Magnetization reversal by spin orbit torque in a perpendicularly magnetized nanomagnet: A micromagnetic study

The discovery that the magnetization of a perpendicularly magnetized nanomagnet can be reversed by an in-plane current via spin orbit torque (SOT) has opened a new way to manipulate magnetization at the nanoscale. This novel switching mechanism has led to a concept of non-volatile magnetic memory MRAM, namely the SOT-MRAM, which combines low power, fast switching, reliability and large endurance. Although numerous experimental works were devoted to the study of magnetization switching by SOT, the mechanism of the magnetization reversal induced by SOT is poorly understood. Magnetization switching experiments were first interpreted in terms of a macrospin model, but although a qualitative agreement could be achieved, the predicted switching current densities were very high compared to the experimental ones. On the contrary, recent experimental works suggested that the magnetization during the switching is inhomogeneous and that switching occurs by domain nucleation followed by domain wall propagation. However, the switching mechanisms were not elucidated. Here we present the results of micromagnetic simulations and analytical modelling which shed a new light on the magnetization reversal in the presence of SOT. We show that the Dzyaloshinskii-Moriya interaction (DMI) plays a key role in the reversal process by leading to a deterministic domain nucleation on the edge of the nanomagnet which triggers the reversal. A simple analytical model, taking into account the magnetization tilting on the edge due to the DMI, allows to calculate the switching current, which is the current for domain nucleation on the edge. Our model provides a much better agreement with experimental results compared to the macrospin approach.