Formation and stability of metastable skyrmionic spin structures with various topologies in an ultrathin film

Abstract: We observe metastable localized spin configurations with topological charges ranging from $Q=-3$ to $Q=2$ in a (Pt${0.95}$Ir${0.05}$)/Fe bilayer on Pd$(111)$ surface by performing spin dynamics simulations, using a classical Hamiltonian parametrized by ab initio calculations. We demonstrate that the frustration of the isotropic exchange interactions is responsible for the creation of these various types of skyrmionic structures. The Dzyaloshinsky--Moriya interaction present due to the breaking of inversion symmetry at the surface energetically favors skyrmions with $Q=-1$, distorts the shape of the other objects, and defines a preferred orientation for them with respect to the underlying lattice.