Field-free spin-orbit-torque switching of perpendicular magnetization aided by uniaxial shape anisotropy.

It has been demonstrated that the switching of perpendicular magnetization can be achieved with spin-orbit torque (SOT) at an ultrafast speed and low energy consumption. However, to make the switching deterministic, an undesirable magnetic field or unconventional device geometry is required to break the structure inverse symmetry. Here we propose a novel scheme for SOT-induced field-free deterministic switching of perpendicular magnetization. The proposed scheme can be implemented in a simple magnetic tunnel junction (MTJ)/heavy-metal system, without the need of complicated device structure. The perpendicular-anisotropy MTJ is patterned into elliptical shape and misaligned with the axis of the heavy metal, so that the uniaxial shape anisotropy aids the magnetization switching. Furthermore, unlike the conventional switching scheme where the switched final magnetization state is dependent on the direction of the applied current, in our scheme the bipolar switching is implemented by choosing different current paths, which offers a new freedom for developing novel spintronics memories or logic devices. Through the macrospin simulation, we show that a wide operation window of the applied current pulse can be obtained in the proposed scheme. The precise control of pulse amplitude or pulse duration is not required. The influences of key parameters such as damping constant and field-like torque strength are discussed as well.