High-field microwave effective linewidth in polycrystalline ferrites: Physical origins and intrinsic limits

High-precision high-field effective (HFE) linewidth measurements have been made at 10GHz on ultradense hot isostatic pressed and conventionally sintered (CS) yttrium iron garnet ferrite materials. The accuracy was increased ten-fold relative to previous data through the use of a high-quality cavity, a modified measurement technique, and a fine adjustment of the gyromagnetic ratio based on the field-frequency response of the loaded cavity. The HFE linewidths for fields well above the region of degenerate bulk spin-wave band decreases with increasing field and extrapolates to known intrinsic single-crystal linewidths in the extreme high-field limit. From a field point shifted up from the high-field band edge dependent on degenerate dipole-exchange spin waves by 2∕3 the saturation induction, the data track closely the computed density of states for electromagnetic spin waves. In the case of the CS material, for fields below this point, one sees a microstructure-related increase related to conventional moderate wave number bulk-band spin waves.