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.
Tailoring Gilbert damping of metallic ferromagnetic thin films is one of the central interests in spintronics applications. Here we report a giant Gilbert damping anisotropy in epitaxial Co50Fe50 (CoFe) thin film with a maximum-minimum damping ratio of 400 %, determined by broadband spin-torque as well as inductive ferromagnetic resonance (FMR). Our CoFe films are deposited via molecular beam epitaxy at room temperature. The films are then fabricated into micron-scale devices. The first sample series, with CoFe(10 nm) and CoFe(10 nm)/Pt(6 nm), are prepared for spin-torque FMR. The second sample series, with CoFe(20 nm), are prepared for vector network analyzer FMR measurements. In addition to the fourfold magnetocrystalline anisotropy, we also find a large fourfold Gilbert damping anisotropy, along with small and consistent inhomogeneous linewidth broadening. In order to exclude the two-magnon scattering influence on linewidth, we have also conducted spin-torque FMR on a CoFe/Pt sample up to 32 GHz and we don’t find any linewidth slope softening in the frequency range. We conclude that the origin of this damping anisotropy is the variation of the spin-orbit coupling (SOC) for different magnetization orientations in the cubic lattice. The large SOC anisotropy may come from the atomic short-range order in disordered Co-Fe alloy, which preserve global cubic symmetry but can have large effects on SOC. The SOC anisotropy is further corroborated from the large crystalline the anisotropic magnetoresistance in CoFe.
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).
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