Specific heat measurements on UNiAl in magnetic fields up to 20 T demonstrate that the Neel temperature (T(N) = 19 K) is drastically reduced by magnetic fields B parallel-to c-axis. AF ordering disappears for B > 11.35 T, the field of the metamagnetic transition. A step in gamma-versus B (to congruent-to 250 mJ/mol K2) observed just above the critical field separates the low-field increasing tendency from the high-field decrease. At 20 T, a gamma-value smaller than the zero-field value of 163 mJ/mol K2 is found.

The electrical resistivity of UNiGa is strongly anisotropic and displays a dramatic decrease in magnetic fields above the metamagnetic transition. The negative d-rho/dT observed above T(c) can be attributed to AF correlations.

UNiGa has been studied by powder neutron diffraction together with UNiAl0.6Ga0.4 and UNiAl. The collinear magnetic structure of UNiGa consists of U magnetic moments in the [001] direction coupled ferromagnetically within (001) sheets and antiferromagnetic stacking (+--++-) of the sheets along the c-axis. In the case of UNiAl0.6Ga0.4, a simple (+-+-) stacking is observed. The very complicated powder pattern of UNiAl prevents determination of the magnetic structure in this case and single-crystal studies are desired.

We compare results of measurements of magnetization, specific heat, electrical resistivity and neutron diffraction on single crystals and oriented polycrystalline powders of various UTX compounds. Very strong anisotropy, persisting also in the paramagnetic state, is a general feature of these materials. Compounds with the hexagonal ZrNiAl-type structure exhibit, as a rule, a uniaxial anisotropy with an easy-magnetization direction along the c axis. Energies of the magnetic anisotropy (of the order of hundreds of kelvins) correlate with the expected development of hybridization strength within planes of the highest coordination of uranium atoms. As a result of this evidence the hybridization-mediated anisotropic exchange is tentatively assumed to be a source of the huge anisotropy. Particular symmetry of anisotropy reflects the type of arrangement of uranium atoms.

High-resolution UV photoelectron spectroscopy on heavy-fermion antiferromagnet UNiAl displays a broad triangular 5f emission spread from E(F) down to at least -1 eV. The presence of 5f states at E(F), in contrast to the low gamma-value of 5 mJ mol-1 K-2, is observed also in local-moment antiferromagnet UPdSn. The considerable degree of itinerancy is demonstrated in both cases by 4f core-level spectra.