Antiferromagnetic Resonance inCr2O3

Abstract

The antiferromagnetic resonance spectrum of single-crystal ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ has been mapped within the wavelength region from 12 mm to 2.8 mm and magnetic field intensities from 0 oersteds to 30 000 oersteds. The resonance frequencies were observed in this region in a temperature interval from about 17°C to 35°C, the Néel temperature, $T_N$. When the [111] crystal axis is perpendicular to the direction of the external magnetic field, the observed resonance locus on a frequency vs magnetic field diagram forms a single curve at each temperature; when the two are parallel, the resonance locus forms a curve of two branches, one of which has not previously been observed for this material. These loci are found to be in good agreement with the resonance theory of Nagamiya, and of Keffer and Kittel. Good theoretical agreement is also found for variations of line width with magnetic field strength and for variations in the peak absorption intensity at resonance. The anisotropy energy $K$ which depends upon the temperature, $T$, as obtained from the data is proportional to ${(T_N-T)}^{0.87}$ over the observable interval. Assuming that $K$ is proportional to a fixed power of sublattice magnetization at all temperatures and that sublattice magnetization behaves as the appropriate Brillouin function of temperature, we can extrapolate to absolute zero and thus find $K=800\mathrm{}000$ ergs/${\mathrm{cm}}^3$.