Atomistic model of magnetisation dynamics and equilibrium properties of magnetic tunnel junctions

The next generation of portable devices requires random access memories (RAMs) which are characterised by high performance and at the same time low power consumption. A promising candidate as replacement in this sector is magnetic RAM (MRAM) based on CoFeB/MgO magnetic tunnel junctions, since it retains the fast operational capability while reducing the energy requirements due to its non-volatile nature. Despite such outstanding features, a complete understanding of the device operation has not been achieved yet. This limits the scaling of the device below the next technological node (< 20nm). The reduction in the number of atoms constituting these devices is responsible for surface and finite size effects as well as a reduced thermal stability, which affects the dynamic properties. Currently, micromagnetism represents the most used theoretical approach to investigate magnetic materials and their properties. The main limitation of this model is the continuum approach. In this approximation the atomic properties are averaged, which makes it unsuitable to describe interface, surface and finite size effects as well as thermal effects. Do deal with these limitations, in this thesis we use an atomistic spin model to investigate the equilibrium and dynamical properties of CoFeB/MgO-based systems, an approach more appropriate to account for such effects. Our results suggest that CoFeB/MgO systems are characterised by more complex magnetic properties than usually assumed. The reversal mechanism is non-uniform at dimensions and temperatures that are technologically relevant. Moreover, at these time scales thermal effects cause a distribution of the dynamic properties that represent an intrinsic limitation to the device reliability and need to be addressed in order to achieve the desired scaling. The results also show the importance of atomistic models to understand and accurately describe the magnetic properties of devices when they are fabricated on the nano scale.