Discovery of quantum confinement effect in sputtered topological insultor films and observation of room-temperature high spin–orbit torque*

In this talk, we will report and discuss the experimental demonstration and first-principles calcuation of quantum confinement in topological insulator. First, we report Bi_xSe_(1-x) films with high SOT at RT grown onto thermally oxidized silicon substrates by magnetron sputtering, which is a semiconductor industry compatible process. The dc planar Hall and ST-FMR methods were used for the characterization of SOT in Bi_xSe_(1-x)/CoFeB heterostructures with in-plane CoFeB. At RT, the of the sputtered Bi_xSe_(1-x) film is up to two orders of magnitude larger than that of HMs. Notably, we developed perpendicular CoFeB multilayers on Bi_xSe_(1-x) films, and we demonstrated switching of the magnetization using SOT arising from the Bi_xSe_(1-x) with very low switching current density in bilayers at RT. Second, we will report our discovery of the quantum confinement effect in topological insulators. The sputter deposited Bi_xSe_(1-x) has granular structure with grain size as small as approximately 6 nm. Electronic band-structure analysis indicates that the reduced dimensionality and quantum confinement strongly influences the spintronic properties of the TI. Our theory identifies the presence of lowly dispersive surface bands with large charge-to-spin conversion efficiency in nanoscale grains, which might explain the experimentally observed enhancement in the figure-of-merit. The demonstrated , ease of growth of the films on a silicon substrate, and successful growth and switching of perpendicular CoFeB multilayers on Bi_xSe_(1-x) film provide an avenue for the use of Bi_xSe_(1-x) as a spin-density generator in SOT-based memory and logic devices.

Mahendra DC, et al, "Room-temperature high spin–orbit torque due to quantum confinement in sputtered Bi_xSe_(1–x) films" Nature Materials. 17, 800-807 (2018); DOI: 10.1038/s41563-018-0136-z