The low field microwave effective linewidth in polycrystalline ferrites

Abstract

High precision measurements on the low and high field effective linewidth ΔHeff at 10GHz have been made on ultradense (UD) and conventionally sintered (CS) polycrystalline yttrium iron garnet (YIG) materials. The high field data confirm previous results on the role of two magnon scattering to low wave number (k) electromagnetic Larmor branch spin waves that lie below the light line. The low field data reveal two important contributions to the effective linewidth. For a field regime from the low k edge of the usual dipole exchange spin wave band down to the point in field (H=HX) where above-the-light-line electromagnetic branch Larmor (EML-HI) spin waves appear, ΔHeff is connected with scattering to relatively high k dipole exchange Larmor (DEL) spin waves. The coupling to these modes comes from grain boundaries in the YIG materials. A grain boundary scattering theory gives reasonable agreement with the data. While the high field effective linewidth due to pseudo in-manifold scattering is larger for the CS samples compared to that for the UD samples, the high k DEL scattering is larger for the UD samples compared to that for the CS samples. This is due to the dominant role of the grain boundaries in the low field ΔHeff. For fields below HX, additional scattering appears for the EML-HI modes. The abrupt appearance of an additional ΔHeff component for H<HX provides direct experimental evidence for the presence of such modes in the spin wave dispersion. Part of the loss contribution for H<HX may be due to subthreshold nonlinear effects.