Microwave effective linewidth in polycrystalline ferrites: Precision measurements and new interpretations

Experimental investigations have been made to resolve the origins and intrinsic limits of microwave loss in polycrystalline bulk ferrites through the enhanced technology and theory. First, the measurement accuracy has been improved ten-fold relative to previous data through the use of a high quality microwave cavity, a metrological ABA method, and a fine adjustment of gyromagnetic ratio. Second, a full theory of classical spin wave excitation and scattering, including the effects of conductivity, damping, a general anisotropy, and grain boundaries, has been developed based on quaternion analysis and operator algebra.;Effective linewidths have been accurately measured at 10 GHz on ultra dense hot isostatic pressed (hipped) and conventionally sintered (CS) polycrystalline yttrium iron garnet ferrite specimens. (1) The high field data have demonstrated the important and previously unappreciated role of spin wave coupling to degenerate electromagnetic spin wave (EM-SW) modes. The data for fields well above the highly degenerate bulk manifold show field dependence and closely track the computed density of such low wave number (k) EM-SW states and extrapolate to known intrinsic single crystal linewidths in the extreme high field limit. Comparison between spheres and disks confirms the corresponding three-dimensional and two-dimensional EM-SW scattering. (2) Below a field point shifted up from the high field manifold edge by two-thirds of the saturation magnetization, the moderately porous CS material shows a microstructure-related increase in the loss related to the conventional moderate k dipole-exchange spin waves. This confirms the pseudo manifold effect due to pore-demagnetizing fields. (3) The low field data show an elbow at 0.23 T, the same field for which the density of degenerate EM-SW modes shows an abrupt change. The data at low field is generally larger than at high field. Such discrepancy has been attributed to the high k scattering due to random grain boundaries, in addition to the pseudo manifold effect. (4) The in-manifold effective linewidth for the hipped material shows a pure anisotropy scattering as expected.