We present theoretical evidence that atomically sharp domain walls in antiferromagnetic materials have qualitatively different properties from their smooth counterparts, in which the reversal of the antiferromagnetic order parameter occurs gradually over many unit cells. A remarkable difference appears when we consider the effect of the domain wall on the propagation of an antiferromagnetic spin wave. Antiferromagnetic spin waves, unlike ferromagnetic ones, have two possible states of circular polarization: left handed and right handed. While a smooth domain wall does not distinguish between the two cases, allowing both types of waves to be transmitted with high probability (tending to 100% in the infinitely smooth limit), an atomically sharp domain wall can act as a spin wave polarizer, i.e., it allows one type of polarization to be almost completely transmitted while the other is almost completely reflected. This remarkable behavior occur in the vicinity of a sharp to smooth transition, which is controlled, within our model, by the ratio of the easy axis anisotropy to the nearest-neighbor exchange constant. The polarization of the transmitted spin wave depends on the orientation of the spins in the sharp domain wall - a property which can be controlled by an external field or spin torque and has no counterpart in a smooth domain wall. Our discovery of the spin-wave polarizing properties of sharp antiferromagnetic domain walls suggests that they could be used as spin polarizers in magnonic circuits.
An intriguing property of a three-dimensional topological insulator (TI) is the existence of surface states with spin-momentum locking. We report the discovery of a new type of Hall effect in a TI Bi2Se3 film [1]. The Hall resistance scales linearly with both the applied electric and magnetic fields and exhibits a π/2 angle offset with respect to its longitudinal counterpart, in contrast to the usual angle offset of π/4 between the linear planar Hall and anisotropic magnetoresistance. At variance with the nonlinear Hall effect due to Berry curvature dipole in time-reversal invariant materials, this novel nonlinear planar Hall effect originates from the conversion of a nonlinear transverse spin current to a charge current due to the concerted actions of spin-momentum locking and time-reversal symmetry-breaking, which also exists in other non-centrosymmetric materials [e.g., WTe2 and the 2DEG on the SrTiO3(001) surface] with a large span of magnitude.
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.
We theoretically investigate a new kind of nonlinear magnetoresistance on the surface of three-dimensional topological insulators (TIs). At variance with the unidirectional magnetoresistance (UMR) effect in magnetic bilayers, this nonlinear magnetoresistance does not rely on a conducting ferromagnetic layer and scales linearly with both the applied electric and magnetic fields; for this reason, we name it bilinear magneto-electric resistance (BMER). We show that the sign and the magnitude of the BMER depends sensitively on the orientation of the current with respect to the magnetic field as well as the crystallographic axes - a property that can be utilized to map out the spin texture of the topological surface states via simple transport measurement, alternative to the angle-resolved photoemission spectroscopy (ARPES).
We present a general drift-diffusion theory beyond linear response to explain the unidirectional magnetoresistance (UMR) observed in recent experiments in various magnetic heterostructures. In general, such nonlinear magnetoresistance may originate from the concerted action of current-induced spin accumulation and spin asymmetry in electron mobility. As a case study, we calculate the UMR in a bilayer system consisting of a heavy-metal (HM) and a ferromagnetic metal (FM), where the spin accumulation is induced via the spin Hall effect in the bulk of the HM layer. Our previous formulation [cf. PRB 94, 140411(R) (2016)] is generalized to include the interface resistance and spin memory loss, which allows us to analyze in details their effects on the UMR. We found that the UMR turns out to be independent of the spin asymmetry of the interfacial resistance, at variance with the linear giant-magnetoresistance (GMR) effect. A linear relation between the UMR and the conductivity-spin asymmetry is revealed, which provides an alternative way to control the sign and magnitude of the UMR and hence may serve as an experimental signature of our proposed mechanism.
The spin Hall effect (SHE) and the inverse spin Hall effect (ISHE) are well established phenomena in current spintronics research. A third important effect is the current-induced spin polarization, which, within the Rashba model for a spin-orbit coupled two-dimensional disordered electron gas, has been predicted by Edelstein in 1990 and it is referred to as the Edelstein effect (EE). This effect is deeply connected to the above two effects thanks to a constraint dictated by the equation of motion. Less known is the inverse Edelstein effect (IEE), which is the Onsager reciprocal of the EE and according to which a charge current is generated by a non-equilibrium spin polarization. The IEE has been recently observed (Nature Commun. 4, 2944 (2013)) in a hybrid ferromagnetic-metal system. In this talk I provide a precise microscopic definition of the IEE and its description within the Rashba model. It turns out that the effect has a surprisingly simple interpretation when the spin-charge coupled drift-diffusion equations governing it are cast in the language of a SU(2) gauge theory, with the Rashba spin-orbit coupling playing the role of a generalized spin-dependent vector potential. After sketching briefly the derivation of the drift-diffusion equations, the latter are applied to the interpretation of the experiments. The role of spin-orbit coupling due to impurities is also considered, by showing that the strenght of the IEE can be controlled by the ratio of the spin relaxation rates associated to the two type of spin-orbit coupling.
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