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

Studying resonance modes in exchange-coupled ferromagnetic bilayers is typically accomplished by a magnetic-field-excited ferromagnetic resonance (FMR) technique, in which case it is usually difficult to excite the optical mode, because the out-of-phase resonance cannot efficiently couple to a uniform excitatory magnetic field. This work shows that spin-torque FMR (ST-FMR) can be used to efficiently excite the optical resonance mode, because the current-induced torque naturally does not act uniformly on the two ferromagnetic layers. The results indicate that ST-FMR is an effective probe of interlayer exchange coupling in such systems, to the benefit of research on spintronic devices.