Higher-order magnetic skyrmions in nonuniform magnetic fields

For a two-dimensional Hubbard model with spin-orbit Rashba coupling in external magnetic field the structure of effective spin interactions is studied in the regime of strong electron correlations and at half-filling. It is shown that in the third order of perturbation theory, the scalar and vector chiral spin-spin interactions of the same order arise. The emergence of the latter is due to orbital effects of magnetic field. It is shown that for nonuniform fields, scalar chiral interaction can lead to stabilization of axially symmetric skyrmion states with arbitrary topological charges. Taking into account the hierarchy of effective spin interactions, an analytical theory on the optimal sizes of such states, the higher-order magnetic skyrmions, is developed for axially symmetric magnetic fields of the form $h(r)\ensuremath{\sim}{r}^{\ensuremath{\beta}}$ with $\ensuremath{\beta}\ensuremath{\in}\mathbb{R}$.