For the preparation of CoO(100) films, Ir(100) was first cleaned and transformed into a Ir(100)-(2×1)O structure (as described above). Afterwards, the sample was annealed in 2 x 10 -7 mbar H2 (Linde, 5.3) at 533 K for 1 min and then at the same temperature in UHV for 1 min. This resulted in a metastable Ir(100)-(1×1) reconstructed surface. Next, a thin layer of metallic cobalt was deposited on the surface (IFil = 2.75 A, UHV = 950 V, IEm = 11 mA, Iion = 30 nA, 6 min) in UHV at a sample temperature of 343 K. After cooling the sample to below 193 K, cobalt was then deposited in a reactive oxygen atmosphere (7 x 10 -7 mbar) for 3 min. Subsequently, the sample was annealed at 373 K for 3 min in UHV to obtain a thin ordered CoO(100) structure.