Magnetic Properties of CoF2

Abstract

Co${\mathrm{F}}_2$ is a simple two-sublattice antiferromagnet and has a rutile crystal structure. Each ${\mathrm{Co}}^{++}$ ion is surrounded by a rhombically distorted octahedron of fluorine anions and the crystal-field parameters are known from an analysis of infrared absorption measurements. A good description of a single ${\mathrm{Co}}^{++}$ system can be obtained in terms of a spin Hamiltonian with $S=\frac{3}{2}$. In this paper, an exchange Hamiltonian for the entire lattice is determined in which the only unknowns are the exchange parameters $J_1$ and $J_2$ describing interactions between nearest- and next-nearest-neighbor ${\mathrm{Co}}^{++}$ ions, respectively. The resulting intermediate coupling problem is attacked by spin-wave methods, introducing operators which represent deviations of spin from the molecular-field ground state. The parameters $J_1$ and $J_2$ are determined by calculating the antiferromagnetic resonance frequency and the temperature dependence of sublattice magnetization, and by fitting theory to experiment. We find that $J_1$ is very small and probably ferromagnetic. With the exchange Hamiltonian now completely determined, a molecular-field treatment is shown to reproduce closely the measured parallel and perpendicular magnetic susceptibilities in the temperature range 0 to 300°K (except in the immediate vicinity of the Néel temperature), and is used to discuss the nuclear magnetic resonance of ${\mathrm{Co}}^{59}$ in Co${\mathrm{F}}_2$.