Limits of the macrospin model in cobalt nanodots with enhanced edge magnetic anisotropy

We report on a theoretical study, by an atomic scale micromagnetic model, of the magnetic configurations and magnetization reversal in cobalt circular nanodots (one and two atomic layers thick, up to 10 nm in diameter) with enhanced magnetocrystalline anisotropy at the edge. According to the dot diameter, out-of-plane, twisted, and in-plane configurations are found. The results are first compared to the macrospin model, usually applied to this type of nanoparticles, and then to a continuous micromagnetic model with a sole radial variation of the magnetization. Field induced magnetization switching is also computed. We observe a process close to coherent rotation for nanodot diameters as small as 2 nm, but with a lower switching field as compared to the macrospin model, that may be called quasiuniform rotation. Our results fix the application limits of the macrospin model, which forgets atomic-scale variation of the properties.