A single crystal of UNiAl was investigated by means of magnetic measurements under pressure up to 1.3 GPa and in fields up to 17 T applied along the hexagonal c axis. At ambient pressure and zero magnetic field UNiAl orders antiferromagnetically at $T_N=19.3\mathrm{}\mathrm{K}.$ Above 50 K, the magnetic susceptibility $\chi \mathrm{}\left(T\right)\mathrm{}$ can be approximated by a modified Curie-Weiss law at all pressures studied with an effective moment decreasing slightly with increasing pressure. At low temperatures, $\chi \mathrm{}\left(T\right)\mathrm{}$ exhibits a well-defined maximum at $T_{\max }$ by several degrees above $T_N.$ Both $T_{\max }$ and $T_N$ decrease linearly with increasing pressure P with a rate of ${\mathrm{dT}}_{\max }/dP=-2.7{\mathrm{K}\mathrm{}\mathrm{GPa}}^{\mathrm{-}1}$ and ${\mathrm{dT}}_N/dP=-1.85{\mathrm{K}\mathrm{}\mathrm{GPa}}^{\mathrm{-}1},$ respectively. Above 10.5 GPa, UNiAl is expected to have a paramagnetic ground state. The critical field $B_c$ of the metamagnetic transition from the antiferromagnetic state towards the forced ferromagnetic state decreases with pressure in a strongly nonlinear way. It falls down with a power of 2.5 of applied pressure. An alternative, two-stage dependence of $B_c$ versus P with two linear ranges of distinctly different slopes can be also argued. It is concluded that the U magnetic moments in UNiAl are more localized than in UCoAl but more delocalized with respect to UNiGa.

• Received 27 August 1998

DOI:https://doi.org/10.1103/PhysRevB.59.8720