Microwave Properties of Fine Grain Polycrystalline Yttrium Iron Garnet

Parallel pump and subsidiary absorption thresholds at 9.1 GHz and effective linewidth (ΔHeff) at 9.9 GHz have been measured for polycrystalline YIG samples with average grain diameter a0 from 1 to 30 μ. The spin-wave linewidths extrapolated to zero wavenumber, ΔHk→0, vary as a0−1. The ΔHk for a0=1 μ shows an inverse k dependence. A simple model in which the spin-wave transit time across individual grains limits ΔHk explains the a0−1 dependence but not the observed k dependences for ΔHk. The high-field ΔHeff is 5.5 Oe for a0=1 μ and varies as a0−1/2. These ΔHeff are significantly larger than single-crystal linewidths. Two-magnon interactions cannot contribute to the high-field ΔHeff. A recently proposed magnetostriction mechanism is consistent with the a0−1/2 dependence, but the predicted magnitudes are much smaller than observed.