To realize high-speed nonvolatile magnetic memory with low energy consumption, electric switching of perpendicular magnetization by spin-orbit torque in the heavy metal/ferromagnetic (HM/FM) structure has recently attracted intensive attention. Conventionally, an external in-plane magnetic field for breaking the symmetry is required for achieving electric switching of perpendicular magnetization. However, electric switching without external field is the prerequisite for the integration of magnetic functionality into the integrated circuit devices. Here, we propose a new method of utilizing a W wedge in the Pt/W/FM structure to induce a spin current gradient, which can result in an in-plane equivalent field along the wedge thickness gradient direction. We experimentally demonstrate the deterministic magnetization switching of perpendicular Co/Ni multilayers without external magnetic field when the electric current is along the wedge thickness gradient direction. Our findings shed light on free field electric switching of magnetization by a new physical parameter-an asymmetric spin current induced by a bilayer wedge structure.
Keywords: free field electric switching; spin current gradient; spin-hall effect; spintronics; spin−orbit torques.