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. We show the observation of a nonlinear magnetoresistance (called bilinear magnetoelectric resistance, BMR) and nonlinear Hall effect in a prototypical topological insulator Bi₂Se₃ [1,2] and Weyl semimetal WTe₂ [3]. 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 BMR and the spin texture was established, which enables a novel transport probe of spin textures. We further extended the observation of BMR effect to the two-dimensional electron gas (2DEG) at a SrTiO₃ (STO) (111) surface [4]. The BMR probes a three-fold out-of-plane spin texture, in addition to an in-plane one at the STO(111) surface 2DEG. 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, applicable to frequency doubling [5] and energy harvesting applications [6].

[1] P. He et al., Nat. Phys. 14, 495 (2018).
[2] P. He et al., Phys. Rev. Lett. 123, 016801 (2019).
[3] P. He et al., Nat. Commun. 10, 1290 (2019).
[4] P. He et al., Phys. Rev. Lett. 120, 266802 (2018).
[5] P. He et al., Nat. Commun. 12, 698 (2021).
[6] D. Kumar et al., Nat. Nanotechnol., https://doi.org/10.1038/s41565-020-00839-3 (2021).