Spin-orbitronics, which takes advantage of spin-orbit coupling (SOC), has expanded the research objects of spintronics to nonmagnetic materials. Here, we report the emerging nonlinear spintronic phenomena in the inversion-asymmetric nonmagnetic materials with SOC. For instance, the surface state of three-dimensional topological insulator (TI) owns helical spin textures with the spin and momentum perpendicularly locked. We show the observation of a nonlinear magnetoresistance (called bilinear magneto-electric resistance, BMER) and nonlinear Hall effect in a prototypical TI Bi2Se3, which scale linearly with both the applied electric and magnetic fields. We further reveal that these effects are originated from the conversion of a nonlinear spin current to charge current under the application of an external magnetic field. A close link between the BMER and the spin texture was established in TI surface states, which enables a novel transport probe of spin textures. We further extended the observation of BMER effect to the d-orbital two-dimensional electron gas (2DEG) at a SrTiO3 (STO) (111) surface. The BMER probes a three-fold out-of-plane spin texture, in addition to an in-plane one at the STO(111) surface 2DEG. This novel spin texture is in contrast to the conventional one induced by the Rashba effect. By performing tight-binding supercell calculations, we find that this 3D spin texture is fully described by the confinement effects of the STO t2g conduction band in the (111) plane. These findings open a new branch in spintronics, which discusses the nonlinear transport effects in spin-polarized nonmagnetic materials, and is therefore referred to as nonlinear spintronics.
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.