Dynamics of a nanoparticle as a one-spin system and beyond

We review some recent results beyond the now established theory of magnetization switching of a nanoparticle within the single-spin approximation. The first extension is that of the Stoner-Wohlfarth model for magnetization static switching under applied magnetic field including the effect of temperature at long-time scales. The second concerns a generalization of the N\'eel-Brown model for thermoactivated dynamic magnetization switching to include the effect of exchange interaction in the framework of Langer's theory in the intermediate-to-high damping limit. We finally argue why the single-spin approximation is not appropriate for very small nanoparticles.