ELECTRONIC-PROPERTIES OF UNIAL IN HIGH MAGNETIC-FIELDS

The ternary compound UNiAl, crystallizing in the ZrNiAl-type structure, is an itinerant antiferromagnet with magnetic moments of less than 0.8mu(B) per U atom and with a high gamma value of 164 mJ/mol K2. The anisotropic response of the electronic properties to magnetic field is closely connected with the huge uniaxial magnetic anisotropy, observed not only in the magnetically ordered state, but also in the paramagnetic region. The origin of this anisotropy can be found in a strongly anisotropic hybridization. The magnetic ordering temperature, T(N) = 19.3 K, is reduced upon applying the field along the c axis of the hexagonal structure, which is the easy-magnetization direction. Antiferromagnetic ordering is suppressed by magnetic fields higher than the critical field of the metamagnetic transition, which is 11.35 T at 1.4 K. The electronic contribution to the specific heat is gradually enhanced by the magnetic field up to the transition, where a gamma value of about 260 mJ/mol K2 is recorded. In higher fields, the gamma coefficient is gradually suppressed even below the zero-field value and reaches 143 mJ/mol K2 at 20 T. The magnetic field applied in the basal plane has a negligible effect on both T(N) and gamma. The pronounced anisotropy is also found in the low-temperature electrical resistivity, which is considerably reduced in fields above the metamagnetic transition. Strong indications of anisotropic magnetic fluctuations can be traced in a number of experimental findings on pure UNiAl and also on Y-, Co-, or Fe-substituted samples, in which a fast decay of magnetic ordering is observed.