Spin wave freezing in Re/Co/Pt multilayers

Abstract

The phenomenon of spin wave (SW) freezing occurs in the Damon-Eshbach mode in thin film magnetic systems, when SW phase and group velocities both go to zero, and the wave ceases to oscillate and move, preserving its shape as a domain structure pattern. This effect is related to the spin reorientation transition, where the magnetization configuration changes between the homogeneous in-plane state and domain structure with the out-of-plane magnetization component state. Here, we study the SW freezing effect in [Re/Co/Pt]$_{20}$ magnetic multilayers, induced by varying the in-plane external magnetic field. The studies were performed on nanostructures with the quality factor $Q$ (ratio of uniaxial anisotropy to demagnetization energies) greater and smaller than one. Domain structures with an out-of-plane magnetization component were observed in these multilayers. The critical field, visible as the saturation field $H_s^{||}$ in the parallel static magnetization curve measured by superconducting quantum interference device (SQUID), is also manifested in the field-dependent vector-network-analyzer ferromagnetic resonance (VNA-FMR) experiment, which measures the homogeneous magnetization oscillations. Brillouin Light Scattering (BLS) spectra, recorded for several values of wave vectors and several field values, probed the field-evolution of the dispersion relation. Micromagnetic simulations allow one to obtain a full dispersion, in good agreement with VNA-FMR and BLS results. Around $H_s^{||}$ the simulated dispersion relations approach the conditions for SW freezing. Below $H_s^{||}$ low and high frequency VNA-FMR modes are related to magnetization oscillations inside domain walls and within domains, respectively. The experimental results of static and dynamic behavior, together with micromagnetic simulations, create an overall consistent picture of the investigated multilayers.