Magnetic properties of UNiAl under pressure

A single crystal of UNiA1 was investigated by means of magnetic measurement s under pressure up to 1.3 GPa and in fields up to 17 T applied along the h exagonal c axis. At ambient pressure and zero magnetic field UNiA1 orders a ntiferromagnetically at T-N = 19.3 K. Above 50 K, the magnetic susceptibili ty chi(T) can be approximated by a modified Curie-Weiss law at all pressure s studied with an effective moment decreasing slightly with increasing pres sure. At low temperatures, chi(T) exhibits a well-defined maximum at T-max by several degrees above T-N. Both T-max and T-N decrease linearly with inc reasing pressure P with a rate of dT(max) /dP = -2.7 K GPa(-1) and dT(N) / dP = - 1.85 K GPa(-1), respectively. Above 10.5 GPa, UNiA1 is expected to h ave a paramagnetic ground state. The critical field B-c of the metamagnetic transition from the antiferromagnetic state towards the forced ferromagnet ic 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 depende nce of B-c versus P with two linear ranges of distinctly different slopes c an be also argued. It is concluded that the U magnetic moments in UNiA1 are more localized than in UCoA1 but more delocalized with respect to UNiGa. [ S0163-1829(99)10613-1].