  Spin-polarized scanning tunneling microscopy has been applied to study
non-collinear spin textures of a Mn monolayer on a four-fold symmetric W(001)
substrate revealing a zero-field spin spiral ground state and two different
types of rotational domain walls. With an applied magnetic field of 9 T, we
observe a coexistence of the spin spiral and the skyrmion phase, even though a
previous theoretical study reported that the phase transition occurs at 18 T.
The skyrmions show a roughly hexagonal arrangement despite the square lattice
symmetry of the W(001) substrate. Based on a reduced set of energy parameters,
we are able to describe the experimental findings and to analyze the
topological properties of the rotational domain walls.
