Magnetization studies in transition-metal niobates. II. FeNb2O6

Abstract

The magnetization and susceptibility of flux-grown Fe${\mathrm{Nb}}_{2}$${\mathrm{O}}_{6}$ single crystals have been measured from 80 K down to 1.4 K. An antiferromagnetic ordering was observed at ${T}_{N}=5.5\ifmmode\pm\else\textpm\fi{}0.5$ K. The antiferromagnetic-paramagnetic phase transition induced by an external field along the orthorhombic $\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ axis seems to be of second order in the whole range from the ${T}_{N}$ down to 1.4 K. The experimental results are interpreted in the mean-field approximation using a four-sublattice model for the ${\mathrm{Fe}}^{2+}$ spins. A nearly accidental doublet ground state is assumed for the ${\mathrm{Fe}}^{2+}$ ions. The $g$ tensors are found to be $g=7.7\ifmmode\pm\else\textpm\fi{}0.4$ in the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}\ensuremath{-}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ plane at angles of \ifmmode\pm\else\textpm\fi{} (${17}^{0}$ \ifmmode\pm\else\textpm\fi{} ${3}^{0}$) to the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ axis. These are also the two easy directions of the four-sublattice ${\mathrm{Fe}}^{2+}$ spin system. The ${\mathrm{Fe}}^{2+}$-${\mathrm{Fe}}^{2+}$ interaction is essentially of the exchange type with a small dipolar contribution. The intrasublattice ferromagnetic interaction is found to be larger than the intersublattice antiferromagnetic coupling.