Hybridization and pressure effects in UTX compounds

Abstract

Abstract:The UTX intermetallic compound (T = transition metal and X = p-electron element) were found to crystallize mainly in two large groups, the orthorhombic TiNiSi and the hexagonal ZrNiAl structure. For both groups, magnetic-ordering phenomena depend sensitively on the 5f-ligand hybridization. The 5f-ligand hybridization is very sensitive to the interatomic distances, which can be effectively controlled by external pressure. Here, we have summarized existing and new pressure studies on various single-crystalline UTX compounds (T = Co, Ni, Ir; X = Al, Ga, Ge). We performed magnetoresistance studies in magnetic fields up to 18 T under external hydrostatic pressure up to 10 kbar. The observed dependencies of the ordering temperatures and the critical fields are compared with the different contributions of the 5f-ligand hybridization, which were deduced from tight-binding calculations. We find relatively weak pressure dependence in compounds, where 5f electrons are more localized, while substantially large pressure effects are found in the more itinerant systems. Uranium compounds have attracted a great deal of attention in the past two decades because of the specific nature of U 5f-electron magnetism. The f electrons of U are found to be intermediate between the delocalized d electrons of the transition metal and the well-localized 4f electrons of the lanthanide. Two mechanisms are known to lead to delocalization the 5f electrons, the direct 5f-5f overlap and the 5f-ligand hybridization.1,2 Comprehensive studies have revealed a large variety of magnetic phenomena starting from weak paramagnetism to various types of (sometimes unusual) long-range magnetic ordering depending on the degree of the 5f-ligand hybridization. Large 5f-ligand hybridization leads to a delocalization of the f electrons, which ultimately leads to a suppression of magnetic moments. On the other hand, in most cases some 5f-ligand hybridization is needed to promote inter-ionic exchange, which confirms a magnetic order ground state. The 5f ligand hybridization depends on several parameters (geometrical surrounding of the 5f atom, coordination, interatomic distances, etc.), and it is determined by the overlap of the respective wavefunctions. The interatomic distances and thus the overlap of wavefunctions can be effectively changed by external pressure.UTX compounds (T = transition metal, X = p-electron element) were found to crystallize mainly in two large groups of crystal structures, the orthorhombic TiNiSi structure and the hexagonal ZrNiAl structure. For both groups, extensive bulk studies revealed a huge magneto-crystalline anisotropy, which is of easy-axis type (c-axis) in the ZrNiAl-structure and easy-plane type (b-c plane) for the TiNiSi-structure compounds. 3-6The huge magnetic anisotropy originates in strong spin-orbit interactions and the presence of large orbital moments, and the magnetic anisotropy was tentatively attributed to the hybridization-mediated anisotropic two-ion interaction. 7In this contribution, we have summarized th...

Thermodynamic and transport properties of single-crystallineUMGa5(M=Fe,Moreno1, Bauer2, Sarrao3 et al. 2005Phys. Rev. B334210View full textAdd to dashboardCite

“…A similar analysis of the UTX and U 2 T 2 X systems is consistent with the occurrence of magnetic or nonmagnetic ground states. 38,39 The tight-binding analysis can be extended to include the isostructural PuMGa 5 ͑M =Co,Rh,Ir͒ materials. PuCoGa 5 exhibits superconductivity at T c = 18.5 K; various thermodynamic measurements indicate a moderate quasiparticle mass enhancement of ␥ ϳ 80 mJ/ mol K 2 .…”