Ferromagnetic resonance linewidth mechanisms in polycrystalline ferrites: Role of grain-to-grain and grain-boundary two-magnon scattering processes

Abstract

The frequency dependence of the ferromagnetic resonance linewidth $\ensuremath{\Delta}H$ in polycrystalline hot isostatically pressed pure and Ca-Ge substituted yttrium iron garnet (YIG) spheres was measured between 1.95 and 6 GHz and analyzed. The linewidths for these ultradense ferrite materials are in the 5--40 and 5--25 Oe ranges for the pure and the substituted YIG samples, respectively. The $\ensuremath{\Delta}H$ vs frequency data show especially sharp Buffler peaks due to the band-edge crossover effect that is the signature for grain-to-grain (GG) two-magnon scattering (TMS) [C. R. Buffler, J. Appl. Phys. 30, S172 (1959)]. Both the GG and recent grain-boundary (GB) TMS models were used to obtain quantitative fits to the data. The fits demonstrate the expected GG TMS contribution to the linewidth for frequencies above the Buffler peak. They also reveal two unexpected results: (1) a dominant GB TMS role for frequencies below the Buffler peak where the GG TMS linewidth contribution is essentially zero and (2) a significant GB TMS role for frequencies above the peak where the linewidth has generally been associated with GG processes only. In the high-frequency regime, the GB scattering term appears to explain about half of the overall linewidth.