Abstract.The ternary silicides RE6T1.67Si3 (RE = Ce, Pr, Nd, Gd, Tb and T = Co, Ni) crystallizing with the hexagonal Ce 6 Ni 1.67 Si 3 -type structure has been recently extensively studied. It has been shown that some of these compounds exhibit interesting magnetocaloric properties. In this work, the magnetic structures of the Nd6Ni1.67Si3 compound were determined by means of neutron powder diffraction. According to previous magnetization and specific heat measurements, this compound exhibits two successive magnetic transitions at 84K and 38K. The first transition corresponds to a ferromagnetic arrangement of the Nd-moments along the c-axis and the second one to a non-collinear ferromagnetic arrangement yielding a conical structure. The magnetic structures of Nd6Ni1.67Si3 are compared to those of the Nd6Co1.67Si3 homologous ternary silicide.
IntroductionRecently, a new series of compounds with the general formula RE 6 Co 1.67 Si 3 (RE = Rare-Earth) was discovered [1,2]. These compounds crystallize with the Ce 6 Ni 1.67 Si 3 aristotype structure [3]. Homologous compounds with nickel instead of cobalt were studied in the past and more recently [3][4][5][6]. To compare the properties of both series with cobalt and nickel, compounds with gadolinium or neodymium as RE were chosen. In the case of Gd 6 M 1.67 Si 3 (M = Co, Ni) compounds, a ferromagnetic behaviour is observed for both silicides around room temperature, with T C = 294K for Gd 6 Co 1.67 Si 3 and 310K for Gd 6 Ni 1.67 Si 3 . They exhibit interesting and similar magnetocaloric properties [6-8] with a same adiabatic temperature change, e.g. ∆T ad = 3.1K for ∆H = 2T [6]. The magnetic properties of Nd 6 Co 1.67 Si 3 and Nd 6 Ni 1.67 Si 3 compounds were also compared and it has been shown that a ferro-or ferrimagnetic transition is observed at 84K for both [9]. Below this magnetic transition a second one was observed at T = 38 and 35K for Nd 6 Ni 1.67 Si 3 and Nd 6 Co 1.67 Si 3 , respectively. The comparison of the temperature dependence of their magnetization M(T) showed significant differences below 40K. To characterize the two magnetic transitions, neutron diffraction experiments were carried out and the results obtained for Nd 6 Ni 1.67 Si 3 will be presented in this paper.