Spin-Wave Instability and Premature Saturation in Antiferromagnetic Resonance

A semiclassical theory of spin-wave instability in antiferromagnetic resonance is developed. The calculation is valid both for simple antiferromagnets and for canted systems when the canting is due either to single ion magnetocrystalline anisotropy or anisotropic exchange. The nonlinear terms leading to the instability are found to originate in the anisotropy and exchange energies. The critical radio frequency field for the onset of instability is calculated and in general is given by hc=4ΔH₀(γΔHₖω₀)1/2, where ΔH₀ and ΔHₖ are, respectively, the uniform mode and spin-wave linewidths; and ω₀ is the zero-field antiferromagnetic resonance frequency. Experimental evidence for the existence of spin-wave instability in the canted antiferromagnet, KMnF₃, is presented and discussed. An anomalous low-level saturation of the rf susceptibility is attributed to the onset of spin-wave instability. A new technique for the measurement of spin-wave linewidths is described. This technique, which is based on the response of the resonant system to amplitude modulated microwave power, yields results in agreement with the instability theory. The spin-wave linewidth as determined by these experiments is approximately three orders of magnitude narrower than the linewidth of the uniform mode, indicating that the broadening mechanisms are grossly different in the two cases.