Magnetic interactions, weak ferromagnetism, and field-induced transitions in Nd2NiO4

Abstract

The magnetic properties of stoichiometric Nd 2 NiO 4 have been investigated by means of dc- and ac-magnetic-susceptibility and isothermal-magnetization measurements. Five different magnetic phase transitions have been identified and characterized. A collinear antiferromagnetic ordering of Ni 2 + magnetic moments exists between T N 1 ≊320 K and T c 1 ≊130 K ( g x mode) where an orthorhombic-to-tetragonal (Bmab to P 4 2 /ncm) structural phase transition occurs. In this temperature range, the Nd 3 + ions behave as a paramagnet being polarized by the effect of an internal magnetic field associated with the Ni-Nd antiferromagnetic superexchange interaction. A weak ferromagnetic component appears below 130 K, which is consistent with the g x c y f z and g x + c y f z magnetic modes for Ni 2 + proposed from a neutron-powder-diffraction experiment.

An additional out-of-plane component of the internal magnetic field on the Nd 3 + ions develops with this structural phase transition and strongly polarizes these ions. Two additional transitions are observed at T c 2 ≊68 K (very prominent) and T c 3 ≊45 K (very smooth), which are characterized by a sudden increase in the internal magnetic field acting on the Nd ions. This internal magnetic field is evaluated and an antiparallel ordering between the Ni and Nd weak ferromagnetic spin components is inferred. A field-induced transition has been identified. A peak on both the differential susceptibility and the real part of the ac susceptibility at T N 2 ≊11 K marks a long-range antiferromagnetic ordering of the Nd 3 + ions. The out-of-plane component of the Ni 2 + magnetic moments is attributed to the antisymmetric interaction D Ni − Ni , which turns out to be quite important ( D Ni − Ni ≊-16.0 meV) as compared to La 2 NiO 4 and La 2 CuO 4 , probably because of a greater tilting angle of the octahedra. Finally, the magnetocrystalline anisotropy associated with Nd ions is found to be high below 20 K.