Spin-resolved scanning tunnelling microscopy measurements were performed for Co double-layer nanostructures on Pt( 1 1 1). High-resolution spectroscopy shows that the electronic structure is highly inhomogeneous due to dislocations induced by the lattice mismatch between Pt and Co. Spin-polarized measurements allow a magnetic state dependent determination of out-of-plane magnetized islands. In the presence of external fields we have observed two different types of Co double-layer islands which exhibit a surprisingly large coercivity depending on their preparation.