Effects of High Pressures on Magnetization of UNiGa

In this work we present the results of magnetization measurements of UNiGa single crystal in magnetic field applied along crystallographic c-axis under high hydrostatic pressures as well as uniaxial stress applied along the c-axis. The UNiGa compound was selected for its complex magnetic behavior and lack of detailed investigations of the impact of high pressures to the magnetic phase diagram as well as for well documented strongly anisotropic properties. UNiGa crystallizes in the hexagonal ZrNiAl – type crystal structure (space group P-62m). It orders antiferromagnetically with TN ≈ 39 K and exhibits three more order-to-order transitions between distinct AF with further decreasing temperature. All these phases are characterized by collinear magnetic structures with uranium magnetic moments locked along the crystallographic c-axis. We measured temperature and field dependencies of magnetization of UNiGa in various conditions in temperatures up to 60 K, magnetic field up to 2.5 T and high pressures up to 8 kbar and 2 kbar for hydrostatic and uniaxial pressures, respectively. Introduction UNiGa is one of the most thoroughly studied materials in the UTX group of compounds (T = transition metal, X = p metal) crystallizing in the hexagonal ZrNiAl – type crystal structure (space group P-62m) [1]. It orders antiferromagnetically with TN ≈ 39 K. With temperature further decreasing, three order-to-order transitions between different AF phases were observed in zero external magnetic field in the temperature range ≈ 34 – 39 K [2]. All these phases are characterized by collinear magnetic structures with uranium magnetic moments (no moment have been detected on Ni and Ga) locked along to the crystallographic c-axis. The magnetic structures consists of basal-plane sheets of ferromagnetically coupled U moments, distinguished from each other just by different “stacking” of the ferromagnetic layers along the c-axis. The ground state was found to be characterized by the + + − + − − coupling with the ordered uranium magnetic moments μ ≈ 1.4 μB [3]. With application of an external magnetic field along the c-axis the transition to the field induced ferromagnetic structure (directly at temperatures below 10 K or via the + + − uncompensated AF structure at higher temperatures) were observed [3]. The effect of pressure on the magnetic phase diagram was previously studied by means of magnetostriction [4], thermal expansion [5] and electrical resistivity [6] measurements. Results of previous studies are summarized in a phase diagram in Fig.1. Fig. 1. Previously published magnetic phase diagram of UNiGa in the magnetic field applied along the crystallographic c–axis [7]. Detailed description of all magnetic structures can be also found in [7]. 101 WDS'09 Proceedings of Contributed Papers, Part III, 101–104, 2009. ISBN 978-80-7378-103-3 © MATFYZPRESS