  We predict the occurrence of metastable skyrmionic spin structures such as
antiskyrmions and higher-order skyrmions in ultra-thin transition-metal films
at surfaces using Monte Carlo simulations based on a spin Hamiltonian
parametrized from density functional theory calculations. We show that such
spin structures will appear with a similar contrast in spin-polarized scanning
tunneling microscopy (SP-STM) images. Both skyrmions and antiskyrmions display
a circular shape for out-of-plane magnetized tips and a two-lobe butterfly
contrast for in-plane tips. An unambiguous distinction can be achieved by
rotating the tip magnetization direction without requiring the information of
all components of the magnetization.
