Microwave-driven ferromagnet–topological-insulator heterostructures: The prospect for giant spin battery effect and quantized charge pump devices

We study heterostructures where a two-dimensional topological insulator TI is attached to two normalmetal NM electrodes while an island of a ferromagnetic insulator FI with precessing magnetization covers a portion of its lateral edges to induce time-dependent exchange field underneath via the magnetic proximity effect. When the FI island covers both lateral edges, such device pumps pure spin current in the absence of any bias voltage, thereby acting as an efficient spin battery with giant output current even at very small microwave power input driving the precession. When only one lateral edge is covered by the FI island, both charge and spin current are pumped into the NM electrodes. We delineate conditions for the corresponding conductances current-to-microwave-frequency ratio to be quantized in a wide interval of precession cone angles, which is robust with respect to weak disorder and can be further extended by changes in device geometry. The origin of the quantization is explained using spatial profiles of local spin and charge currents in the reference frame rotating with the magnetization, which concomitantly reveals how current exiting from the chiral spin-filtered edge states within the TI region remains largely confined to a narrow flux within the NM electrodes that is refracted at the TI NM interface.