Nonlocal feedback in ferromagnetic resonance

Ferromagnetic resonance in thin films is analyzed under the influence of spatiotemporal feedback effects. The equation of motion for the magnetization dynamics is nonlocal in both space and time and includes isotropic, anisotropic, and dipolar energy contributions as well as the conserved Gilbert and the nonconserved Bloch damping. We derive an analytical expression for the peak-to-peak linewidth. It consists of four separate parts originated by Gilbert damping, Bloch damping, a mixed Gilbert-Bloch component, and a contribution arising from retardation. In an intermediate frequency regime the results are comparable with the commonly used Landau-Lifshitz-Gilbert theory combined with two-magnon processes. Retardation effects together with Gilbert damping lead to a linewidth whose frequency dependence becomes strongly nonlinear. The relevance and the applicability of our approach to ferromagnetic resonance experiments are discussed.