Microwave damping in polycrystalline Fe-Ti-N films: Physical mechanisms and correlations with composition and structure

Ferromagnetic resonance (FMR) derivative linewidths were measured from 3 to 12 GHz on 50 nm thick sputtered polycrystalline Fe-Ti-N films with 3 at. % titanium and a nitrogen content (x(N)) from 1.9 to 12.7 at. %. The measurements were made with both stripline and waveguide FMR spectrometers. Linewidths were generally lowest at x(N)=7 at. %, with derivative linewidth (Delta H) values in the 15-25 Oe range and a nominally linear increase with frequency (f). This minimum linewidth composition is connected with the bcc to bct structural transition in the Fe-Ti-N system. Linewidths increased at both larger and smaller x(N) values and were accompanied by the development of a more rounded frequency profile that is indicative of two-magnon scattering. All of the Delta H vs f data could be fitted successfully with a constant inhomogeneity broadening linewidth of 8-11 Oe, a two-magnon scattering (TMS) linewidth from the random grain-to-grain fluctuations in the effective anisotropy field directions for the polycrystal, and a magnon-electron (m-e) intrinsic relaxation term modeled through Gilbert damping with a single alpha value of 0.003. The actual fits were done through the convolution of a Gaussian linewidth for the inhomogeneity term and a Lorentzian linewidth for the TMS and m-e terms. The fitted anisotropy field parameters from the TMS analysis ranged between 398 and 883 Oe, with the minimum also at the bcc to bct structural transition at x(N)=7 at. %.