Theory of Magnetic Excitations in Random Antiferromagnets. I. Antiferromagnetic Resonance

A theory for obtaining the dynamic susceptibility χ(ω, H ) of random magnets is developed by using distribution functions and applied to analyses of the antiferromagnetic resonance of dilute magnets and mixtures of two magnetic ions. It is shown that χ''(ω, 0) of the dilute magnet with small anisotropy exhibits a single resonant peak which decreases in frequency with concentration of magnetic ions, while that with large anisotropy exhibits some peaks which do not decrease in frequency but change their intensities with decreasing concentration. On the other hand, χ''(ω, 0) of the mixture generally exhibits two peaks with structure. Our results are found to be qualitatively in good agreement with recent experimental data for T x S 1-x F 2 and Rb 2 T x S 1-x F 4 , where T =Mn or Co and S =Zn, Mg, Co or Ni.