Antidamping torques from simultaneous resonances in ferromagnet-topological insulator-ferromagnet heterostructures

We studied the magnetodynamics of ferromagnetic films coupling across a topological insulator (TI) Bi2Se3 layer using ferromagnetic resonance (FMR). TIs have attracted much attention across the physics community as they hold the potential for dissipationless carrier transport, extremely high spin-orbit torques, and are host to novel quantum effects. To investigate the coupling between the ferromagnetic (FM) layers, vector network analyzer (VNA)-FMR measurements of the resonance linewidth were performed as a function of bias field angle. By bringing the resonances of the two FM layers into close proximity, it was possible to observe antidamping torques that lead to a narrowing of linewidth, a characteristic of spin pumping. The element- and hence layer-specific technique of x-ray detected ferromagnetic resonance (XFMR) was used to circumvent the difficulty of obtaining accurate fits to the two overlapping resonances in close proximity. Our results confirm that the interaction across the TI is a dynamic exchange mediated by spin pumping, as opposed to a self-coupling of the surface state or similar, more unconventional mechanisms