Výsledky projektu Chování systému s f elektrony na pokraji existence magnetismu

Abstrakt:
Strong magnetocrystalline anisotropy (MA) is a well-known property of uranium compounds. The almost isotropic ferromagnetism in U4Ru7Ge6 reported in this paper represents a striking exception. We present results
for magnetization, ac susceptibility, thermal expansion, specific heat, and electrical resistivity measurements performed on a U4Ru7Ge6 single crystal at various temperatures and magnetic fields; we discuss the results in relation to first-principles electronic structure calculations. U4Ru7Ge6 behaves as an itinerant 5f-electron ferromagnet. The ground-state easy magnetization direction is along the [111] axis of the cubic lattice. The anisotropy field along the [001] direction is only about 0.3T, which is at least three orders of magnitude smaller than for other U ferromagnets. At Tr=5.9 K the easy magnetization direction changes to [001], and remains [001] up to TC. The magnetoelastic interaction induces a rhombohedral (tetragonal) distortion of the paramagnetic cubic crystal lattice in the case of the [111] ([001]) easy magnetization direction. The rhombohedral distortion is connected with two crystallographically inequivalent U sites. Our density functional theory calculations, including spin-orbit interaction (SOI) of the U 5f electrons, also produces two inequivalent U sites, because SOI leads to a reduction of the symmetry of the former cubic structure. The calculated ground state is in agreement with the experimentally observed [111] easy magnetization direction. The first excited state has moments along the [001] direction, which agrees with the moment orientation for T>Tr. The energy of the first excited state is 0.9meV above the ground state, which is comparable to the value of 0.51 meV, corresponding to kBTr. We propose that weak MA of the U4Ru7Ge6 compound is due to the lack of direct overlap of the 5f orbitals of the nearest U ions, which is screened out by the closed Ru and Ge cuboctahedra coordinating each U ion.

Abstrakt:
We report on a microscopic study of the evolution of ferromagnetism in the Ru substituted ferromagnetic superconductor (FM SC) UCoGe crystallizing in the orthorhombic TiNiSi-type structure. For that purpose, two single crystals with composition UCo0.97Ru0.03Ge and UCo0.88Ru0.12Ge have been prepared and characterized by magnetization, AC susceptibility, specific heat and electrical resistivity measurements. Both compounds have been found to order ferromagnetically below TC = 6.5 and 7.5 K, respectively, which is considerably higher than the TC = 3 K of the parent compound UCoGe. The higher values of TC are accompanied by enhanced values of the spontaneous moment μspont = 0.11 μB/f.u. and μspont = 0.21 μB/f.u., respectively in comparison to the tiny spontaneous moment of UCoGe (about 0.07 μB/f.u.). No sign of superconductivity was detected in either compound. The magnetic moments of the samples were investigated on the microscopic scale using polarized neutron diffraction (PND) and for UCo0.88Ru0.12Ge also by soft X-ray magnetic circular dichroism (XMCD). The analysis of the PND results indicates that the observed enhancement of ferromagnetism is mainly due to the growth of the orbital part of the uranium 5f moment μULμLU, reflecting a gradual localization of the 5f electrons with Ru substitution. In addition, the parallel orientation of the U and Co moments has been established in both substituted compounds. The results are discussed and compared with related isostructural ferromagnetic UTX compounds (T: transition metals, X: Si, Ge) in the context of a varying degree of the 5f-ligand hybridization.

Abstrakt:
We have investigated the impact of Ru substitution for Co on the behavior of the ferromagnetic superconductor UCoGe by performing x-ray diffraction, magnetization, specific heat, and electrical resistivity measurements on polycrystalline samples of the UCo1−xRuxGe series (0≥x≤0.9). The initial Ru substitution up to x≈0.1 leads to a simultaneous sharp increase of the Curie temperature and spontaneous magnetization up to maximum values of TC=8.6K and MS=0.1μB per formula unit, respectively, whereas superconductivity vanishes already for x≈0.03. Further increase of the Ru content beyond x≈0.1 leads to a precipitous decrease of both TC and MS towards a ferromagnetic quantum critical point (QCP) at xcr=0.31. Consequently, the T−x magnetic phase diagram consists of a well-developed ferromagnetic dome. We discuss the evolution of ferromagnetism with x on the basis of band structure changes due to varying 5f-ligand hybridization. This scenario is supported by the results of electronic structure calculations and consideration of the simplified periodic Anderson model. The analysis of the temperature dependencies of the electrical resistivity and heat capacity at low temperatures of the samples in the vicinity of the QCP reveals a non-Fermi-liquid behavior and assigns the ferromagnetic quantum phase transition to be most likely of a continuous Hertz-Millis type.