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The topographic and magnetic structures of patterned 20-nm-thick Fe films grown on Si(001), polycrystalline NiO, and SiO2 substrates are compared by scanning tunneling microscopy/magnetic force microscopy measurements under ultrahigh vacuum conditions to investigate the influence of the different substrates. Iron grows as a polycrystalline film. The size of the crystallites decreases from Si to NiO and SiO2 substrates, respectively, while the surface roughness increases. The increased disorder in the film structure does not necessarily lead to an increased disorder in the magnetic domain structure. Instead, the magnetic structure of iron on NiO is the most complex, which can be attributed to the exchange coupling to the substrate. The antiferromagnetic NiO crystallites exhibit no long range crystallographic ordering which leads to random magnetic orientation. More evidence of the influence of the exchange coupling is found when comparing the magnetic structures of iron on NiO and SiO2 as a function of film thickness in the range of 0-10 nm.