Magnetization studies in transition-metal niobates: I. NiNb2O6

Abstract

The principal magnetic susceptibilities of Ni${\mathrm{Nb}}_{2}$${\mathrm{O}}_{6}$ were measured from ambient down to 1.4 K. At ${T}_{N}=6.0\ifmmode\pm\else\textpm\fi{}0.3$ K an antiferromagnetic ordering was observed. An antiferromagnetic-paramagnetic phase transition was induced by external magnetic fields parallel to the crystallographic $\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ axis in the temperature range below ${T}_{N}$. The experimental results are interpreted in terms of anisotropic bilinear interactions between ${\mathrm{Ni}}^{2+}$ ions within the manifold of the ground term $^{3}A_{2}$. A two-sublattice uniaxial antiferromagnet model is used to describe the ${\mathrm{Ni}}^{2+}$ spins in the mean-field approximation. The bilinear ${\mathrm{Ni}}^{2+}$ -${\mathrm{Ni}}^{2+}$ interactions are essentially of the exchange type with a small dipolar contribution. Along all three orthorhombic directions the ferromagnetic intrasublattice interaction is found to be stronger than the intersublattice antiferromagnetic coupling. The ${\mathrm{Ni}}^{2+}$ $g$ tensor, taken to be isotropic, is $g=2.4\ifmmode\pm\else\textpm\fi{}0.1$ and the uniaxial anisotropy term $\ensuremath{-}K{S}_{z}^{2}$ in the effective spin Hamiltonian with $S=1$ yields $K=7.9\ifmmode\pm\else\textpm\fi{}0.7$ K/spin.