Spin wave instability processes in anisotropic ferrite materials

Abstract

A theoretical and experimental study of spin wave instability processes in anisotropic ferrite materials has been performed. The theory of spin wave instability for ferromagnetic insulators is extended to include generalized anisotropy based on a tensor formulation of the static and dynamic effective fields. The formalism is set up for saturated magnetic ellipsoids and a general microwave field configuration. The analysis yields working formulae for the threshold microwave field amplitude evaluations and critical mode determinations. Numerical simulations are used to calculate threshold field and critical modes for first order oblique pumping processes in a thin disk with easy plane anisotropy. The theory was successfully applied to the analysis of comprehensive experimental data on anisotropic ferrite materials for various static and microwave field configurations.

The dependence of the threshold field amplitude on the external field was investigated experimentally at 9 and 16.7 GHz. The high power thresholds were measured for different geometrical configurations and spin wave relaxation rates were determined. The data showed qualitative agreement with the theoretical calculations. The spin wave linewidth was found to be independent of the spin wave propagation direction. The specific results are presented for three different ferrite materials, liquid phase epitaxy (LPE) yttrium iron garnet (YIG) single crystal thin films, polycrystalline hipped YIG, and Y-type zinc (Zn-Y) single crystal easy plane hexagonal ferrite.

In LPE YIG films, the spin wave linewidth was 0.2 Oe at 9 GHz and the influence of thin film geometry and small magnetocrystalline anisotropy on the high power thresholds was found. In hipped YIG, the spin wave linewidth was determined by the grain size and was 1.2 Oe at 9 GHz and 0.6 Oe at 16.7 GHz. In Zn-Y, 9 GHz data demonstrated related sample size effects, so that high power thresholds were inversely proportional to the sample lateral size. At 16.7 GHz, there were no sample size effects and oblique pumping measurements were performed. It was found that the spin wave linewidth increases from 12 Oe to about 18 Oe if the magnetization is pulled out of the easy crystallographic plane.