Magnetic phases at the molecular scale: the case of cylindrical Co nanoparticles

The magnetic phases of cobalt nanocylinders at the molecular scale have been studied by means of density functional theory together with micromagnetism. Diameters of the objects are under 1 nm. The magnetic phases resulting from first-principle calculations are far from obvious and quite different from both semiclassical results and extrapolations from what is measured for larger objects. These differences reinforce the importance of the quantum mechanical approach for small nanoscopic particles. One of the main results reported here is precisely the unexpected order in the last filled orbitals, which produce objects with alternating magnetic properties as the length of the cylinder increases. The resulting anisotropy is not obvious. The vortex phase is washed out due to the aspect ratio of the systems and the strength of the exchange constants for Co. Nevertheless, we do a pedagogical experiment by turning gradually down the exchange constants to investigate the kind of vortex states which are hidden underneath the ferromagnetic phases.