Karel Prokes

Summary

This thesis describes the electronic properties of several ternary uranium intermetallic compounds, studied by means of bulk measurements (magnetic susceptibility, magnetization, specific heat, electrical resistivity and thermal expansion) and experiments probing magnetic moments on a microscopic scale (neutron scattering and muon spin rotation). The magnetic structures were determined by means of powder neutron diffraction or neutron diffraction on single crystals produced at the facility FOM-ALMOS at the University of Amsterdam or at the EITU Karlsruhe. Inelastic neutron scattering was used to investigate dynamic processes involving U magnetic moments.

After a very brief introduction in Chapter 1, Chapter 2 introduces the theoretical concepts and models that are the most widely used in the description of U-based compounds. The 5f states are intermediate between the delocalized d-states in the transition-metals and the localized 4f states in the lanthanides. There are two main mechanisms which delocalize 5f states, firstly the direct overlap of 5f wavefunctions and, secondly the 5f-ligand hybridization.

Chapter 3 describes the main experimental techniques used in the investigation of the electronic bulk properties of the compounds studied. Sample preparation and characterization os samples described in this thesis is included in this chapter.

The neutron-scattering techniques used to determine the geometrical arrangements and the dynamic processes involving uranium magnetic moments are briefly described in Chapter 4. In this chapter, also the technique of muon spin rotation spectroscopy is briefly summarized.

Experimental results obtained on four UTX compounds (UNiGa, UNiAl, UCoGa and URhAl) crystallizing in a crystal structure with the hexagonal symmetry (the ZrNiAl-type of structure) are described in Chapter 5. Due to the strong magnetocrystalline anisotropy, the majority of the experiments was performed on single crystals. By means of susceptibility and high-field magnetization measurements, it was found that the c-axis is the easy-magnetization direction. The anisotropy energy determined from the bulk measurements is of the order of 300 - 500 K in all four systems. Inelastic-neutron-scattering experiments on polycrystalline URhAl reveal an anisotropy energy of about 470 K ( 700 T). In addition, a broad quasielastic signal suggesting hybridization between the 5f states and the conduction-electron electrons is found (also in the case of UNiAl). The magnetic structures were determined by neutron-diffraction experiments. UNiGa and UNiAl are found to order antiferromagnetically at low temperatures, whereas UCoGa and URhAl order ferromagnetically. The strong magnetic anisotropy locks the uranium moments to be oriented along the c-axis. The interaction between the uranium moments within the basal plane is in principle ferromagnetic. The interaction along the c-axis yields the type of the magnetic ordering. In the case of UNiGa, the nearest-neighbour and the next-nearest-neighbour interactions are in delicate balance leading to a rich variety of stacking of ferromagnetical basal planes along the c-axis. Rather complicated magnetic phase diagrams at ambient pressure and at 9 kbar were constructed. Pressure is found to promote antiferromagnetic interactions along the c-axis. The other three systems reveal only one magnetically ordered phase at low temperatures. The low-temperature specific heat coefficient g is enhanced in all systems. The magnetic fluctuations in UNiAl are found to be responsible for the highest g value (169 mJ/mol_UK²) among systems crystallizing in the ZrNiAl-type of structure. In the majority of the cases it is impossible to assign one single value of the Debye temperature to both low- and high-temperature parts of the specific heat, if the the electronic contribution g is kept temperature independent. The electricaltransport properties are highly anisotropic with rather high values of the residual resistivity for the current along the c-axis. The electrical resistivity of the antiferromagnetically (AF) ordered compounds can enormously be reduced upon application of a magnetic field that is strong enough to align magnetic moments ferromagnetically (F). Specific-heat, the Hall effect, magnetocaloric and thermopower measurements on a UNiGa single crystal suggest that most probably the reconstruction of the Fermi surface on going from the AF to the F state plays an important role, particularly, the disappearance of the Fermi surface gapping is responsible for the reduction of the electrical resistivity in these materials.

In Chapter 6, the electronic properties of few UTX systems crystallizing in orthorhombic crystal structures (TiNiSi and CeCu₂-type of structure) are presented. Among these materials, only UNiGe was studied as single crystal, while UNiSi, URhSi, UPdSi and UPtSi were studied as polycrystals. By means of bulk measurements, it was found , in contrast to hexagonal UTX compounds, that the magnetic anisotropy is multiaxial with the a-axis as hard-magnetization direction. The anisotropy energy determined from the bulk measurements is of the order of 100 K in all systems. No inelastic neutron-scattering experiments, which are highly desirable, were performed up to now. UNiGe, UPdSi and UPtSi are found to order antiferromagnetically at low temperatures and UNiSi and URhSi most probably ferromagnetically. These conclusions were made on the basis of bulk measurements and in the case of the first three systems, they were verified by neutron diffraction. In the latter two systems a strongly reduced U magnetic moments are found. Also, in these systems anomalous temperature dependence of the specific heat is found. On the basis of the bulk measurements it is not possible to decide regarding the origin (the crystal field or magnons with a gap in the dispersion relation). In the case of UNiGe, complex magnetic phase diagrams for the b- and c-axis orientations have been established. It has been found that the unusually high electrical resistivity (in zero field) undergoes sudden changes at critical fields where metamagnetic transitions occur and is reduced by 80% of its zero-field value for both orientations. It has been found that there is significant a-axis component (0.35 m _B/U) of the uranium magnetic moments, which is a hard-magnetization direction. The fact that the a-axis components cannot be aligned by magnetic field of 38 T, while the b- and the c- components do that at lower fields, points to the existence of anisotropic exchange interactions.

Chapter 7 is devoted to description of the magnetic properties of magnetically ordered U₂T₂X compounds. The low-temperature specific-heat coefficient g is enhanced in all systems, reaching a maximum value in U₂Pt₂In (425 mJ/K²mol_U) which is, however nonmagnetic at least down to 1.3 K. Systems which do exhibit magnetic order at low temperatures (U₂Ni₂In, U₂Pd₂In, U₂Ni₂Sn, U₂Rh₂Sn, U₂Pd₂Sn and U₂Pt₂Sn) order antiferromagnetically with magnetic unit cell either of the same size as the crystallographic one or doubled along the tetragonal axis. The direction of U magnetic moments in U₂Ni₂Sn and U₂Rh₂Sn contradicts the generally accepted empirical rule that the uranium magnetic moments are directed perpendicularly to the shortest U-U link. In U₂Ni₂In and U₂Rh₂Sn, strongly reduced uranium magnetic moments are found. In some cases, the specific heat measurements suggest changes in the density of states at the Fermi level in the vicinity of magnetic phase transitions, which can be understood in terms of the Fermi surface gapping. The high-field magnetization shows as a rule for all U₂T₂X compounds no tendency for saturation pointing to field-induced moments. Magnetic measurements on a single crystal of U₂Pd₂In show highly anisotropic high-field magnetization as well a change of the easy-magnetization direction at 25 K.

The experimental results presented in this thesis yield a clear general tendency in the development of the magnetic properties of UTX and U₂T₂X compounds as a consequence of the 5f-ligand hybridization.