Field Dependence of the Effective Linewidth Due to Porosity and Anisotropy in Polycrystalline Yttrium Iron Garnet and Calcium-Vanadium-Substituted Yttrium Iron Garnet at Microwave Frequencies
Authors: Patton Carl E. 
Published: February 2004

The field dependence of the uniform precession relaxation rate, or equivalently, an effective linewidth ΔHeff, can provide valuable information concerning the relaxation which is inaccessible from standard linewidth determinations. Determinations of ΔHeff vs static field H0 have been made as a function of porosity in polycrystalline yttrium iron garnet (YIG) and as a function of anisotropy in polycrystalline calcium-vanadium-substituted yttrium iron garnet (YCVF). The experimental results were obtained from microwave susceptibility data taken on spherical samples at room temperature and 10 GHz. Porosity ranged from 21% to less than 1% for the YIG. The YCVF materials had anisotropy fields from 114 to 350 Oe. Far from resonance ΔHeff is small, on the order of a few Oe, and field independent. Near resonance, ΔHeff increases sharply and exhibits a strong peak. The size and symmetry of the peak is a strong function of both porosity and anisotropy. In general, ΔHeff peaks at fields above the field for resonance Hres for the porous materials. The magnitude of the peak increases and the curve broadens considerably with increasing porosity. For the YCVF materials, ΔHeff peaks below Hres for the low-anisotropy samples, and peaks above resonance for the high-anisotropy materials. The effective linewidth increases and the curves broaden with increasing anisotropy. These results indicate that a simple linewidth parameter is only a qualitative measure of the relaxation rate for the uniform precession near resonance and that the actual field dependence of the relaxation is significant. The experimental results can be explained on the basis of a two-magnon scattering relaxation rate associated with inhomogeneties due to porosity and the anisotropy in randomly oriented crystallites for the polycrystalline sample. The increase in ΔHeff for a field interval around resonance is due to the degeneracy of long-wavelength spin waves with the uniform precession mode at the excitation frequency for field values inside the interval. The small field independent ΔHeff outside this interval is due to the absence of a large scattering contribution to the relaxation rate for such fields. The differences in the size and symmetry of the ΔHeff peak for the different materials are due to differences in the nature of the scattering interactions in each case. Anisotropy-related scattering results in a peak below resonance and porosity related scattering results in a peak above resonance. The broadening of the curves with increasing porosity and anisotropy is due to secondary scattering interactions, which do not involve the uniform precession mode at the excitation frequency and which allow a scattering contribution to the relaxation rate even outside the field interval for degeneracy. Theoretical curves have been obtained for ΔHeff vs static field utilizing a recent calculation by Schlömann for the scattering relaxation rate due to anisotropy which includes secondary scattering. For high anisotropy materials, crystallographic resonances modify the experimental results appreciably so that the analysis used to extract ΔHeff from the data, as well as the theory, are not strictly applicable. For the low-anisotropy YCVF materials, the agreement is quite good.