We report measurements of the frequency and temperature dependence of ferromagnetic resonance (FMR) for a 15-nm-thick yttrium iron garnet (YIG) film grown by off-axis sputtering. Although the FMR linewidth is narrow at room temperature [corresponding to a damping coefficient $\alpha =(9.0\pm 0.2)\times {10}^{-4}]$, comparable to previous results for high-quality YIG films of similar thickness, the linewidth increases strongly at low temperatures, by a factor of almost 30. This increase cannot be explained as due to two-magnon scattering from defects at the sample interfaces. We point out that the increased low-temperature linewidth can be explained by impurity relaxation mechanisms that were elucidated 50 years ago in bulk YIG samples. High-purity starting materials and careful optimization of growth protocols to avoid nonstoichiometries should therefore be employed for making low-temperature thin-film YIG devices.

• Received 6 December 2016
• Revised 11 April 2017

DOI:https://doi.org/10.1103/PhysRevB.95.174411