Spin-Circuit Representation of Spin Pumping Into Topological Insulators and Determination of Giant Spin Hall Angle and Inverse Spin Hall Voltages

Topological insulators and giant spin-orbit torque switching of nanomagnets are among the frontier topics for the development of energy-efficient spintronic devices. Spin-circuit representations involving different materials and phenomena are quite well established now for their prowess of interpreting experimental results and then designing complex and efficient functional devices. Here, we construct the spin-circuit representation of spin pumping into topological insulators, considering both the bulk and surface states with parallel channels, which allows for the interpretation of practical experimental results. We show that the high increase in effective spin mixing conductance and inverse spin Hall voltages cannot be explained by the low-conductive bulk states of topological insulators. We determine a high spin Hall angle close to the maximum magnitude of one from experimental results and address the controversy in the literature by correctly estimating the parameters involved in the system. With an eye to designing energy-efficient spin devices, we further employ a spin-sink layer in the spin-circuit formalism to increase the effective spin mixing conductance at low thicknesses and double the inverse spin Hall voltage.