Spin-Pumping-Induced Non-Linear Electric Current on the Surface of a Ferromagnetic Topological Insulator

Abstract: We investigate the spin-pumping-induced electric current on the surface of a three-dimensional topological insulator hybridized with a ferromagnet, namely, ferromagnetic topological insulator. In order to do this, we establish the microscopic formalism and construct the perturbation theory using a Keldysh Green's function approach. We analyze how this electric current is generated by an exchange interaction and an external ac magnetic field, which is the driving force of ferromagnetic resonance as well as the spin pumping. The mechanism is as follows. First, the ferromagnetic resonance is driven and a zero-momentum magnon emerges. It is the fluctuation from the saturation magnetization pointing parallel to the precession axis of the ferromagnetic resonance. After then, the spin pumping is generated with the zero-momentum magnon being the carrier of spin. The zero-momentum magnon and the topological insulator surface state couples through the exchange interaction and the spin carried by the magnon is transferred to it. Owing to the spin-momentum locking, the transferred spin is converted into the momentum of topological insulator surface state leading to the generation of electric current flowing perpendicular to the precession axis of the ferromagnetic resonance. It is quadratic in the amplitude of external ac magnetic field whereas it is linear to the strength of the exchange interaction. The associated electric voltage is described by the spectrum of zero-momentum magnon. The non-linearity of spin-pumping-induced electric current in the ac magnetic field as well as the linearity in the exchange-interaction strength reflects that the surface of ferromagnetic topological insulator has a high-performing functionality of generating the electric charge current by magnetic controlling.