0.45 μB/U consists of 2.59 μS and -3.04 μL.
This work was supported by the Czech Science Foundation under the grant No. 18-02344S. The work at JRC Karlsruhe was supported by the European FP7 TALISMAN project, under contract with the European Commission. Part of the work was supported by the project “Nanomaterials centre for advanced applications”, Project No. CZ.02.1.01/0.0/0.0/15_003/0000485, financed by ERDF.