Competing effect of spin-orbit torque terms on perpendicular magnetization switching in structures with multiple inversion asymmetries.

Current-induced spin-orbit torques (SOTs) in structurally asymmetric multilayers have been used to efficiently manipulate magnetization. In a structure with vertical symmetry breaking, a damping-like SOT can deterministically switch a perpendicular magnet, provided an in-plane magnetic field is applied. Recently, it has been further demonstrated that the in-plane magnetic field can be eliminated by introducing a new type of perpendicular field-like SOT via incorporating a lateral structural asymmetry into the device. Typically, however, when a current is applied to such devices with combined vertical and lateral asymmetries, both the perpendicular field-like torque and the damping-like torque coexist, hence jointly affecting the magnetization switching behavior. Here, we study perpendicular magnetization switching driven by the combination of the perpendicular field-like and the damping-like SOTs, which exhibits deterministic switching mediated through domain wall propagation. It is demonstrated that the role of the damping-like SOT in the deterministic switching is highly dependent on the magnetization direction in the domain wall. By contrast, the perpendicular field-like SOT is solely determined by the relative orientation between the lateral structural asymmetry and the current direction, regardless of the magnetization direction in the domain wall. The experimental results further the understanding of SOTs-induced switching, with implications for spintronic devices.

Switching of perpendicular magnetization by spin-orbit torques in the absence of external magnetic fields.

Magnetization switching by current-induced spin-orbit torques is of great interest due to its potential applications in ultralow-power memory and logic devices. The switching of ferromagnets with perpendicular magnetization is of particular technological relevance. However, in such materials, the presence of an in-plane external magnetic field is typically required to assist spin-orbit torque-driven switching and this is an obstacle for practical applications. Here, we report the switching of out-of-plane magnetized Ta/Co(20)Fe(60)B(20)/TaO(x) structures by spin-orbit torques driven by in-plane currents, without the need for any external magnetic fields. This is achieved by introducing a lateral structural asymmetry into our devices, which gives rise to a new field-like spin-orbit torque when in-plane current flows in these structures. The direction of the current-induced effective field corresponding to this field-like spin-orbit torque is out-of-plane, facilitating the switching of perpendicular magnets.

Voltage-induced ferromagnetic resonance in magnetic tunnel junctions.

We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have similar magnitudes, and thus that both torques are equally important for understanding high-frequency voltage-driven magnetization dynamics in MTJs. As an example, we show that VCMA can increase the sensitivity of an MTJ-based microwave signal detector to the sensitivity level of semiconductor Schottky diodes.