Magnetic Properties of the Canted Antiferromagnetα-CoSO4

It is known from neutron scattering experiments that $\alpha$-CoS${\mathrm{O}}_4$ is a four-sublattice canted antiferromagnet with no net magnetic moment. In this paper the magnetic properties of this material are analyzed using a model in which an isotropic fictitious spin of ½ is assigned to each ${\mathrm{Co}}^{++}$ ion. The large canting angle of 25° is interpreted in terms of antisymmetric terms in the Hamiltonian due to anisotropic superexchange and the large anisotropy in the $g$ values. Corresponding to the four sublattices, there are four spin-wave modes at $k=0\mathrm{}$. Calculations indicate that the resonant frequencies of these modes should lie in the far infrared, and that only three of the modes should be observable spectroscopically. In addition, static susceptibilities of the system have been calculated for $T=0\mathrm{}$ and $T\gg T_N$ (12°K), and the $g$ values have been estimated. Far-infrared transmission experiments have resulted in the observation of three lines at 20.6, 25.4, and 35.8 ${\mathrm{cm}}^{-1\mathrm{}}$, with relative intensities 1:1:0.1. Although these lines are presumed to be the three expected resonances, an unambiguous fitting for all the parameters of the model has not been possible. The temperature dependence of the resonance lines is anomalous.