Effect of Grain Size on the Microwave Properties of Polycrystalline Yttrium Iron Garnet

Abstract

Data on the parallel pump and subsidiary absorption threshold at 9.1 GHz and effective linewidth ΔH_eff at 9.9 GHz have been obtained for polycrystalline YIG materials with an average grain diameter a₀ from 1 to 30 μ. The fine‐grain samples (1–10 μ) were made by hot pressing. The parallel pump spin‐wave line‐widths extrapolated to zero wavenumber, ΔH_k→0, vary as a₀⁻¹, with ΔH_k→0=25 Oe at a₀=1 μ. The ΔH_k for the fine‐grain samples (a₀ ≈ 1 μ) show an inverse k‐dependence. A simple model in which the spin‐wave transit time across individual grains limits ΔH_k appears to explain the a₀⁻¹ dependence but not the observed k‐dependences for ΔH_k. The high‐field ΔH_eff is 5.5 Oe for a₀=1 μ and varies as a₀^−1/2. The low‐field values are slightly larger, due to a small two‐magnon contribution arising from coupling to degenerate short‐wavelength spin waves. Even though the high‐field ΔH_eff contains no two‐magnon contribution, it is significantly larger than linewidths in single crystals. A recently proposed magnetostriction mechanism is consistent with the a₀^−1/2 dependence, but the predicted magnitude of ΔH_eff is much smaller than observed.