We report on the effect of preparation temperature in the magnetic properties of oxidized iron thin films deposited by dc-magnetron sputtering below room temperature. Films prepared at 300 K show a typical thin film magnetic behavior, whereas samples prepared at 200 K present visible features of granular core-shell system formed by an oxide shell surrounding a ferromagnetic core and displaying exchange anisotropy. These differences are directly linked to the film microstructure and composition. We present results of microstructure, composition, and chemical analysis in order to discuss the observed magnetic behavior. Size and shape of iron crystallographic grains were characterized by transmission electron microscopy. Depth-resolved compositional characterization, obtained by ion-beam analysis techniques, has probed the penetration of oxygen along the thickness of the films and x-ray photoelectron spectroscopy was used to identify the different iron oxide species present in the structures.