Spin-Torque Ferromagnetic Resonance in W/Co - Fe - B/ W/Co - Fe - B/Mg O Stacks

We investigate the magnetic properties of as-grown and annealed W/Co-Fe-B/W(insertion)/Co-Fe-B/MgO stacks with different thicknesses of Co-Fe-B layers and W insertion layers using spin-torque ferromagnetic resonance (ST FMR) technique. The dependences of perpendicular magnetic anisotropy (PMA), damping constant, and interlayer exchange coupling on the annealing, Co-Fe-B layer thicknesses, and W insertion layer thicknesses are systematically studied. The PMA is strongly enhanced after annealing at 400 °C. Nevertheless, the Gilbert damping constant remains nearly unchanged after annealing. The PMA also increases with the thickness of the W insertion layer. Both acoustic (in-phase) mode and optical (out-of-phase) mode are observed. The optical mode, which is typically weak when using the conventional FMR measurement, shows a comparable magnitude to the acoustic mode. The effective excitation of the optical mode is ascribed to the nonuniform spin-orbit torque acting on the two coupled layers. Furthermore, the interlayer exchange coupling (ferromagnetic or antiferromagnetic) can be identified through analyzing the two resonance modes, which is found to depend on the W insertion layer thickness, Co-Fe-B layer thickness, and annealing conditions. The experimental results will be useful for developing high-frequency magnetic devices based on magnetic multilayer films with high PMA and thermal stability. Our experimental results also show that the ST FMR is an effective methodology for studying interlayer exchange coupled systems.