New magnetic phenomena in TbNi₂

Structural investigations at room temperature revealed that TbNi₂ does not crystallize in the Laves phase structure, but shows a superstructure of the Laves phase with the space group F-43m. Susceptibility, specific heat, magnetostriction and magnetoresistance measurements on polycrystalline specimens showed an additional magnetic phase transition at T_R = 14 K below the Curie temperature of T_C = 36 ± 0.2 K. In order to clarify the nature of this magnetic phase transition at 14 K, elastic neutron diffraction below and above T_R and T_C was performed. The analysis of these data showed that this transition at T_R is due to the rotation of the Tb moments on three of the total of eight non-equivalent Tb sites in the rhombohedrally distorted unit cell in the magnetic ordered state. This rotation of these Tb moments is out of the [111] direction into a plane perpendicular to the space diagonal. The cause for this magnetic instability is due to an interplay of the regularly arranged vacancies in the superstructure and the crystal field level position which has been studied by inelastic neutron scattering.