Ferromagnetic resonance line broadening and shift effect in nanocrystalline thin magnetic films: Relation with crystalline and magnetic structure

With the rapid development of telecommunication technologies and highly integrated electronic devices, researchers show great interest in nanocrystalline soft magnetic thin films with unique characteristics for microwave applications. An important direction of the current research in this field is the study of high-frequency magnetization dynamics that directly depends on the damping processes in a magnetic medium. This paper reports on the effect of sharp broadening and shift of the ferromagnetic resonance (FMR) line revealed experimentally in a 40-nm-thick nanocrystalline permalloy (Fe ₂₀Ni ₈₀ ) thin film at a frequency of about 5 GHz. The effect arises only in films with crystallite size exceeding some critical value D _cr . The micromagnetic simulation demonstrates that exchange and dipolar interactions between randomly oriented crystallites form in the film a quasiperiodic magnetic structure with a characteristic wavelength in the range from 36 nm to 3.3 µm. An analysis of the two-magnon scattering model and simulation results shows that the formed magnetic structure provides the energy transfer from uniform magnetization oscillations (uniform FMR) to spin waves, which results in an additional energy dissipation channel and, consequently, sharp FMR line broadening. A theoretical estimate of the critical crystallite size D _cr based on this model yields a value of ~14.3 nm for 40-nm-thick Fe ₂₀Ni ₈₀ films.