Abstract
Magnetic random access memory (MRAM) has got intensive attention during these years. For spin-orbit-torque (SOT) MRAM, it possesses lower write current compared to spin-transfer-torque (STT) MRAM, which improves the energy efficiency and the scalability in logic circuit. The spin-orbit-torque arises mainly from two difference sources, one is the spin current generated by bulk spin Hall effect (SHE), the other one is the effective electrical field induced by the heterostructure interfaces. With the basic structure composed of a ferromagnetic (FM) layer and an adjacent heavy mental layer, when the in-plane current flowing into the structure will generate the pure spin current perpendicular to the film acting on the magnetization of FM. In the FM materials performing the perpendicular magnetic anisotropy (PMA), generally, we need an additional in-plane magnetic field to allow the deterministic switching. Despite this in-plane field may can be integrated into the devices by back-end process, like package. It still rises the complexity and provides additional steps for practical application. Besides, in most material system with PMA, the degradation of thermal stability is a huge problem when we want to scale down the magnetic devices. We have known that acquiring a build-in field or other forces in the structure may be a way to solve the issue of external field for engineering in SOT-MRAM. So in this work, we used an antiferromagnetic (AFM) materials adjacent the FM layer to provide an in-plane bias field by interlayer coupling. The field-free binary switching was accomplished successfully after the post field-annealing along the in-plane direction. Surprisingly, we found that the interfacial spins at FM/AFM interface were also reversed by the SOT with the magnetization of FM layer. This shift in perpendicular hysteresis loop displayed the unidirectional anisotropy at zero external field after the electric pulses. This innovative method to switch the direction of exchange bias tremendously increased the thermal stability for magnetic state in practical applications. As a result, our researches offer a great improvement and a significant breakthrough for SOT-MRAM, as well as other spintronic devices.