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

Abstract

Ferromagnetic resonance (FMR) derivative linewidths were measured from $3\text{to}12\mathrm{GHz}$ on $50\mathrm{nm}$ thick sputtered polycrystalline Fe-Ti-N films with $3\mathrm{at.}\%$ titanium and a nitrogen content $\left(x_N\right)$ from $1.9\text{to}12.7\mathrm{at.}\%$. The measurements were made with both stripline and waveguide FMR spectrometers. Linewidths were generally lowest at $x_N=7\mathrm{at.}\%$, with derivative linewidth $\left(\Delta H\right)$ values in the $15–25\mathrm{Oe}$ range and a nominally linear increase with frequency $\left(f\right)$. 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 $8–11\mathrm{Oe}$, 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 $883\mathrm{Oe}$, with the minimum also at the bcc to bct structural transition at $x_N=7\mathrm{at.}\%$.