From corrosion layers to high-performance storage devices, the Fe-Cr-O ternary system contains typical “old” materials with widespread applications. In this PhD thesis, the macroscopic behavior of this multifaceted system is analyzed in the light of their microscopic properties. To do so, a comprehensive investigation of high-purity, epitaxial Fe-Cr-O thin films is performed in combination with theoretical calculations. Focus is placed on two stable structures of the Fe-Cr-O system: ∝-Cr₂O₃ and (Fe,Cr)₃O₄ series. The first part of this thesis holds a discussion about the effects of lattice strain in the electronic structure of ∝-Cr₂O₃ thin films. Here epitaxial ∝-Cr₂O₃(0001) thin films were synthetized in three model strain scenarios: completely relaxed, under compressive or under tensile in-plane strain. For each scenario, Cr 2p X-ray photoemission (XPS) spectra were recorded and their features were analyzed by crystal field multiplet (CFM) calculations. According to this approach, the lattice strains introduce distortions in the Cr local environment that split the 3d valence orbitals triplet t2g into a1 + e orbitals. For relaxed thin films, the XPS spectroscopic signatures were well reproduced by CFM calculations when the t2g subset was fully degenerated. Meanwhile, spectra of films under tensile (or compression) in-plane strain were simulated properly when a1 orbitals was stabilized (or destabilized) with respect to e orbitals. Therefore, one proved that subtle changes observed in the lines-shape of the Cr 2p XPS spectrum can be used to identify stress states on complex oxides. This convenient methodology (XPS+CFM), which was first optimized using model samples, was then used to successfully identify stress states on polycrystalline ∝-Cr₂O₃ layers grown during the oxidation of a commercial chromia-forming alloy. The second part of this thesis holds a discussion about the effects of composition, cation disorder and microstructure on the physical properties of (Fe,Cr)₃O₄ thin films. Here stoichiometric series of epitaxial (Fe,Cr)₃O₄(111) thin films were prepared by molecular beam epitaxy (MBE) and their structural, chemical and physical properties were studied by a number of characterization methods. These thin films are composed by Cr³⁺ ions sitting exclusively at octahedral (Oh) interstices and Fe²⁺ and Fe³⁺ ions distributed at Oh and tetrahedral (Td) interstices of the spinel structure. Once again, spectroscopic signatures (here from X-ray magnetic circular dichroism or XMCD) of Cr and Fe were explored by CFM calculations in order to obtain their valence and local environment. More importantly, we were able to determine the cation site distribution, i.e. the relative concentration of each specie on each interstice of the spinel structure. This information is crucial in the analysis of physical properties. In general, we observed that increasing Cr content steadily decreases the magnetic moment and increases the resistivity of the (Fe,Cr)₃O₄ thin films, which is consistent with previous studies in bulk samples. However, unlike bulk structures, the thin films kept an intermediate spinel structure (i.e. Fe²⁺ and Fe³⁺ ions in both Oh and Td-sites) for high Cr content. Because of that, all thin films presented Curie temperatures (Tc) above the room temperature, which is further proved using another theoretical model: the atomistic spin simulations. Furthermore, three possible conductive paths (involving electron hopping between Fe²⁺ and Fe³⁺ sites) were analyzed for the (Fe,Cr)₃O₄( thin films. Because both Fe²⁺ and Fe³⁺ ions sit at Oh-sites (even at high Cr content), these films are less resistive than the bulk as the favored Oh-to-Oh electron-hopping path is still available. In conclusion, we showed that tuning the physical properties of the (Fe,Cr)₃O₄ series can be achieved through the composition and size modulation.