Unidirectional orbital magnetoresistance in light-metal–ferromagnet bilayers

Abstract

We report the observation of a unidirectional magnetoresistance (UMR) that originates from the nonequilibrium orbital momentum induced by an electric current in a naturally oxidized Cu/Co bilayer. The orbital-UMR scales with the torque efficiency due to the orbital Rashba-Edelstein effect upon changing the Co thickness and temperature, reflecting their common origin. We attribute the UMR to orbital-dependent electron scattering and orbital-to-spin conversion in the ferromagnetic layer. In contrast to the spin-current induced UMR, the magnon contribution to the orbital-UMR is absent in thin Co layers, which we ascribe to the lack of coupling between low energy magnons and orbital current. The magnon contribution to the UMR emerges in Co layers thicker than about 5 nm, which is comparable to the orbitalto-spin conversion length. Our results provide insight into orbital-to-spin momentum transfer processes relevant for the optimization of spintronic devices based on light metals and orbital transport.

Large Interfacial Rashba Interaction Generating Strong Spin–Orbit Torques in Atomically Thin Metallic Heterostructures

“…Moreover, recent theory has evidenced additional orbital currents in light elements from orbital Hall effects (OHE) and/or orbital Rashba effects (ORE) superimposing to the spin currents . These orbital currents may result in the generation of orbital torques , and orbital Rashba magnetoresistance. , This questions the actual origin of the magnetic torques in multilayers with light elements.…”

“…30 These orbital currents may result in the generation of orbital torques 12,31 and orbital Rashba magnetoresistance. 13,32 This questions the actual origin of the magnetic torques in multilayers with light elements.…”

Efficient Noncollinear Antiferromagnetic State Switching Induced by the Orbital Hall Effect in Chromium

“…However, recent theoretical work unveils that, other than a transverse spin current from SHE, an electric field applied along the plane of a metallic film can also induce a transverse orbital current due to orbital Hall effect (OHE). − Unlike the SHE, the OHE does not require SOC, and therefore, it was predicted to exist in a wide range of materials including light metals such as Al, Cu, and Cr. − Although the OHE is perceived to be more fundamental than SHE, experimental investigations on the OHE are relatively recent because of primarily two reasons: difficulty in separating the two effects that typically occur concurrently in materials with the SOC and absence of direct coupling between orbital Hall current and magnetization. Nevertheless, there is increasing evidence that the OHE is present in various light metals with negligible or weak SOC, − including the orbital Rashba–Edelstein effect. − Similar to SHE, the OHE can also induce SOT, − , orbital Hall magnetoresistance, and unidirectional orbital Hall magnetoresistance in ferromagnet (FM)/nonmagnetic metal (NM) bilayers. Notably, SOT-induced magnetization switching has been demonstrated in CoFeB/Cr without a heavy metal layer .…”

“…Nevertheless, there is increasing evidence that the OHE is present in various light metals with negligible or weak SOC, 12−15 including the orbital Rashba− Edelstein effect. 16−18 Similar to SHE, the OHE can also induce SOT, [12][13][14]16 orbital Hall magnetoresistance, 17 and unidirectional orbital Hall magnetoresistance 19 in ferromagnet (FM)/ nonmagnetic metal (NM) bilayers. Notably, SOT-induced magnetization switching has been demonstrated in CoFeB/Cr without a heavy metal layer.…”

Current-Induced Magnetization Switching in Light-Metal-Oxide/Ferromagnetic-Metal Bilayers via Orbital Rashba Effect

“…Despite its similarity to the SRE, ORE is independent of SOC, and inversion symmetry breaking due to hybridization between O p states and Cu d states is sufficient for its emergence. Recent experiments provide evidence for the existence of an orbital torque by measuring spin-torque ferromagnetic resonance and magnetoresistance in FM/CuO x bilayers that do not contain any heavy elements. − However, crucial current-induced magnetization switching using ORE has not yet been realized, although it is the prerequisite for the application of spintronic devices.…”