Magnetic Properties and Hybridization Effects in Utx Compounds

Systematic trends in the results of magnetic and specific-heat studies of UTX compounds (T = transition metal, X = p element) are discussed in terms of the degree of the U-Sf electron delocalization which is controlled by the strength of 5f-ligand hybridization. Continuous changes of this parameter and its influence on ground-state properties were investigated in solid solutions of selected UTX compounds. The main feature in the electronic structure of most of uranium compounds is a more or less narrow 5f band in the vicinity of E p . Therefore the magnetic properties (formation of a 5f magnetic moment, magnetic ordering), as well as other electronic properties (transport properties, specific heat) are intimately connected with the degree of Sf-electron localization. The direct overlap of Sf-wave functions d neighbouring atoms in case of compounds with sufficiently short inter-uranium distances is one of main delocalizing mechanisms. The further mechanism delocalizing the 5f electrons in compounds is the ubiquitous hybridization of 5f states with the s, p or d states brought to the valence band by other-component atoms. This so called Sf-ligand hybridization [I] is the principal delocalizing process in compounds where a larger separation of U atoms prevents the direct 5f-5f overlap. The considered trends in hybridization can be tested by studying magnetic and other electronic properties within extended groups of isostructural compounds with different ligand elements. In this contribution we discuss the development of the 5f-ligand hybridization and its influence on electronic properties of UTX compounds (T = transition metal, X = element from the group 111. V. of the Periodic Table). These compounds crystallize in different structure types. The two most extended isostructural groups are characterized by the structure types ZrNiAl (hexagonal) or CeCuz (orthorhombic). In the case of a relatively weak 5f-ligand hybridization, i.e. for late d metals and for larger X and T constituents, magnetic ordering of the ferromagnetic type is found in the hexagonal compounds. One of the prominent features of compounds with this structure is a strong anisotropy of the magnetic properties (uniaxial with the easy-magnetization axis being along the c axis in all compounds except in UPdIn where it is probably located in the basal plane), which is one of arguments for the 5f origin of magnetism. In ferromagnetic compounds anisotropy fields as high as 200 T were estimated from magnetization measurements on UCoGa and UNiGa [2,3]. Our high-field experiments on most of hexagonal compounds proved the presence of such a strong anisotropy to be a general feature irrespective to the type of ground state. The compounds with stronger hybridization (URuA1, URuGa) show spin-fluctuation (SF) character of the magnetic susceptibility and no magnetic ordering is found down to lowest temperatures. As the Rh isotypes are ferromagnetic, the details of the SF -+ F transition can be studied on the U(Ru,Rh)AI and U(Ru,Rh)Ga pseudoternary systems. The development of the temperature dependences of magnetic susceptibility in U R U ~ ~ R ~ , A I compounds is seen in figure 1. Although the character of the lowtemperature susceptibility is different for URuGa and URuAl[3], the substitution of Rh smears out the characteristic low-temperature deviations from the CW law in both systems already for x > 0.1. The onset offerromagnetism is observed around x = 0.2. The susceptibility at higher temperatures is well described by a modified Curie-Weiss law over the whole concentraGon region. The concentration dependences of Tc and 8, ,