Magnetic properties of UNiAl under pressure

Abstract

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 ${T}_{N}=19.3\mathrm{K}.$ Above 50 K, the magnetic susceptibility $\ensuremath{\chi}(T)$ 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, $\ensuremath{\chi}(T)$ exhibits a well-defined maximum at ${T}_{\mathrm{max}}$ by several degrees above ${T}_{N}.$ Both ${T}_{\mathrm{max}}$ and ${T}_{N}$ decrease linearly with increasing pressure P with a rate of ${\mathrm{dT}}_{\mathrm{max}}/dP=\ensuremath{-}2.7{\mathrm{K}\mathrm{}\mathrm{GPa}}^{\mathrm{\ensuremath{-}}1}$ and ${\mathrm{dT}}_{N}/dP=\ensuremath{-}1.85{\mathrm{K}\mathrm{}\mathrm{GPa}}^{\mathrm{\ensuremath{-}}1},$ 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.