Two‐Magnon‐Scattering Contributions to FMR Linebrdadening in Polycrystalline Garnets

Measurements of the FMR linewidth, ΔH, of different Walker modes and of the lineshift, δf, of the UPR mode have been made as a function of frequency and temperature on spheres of polycrystalline garnets of pure YIG and CaVIn substituted YIG with reduced anisotropy field H_a. The experimental results can be explained on the basis of anisotropy and porosity (p) induced two‐magnon‐ scattering contributions, ΔH_a and ΔH_p. Excellent quantitative agreement is found with Schlömann's theory for ΔH_a and δf, apart from the region close to the top of the spinwave manifold (SWM). The deviations are related to the inhomogeneity ratio, ($\text{4π}{\mathrm{M}}_{\mathrm{s}}/{\mathrm{H}}_{\mathrm{a}}$ ), and may be explained by the pertubation of the SWM, by approach to saturation effects, and by the density of magnetostatic modes (MSM) evaluated from numeric calculations. The frequency and field dependence of the separated porosity scattering contribution ΔH_p show an excellent quantitative agreement with ΔH_eff measurements reported by Vrehen, Broese van Groenou and Patton but contradict known theories. ΔH_p consists of a nearly field independent fraction arising from scattering to high‐k states and a usually much larger fraction, showing a distinct dependence on cos θ_o of low‐k spinwaves. Possible coupling models, based on these results, are presented. The experimental findings provide a guide for the evaluation of H_a and p of polycrystalline ferrites from ΔH‐measurements.