Micromagnetic simulations of the main state of ferromagnetic nanodots with uniaxial anisotropy

The work performed numerical simulations of the ground state of the ferromagnetic nanodots with uniaxial anisotropy. The calculation of the ground state was performed using a three-dimensional micromagnetic simulation package OOMMF for two types of sample geometry - cubic and ellipsoidal with a simple cubic lattice. Cases were analyzed as a ferromagnet of the “easy axis” and “easy plane”. In the simulation permalloy material parameters and materials such as SmCo were used. The latter allowed values of the uniaxial anisotropy which is several orders higher than technologically realizable values from permalloy. It is shown that for an ellipsoidal nanodots with permalloy material parameters, there is a transition region, the size of the sample in which the ground state is highly degenerated. It is found that above the upper boundary of the transition region there is a tendency to vortex stabilization of the ground state. For any sample size permalloy effects depending on the topology of the ground state of the system on the magnitude and sign of the constant uniaxial magnetic anisotropy have been identified. For a cubic specimen geometry permalloy existence of a stable vortex in the ground state is possible if the size of nanodots is several times higher than that of the ellipsoidal case. For samples with material parameters SmCo large uniaxial anisotropy has a stabilizing effect on the vortex structure in the ground state nanodots. Vortex state is feasible for much smaller nanodots, in comparison with the permalloy. Under specific size of nanodots SmCo ground state (in the case of easy-axis) acquires skyrmion-like character.