Widespread applications of spintronic devices require an efficient method to manipulate the local magnetization. Topological insulators, such as Bi2Se3, are an emerging state of quantum matter, expected to exhibit more efficient charge-to-spin conversion and thus spin-obit torques (SOTs) compared to that of heavy metals.
Here, we investigate the SOTs in the Bi2Se3/ferromagnet hetrostructures at different temperatures by the spin torque ferromagnetic resonance (ST-FMR) technique. We find that the SOTs efficiency abruptly increases from 0.047 at 300 K to 0.42 below 50 K. Moreover, we observe a significant out-of-plane SOT efficiency in the low temperature range. We identify that the spin-momentum-locked topological surface states (TSS) play an important role in the strong SOTs. By decreasing the Bi2Se3 thickness, we achieve TSS dominant SOT. The SOT efficiency is enhanced by more than 5 times below 8 quintuple layers and reaches a maximum value of ~1.75 at 5 quintuple layers at room temperature. In addition, the electron-mediated spin torque induced magnetization switching in the Bi2Se3/NiFe heterostructure is successfully achieved at room temperature. The switching current density, JC, is extremely low (~6 × 10^5 A/cm^2), which is much smaller than that in heavy metals. Our results illustrate that the thinner topological insulators possess a strong capability for generating SOTs.
We also report the magnetization reorientation induced by magnon-mediated spin torque in topological insulator devices. Much less Joule heating is expected by taking advantage of magnons, which can carrier spin angular momentum without involving moving charges. Our observation is of importance for magnon-based future spin-devices.