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 spin spirals but with a minute (of the order of a few mu eV/atom) energy difference with skyrmionic states. The spontaneous low-temperature skyrmions, with similar to 15 nm to similar to 35 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 and more dispersed with the enhancement of the Ir buffer thickness. Also, with increasing n, the skyrmions' stability decreases when an external magnetic field is applied or the temperature is raised. For n = 0 and n = 1, we find that metastable skyrmioniums can occur, which are characterized by a slightly lower stability with respect to the external fields and larger critical currents, compared to skyrmions.