Influence of the inhomogeneous distribution of components on itinerant-5f-electron metamagnetism in UCoAl under high pressures

Abstract

We have performed a comparative study of the magnetization process and the magnetic susceptibility under pressure on a stoichiometric UCoAl single crystal and an off-stoichiometric U₀₉Co_1.05Al_1.05 single crystal of the anisotropic itinerant-5f-electron metamagnet UCoAl. Both UCoAl and U_0.9Co_1.05Al_{1. 05} exhibit a metamagnetic transition to the ferromagnetic state with a magnetic moment M ≈ 0.3μ_B (U atom)⁻¹ in magnetic fields applied along the c axis. The critical field B_c of the transition increases with increasing pressure and temperature. The temperature dependence of the susceptibility shows a broad maximum at a temperature T _max that gradually increases with increasing pressure. At ambient pressure, the transition in UCoAl is of first order below 12 K and of second order at 12–20 K. Broadeningof the transition inU_0.9Co_1.05Al_1.05 at low temperatures is attributed rather to a variation in the B _c value over the crystal (which can be described by Gaussian distribution with a half-width of 0.7 T at ambient pressure) due to a difference in ligand neighbourhood of U atoms rather than to a second-order transition. This conclusion is corroborated by an observed hysteresis in the transition that compares well with the hysteresis of the transition in UCoAl. The experimental data have been analysed within a theory of itinerant-electron metamagnetism considering anisotropic thermal fiuctuations in the magnetic moment. For homogeneous UCoAl, quantitative agreement between theory and experiment has been found, whereas the behaviour of the heterogeneous U_o.9Co_1.05Al_{1 05} sample can be explained within the theory only qualitatively.