Magnetic and transport properties of UBi₂and USb₂single crystals

The thermoelectric power, S(T), of USb2 and UBi2, which are tetragonal, uniaxial antiferromagnets below T N = 202 K and 180.8 K, respectively, have been examined between 0.4 K and 300 K. The values of S(T), up to now known above 70 K for USb2 and unknown for UBi2, are positive along the a-axis for both compounds in the whole examined temperature range. The S(T) data for the c-axis (the easy magnetization axis) are positive near room temperature for USb2 and UBi2 but becomes negative below 120 K and 170 K, respectively, with two very deep minima in S(T) dependence for USb2. In the latter compound the Fermi surface, known from literature, is composed of the only cylindrical sheets that are slightly corrugated and parallel to the c-axis. UBi2, the Fermi surface of which is composed of one spherical and two cylindrical sheets, shows corresponding minima although less pronounced than those in USb2. Having at disposal the highest purity single crystals in comparison to those for which the resistivity, ρ(T), has been reported in literature, the ρ(T) anisotropy was re-examined for these two systems. Magnon and phonon contributions to their total electrical resistivity have been determined and the critical fluctuation behaviour of the resistivity near T N for both dipnictides has been analysed. Although the magnetic susceptibilities of UBi2 and USb2 reveal a similarity, their transport properties are significantly different due to the difference in the Fermi surface topology.