Antiferromagnetic Resonance in MnO: Co: A Measurement of the Magnetoelastic Properties of theCo2+Ion

The "out-of-plane" antiferromagnetic resonance frequency ${\omega }_1$ of MnO has been followed as a function of Co-impurity concentration in the range 0.5-6.0 mole%. Using far-infrared techniques, ${\omega }_1$ is observed to shift from 27.7 ${\mathrm{cm}}^{-1\mathrm{}}$ in pure MnO to 38.0 ${\mathrm{cm}}^{-1\mathrm{}}$ for 6.0% Co doping. Line broadening precluded the possibility of following the resonance to higher impurity concentrations. Measurements of ${\omega }_1$ as a function of temperature confirm the magnetic nature of the resonance absorption lines, and are consistent with existing data which show that the Néel temperature of the mixed system is a linear function of impurity concentration. The perpendicular susceptibility, on the other hand, appears to change very little as Co is added to MnO. The square of the frequency ${\omega }_1$ is found to be proportional to the Co concentration, suggesting that ${\mathrm{Co}}^{2+}$ single-ion anisotropy is primarily responsible for the shift, and an anisotropy contribution of 32.8 ${\mathrm{cm}}^{-1\mathrm{}}$/ion is deduced from the results. The theory of the magnetostriction of CoO developed by Kanamori is modified to deal with the present case, and it is shown that the observed anisotropy can be readily understood in these terms. The relevance of this result to the properties of CoO is discussed, in particular, the problem of magnetic anisotropy and the possibility of a dynamic Jahn-Teller effect in the paramagnetic state. Some features of antiferromagnetic resonance (AFMR) and anisotropy in doped antiferromagnets relevant to the interpretation of the experimental results are developed in two appendixes.