Complex magnetic textures in Ni/Ir$_{n}$/Pt(111) ultrathin films

A combined approach using first-principles calculations and spin dynamics simulations is applied to study Ni/Ir$_{n}$/Pt(111) ($n=0,1,2$) films. The lowest-energy states are predicted to be almost degenerate with negligble energy differences between pure spin-spiral and skyrmionic states. Moreover, for $n=0$ and $n=1$, we found that metastable skyrmioniums can occur, which are characterized by a slightly lower stability with respect to the external fields, enhanced lifetime, and the same critical current density as skyrmions. The spontaneous low temperature skyrmions, with $sim$10 nm to $sim$20 nm size, arise from a large Dzyaloshinskii-Moriya (DM) and Heisenberg exchange interactions ratio and, in particular, from a large in-plane DM vector component for nearest neighbors. The skyrmions become larger, faster and more dispersed with the enhancement of the Ir buffer thickness. Also, with increasing textit{n}, the skyrmions stability decrease when an external magnetic field is applied or the temperature is raised.