UCLA

During recent years, enormous progress has been made in the spintronics research field which utilizes the spin degree of freedom of electrons in addition to their charge for information processing with the goal to achieve non-volatile spintronic memory and logic devices with fast speed, high density, good reliability and low power consumption. In particular, novel materials have been incorporated in the magnetic structures to realize manipulation and/or switch of magnetic moment using the least possible power. Among them, heavy metals and topological insulators (TIs), which exhibit strong spin-orbit coupling, have been employed to generate spin-orbit torques (SOTs) to enable efficient switching of magnetic moment, which may thus lead to the next generation green spintronic devices.

In this work, we will explore the current-induced SOTs in TI-based magnetic structures. First, we present the magnetization switching through giant SOT induced by an in-plane current in a TI/Cr-doped TI bilayer heterostructure. The critical current density required for switching is below 8.9�10^4 A/cm^2 at 1.9 K. Both the effective spin-orbit field to current ratio and the spin-torque efficiency are found to be three orders of magnitude larger than those reported for conventional heavy metal/ferromagnet heterostructures.

Second, we show the effective electric-field control of the giant SOT in a uniformly Cr-doped TI thin film using a top-gate field-effect transistor structure. We demonstrate that the SOT strength can be modulated by a factor of 4 within the accessible gate voltage range, and it shows strong correlation with the spin-polarized surface current arising from surface spin-momentum locking in the film. Furthermore, we demonstrate the magnetization switching by scanning gate voltage with constant current and in-plane magnetic field applied in the Cr-doped TI thin film.

Last but not least, we summarize the research results and discuss future potential research opportunities and challenges in this field. There are still many unresolved questions in this new research field. Nevertheless, the giant current-induced SOT we have observed in TI-based magnetic structures suggests that it might have wide implications in the next generation gate-controlled, ultralow power spintronic devices.