Andrei Lacerda

We report measurements of electrical resistivity on a UNi1/3Ru2/3Al single crystal as a function of temperature and magnetic field performed under hydrostatic pressure up to 9 kbar. Magnetization curves of UNi1/3Ru2/3Al are very similar to many other UTX with ferromagnetic order. Temperature dependence of electrical resistivity of UNi1/3Ru2/3Al, however, is characteristic for antiferromagnets, and drop of resistivity in field resembles its behavior at metamagnetic transition. Two clear anomalies in the resistivity curves are attributed to the magnetic phase transitions at TC = 80 K and TM = 24 K. TC is shifted to higher temperature with increasing pressure at an approximate rate of 0.2 K/kbar while TM is slightly shifted toward lower temperatures. The results are discussed in terms of moment instability and variation of the exchange due to increased 5f-ligand hybridization. If UNi1/3Ru2/3Al is indeed antiferromagnet, the field-induced ferromagnetic state remains stable in zero and even in negative field, which is rather rare case. A detailed neutron diffraction study is needed to solve the problem.

The intermetallic compound UNi0.39Rh0.61Al crystallizes in the hexagonal ZrNiAl-type structure, like the parent compounds UNiAl (antiferromagnet, TN = 19K) and URhAl (ferromagnet, TC = 27K) [1]. UNi0.39Rh0.61Al undergoes an antiferromagnetic (AF) transitions at TN = 13K. Here, we report on the results of resistance and magnetoresistance studies on single-crystalline UNi0.39Rh0.61Al in magnetic field up to 18T applied along and perpendicular to the c-axis. This compound is found to exhibit highly anisotropic properties with the easy-magnetization axis along the c-axis. While for fields applied along c-axis, 3T completely suppresses the AF ordering, antiferromagnetism persists up to the highest field of 18T applied perpendicular to the c-axis. The results are discussed in terms of variation of the 5f-ligand hybridization compared to the one of its parent compounds. [1]. A.V. Andreev, J. Alloys and Compound, 282, 46 (1999).