Contribution of the Two-Magnon Process to Magnetostatic-Mode Relaxation

Abstract

The linewidths of the magnetostatic modes have been measured at $X$ band with the static magnetic field along the [111] direction in several highly polished spheres of single-crystal pure yttrium iron garnet (YIG). The large variations of the magnetostatic-mode linewidths previously found in larger linewidth spheres were not observed in the highly polished YIG spheres. However, there is a residual variation of mode linewidths at both 300\ifmmode^\circ\else\textdegree\fi{}K and 4.2\ifmmode^\circ\else\textdegree\fi{}K, the variation being somewhat larger at 4.2\ifmmode^\circ\else\textdegree\fi{}K. It was found that the essential features of the variation of mode linewidths in a highly polished YIG sphere can be explained by the surface-pit-scattering theory of Sparks, Loudon, and Kittel when the linewidth predicted by this theory is multiplied by an enhancement factor ${F}_{\mathrm{nmr}}$ to include the effect of the spatial variation of magnetization intensity for the individual magnetostatic modes. The results of calculations based on the Sparks-Loudon-Kittel theory for the linewidths due to scattering by fractional-micron-size pits are presented, as well as computed values of the enhancement factor ${F}_{\mathrm{nmr}}$ for most of the low-order modes. From the measurements it is estimated that at both temperatures the contribution of the two-magnon process to the uniform-precession linewidth is 0.10 Oe or less. This indicates that the major relaxation channel for the uniform-precession mode is not via degenerate spin-wave interactions. However, at 4.2\ifmmode^\circ\else\textdegree\fi{}K this may be due to rare-earth impurities broadening the linewidth. The conclusions reached regarding the contribution of the two-magnon process to magnetic resonance relaxation of the uniform-precession mode in highly polished YIG spheres are in substantial agreement with the modulation experiments of Fletcher, LeCraw, and Spencer.