Uniform Mode Resonance and Spin-Wave Instability in RbMnF3

Abstract

Low‐power resonance linewidth and high‐power second‐order spin wave instability measurements have been performed at 4.2°K on the cubic antiferromagnet RbMnF₃. With increasing power level the uniform mode susceptibility shows a small initial rise, followed by a hyperbolic decline from a critical rf field of 0.32 Oe. The very large low‐power resonance linewidth of 140 Oe, much greater than the width of the spin‐wave band, can be understood by assuming a variation of anisotropy over the sample in the manner suggested by Eastman. The instability thresholds for spin waves excited in the regions of large amplitude precession in nonuniform samples is derived by a iterative solution of the coupled equations of motion. To the first order, the departure from the uniform sample threshold depends upon the range of correlation r₀ of the anisotropy fluctuation. When r₀≫10⁻⁵ m, simple formulas for the susceptibility and critical field can be used. The theory has been fitted to the observed decline of susceptibility curve with the value ΔH_k = 7.6 Oe. This result may be compared with the value ΔH_k = 3×10⁻² Oe which is obtained from a uniform sample theory, and assuming the relaxation rate of the uniform mode to be given by the resonance linewidth.