Two-dimensional atomic-scale magnetic skyrmion lattice of Fe/Ir(111)

Skyrmions are topologically protected field configurations with particle-like properties which were believed to crystallize only under non-equilibrium conditions. Recently, it has been predicted that they can also become ground states in magnetic systems with broken inversion symmetry [1]. We give theoretical evidence of a two-dimensional lattice of atomic-scale skyrmions as the magnetic ground state of a monolayer Fe on Ir(111), a system which has been studied by spin-polarized scanning tunneling microscopy [2]. We apply an extended Heisenberg model based on parameters determined from first-principles to identify the four-spin interaction as its microscopic origin. This interaction couples degenerate spin spirals and enforces a square-symmetry on the skyrmionic spin texture decoupled from the underlying hexagonal atomic lattice. [1] U. K. R\"o{\ss}ler et al., Nature 442, 797 (2006). [2] K. von Bergmann et al., PRL 96, 167203 (2006).