The switching paths of spin transfer torque magnetic random access memories.

Summary In magnetic spin transfer memories, the magnetization of a ferromagnetic thin film is flipped under the effect of a polarized current. In this manuscript, we study the way this reversal occurs, called the reversal path. After having established the basic theoretical concepts and performed a state of the art of the reversal path, I present the results of our micromagnetic simulations. We have studied the rollover path as a function of the device diameter. These numerical calculations predict a rollover composed of a coherent phase followed by the nucleation and propagation of a domain wall. This rollover path is expected for devices the 20 to 100 nm at room temperature, thus in our future measurements. The domain wall propagation observed in the simulations exhibits complex Walker oscillations that are not explained by the state-of-the-art models. Also I present a more complete wall dynamics model, where the exact geometry of the system is taken into account. In this geometry the elasticity of the wall gives rise to a new field that we call the stretching field. This stretching field plays a crucial role in the wall dynamics and will allow us to understand and predict the complex Walker oscillations. Our measurements are performed on the latest generation of magnetic spin transfer memory devices, based on a perpendicular anisotropy magnetic tunnel junction. The diameter of our devices varies between 26 and 200 nm. We perform magnetometry, ferromagnetic resonance and time-resolved electrical measurements of the switching. The measured turning path in the latter exhibits the signatures of a coherent initial phase followed by a domain wall shift, as calculated in our simulations. The strong Walker oscillations predicted by our models are observed for specific samples where the free layer has few defects, but not in our most standard samples. This highlights the value of our analytical work in understanding rollover in devices for industrial applications.