These results suggest the formation of an FeOx shell encasing the Fe-based metallic alloy in the reduced SFMNR (Figure ). Furthermore, depth-profiling XPS indicates that the Mo cation compensates for the reduction in Fe valence during nanoparticle formation, thereby stabilizing the perovskite host structure. The averaged valence state of Mo in the reduced SFMNR (approximately 5.85 in the bulk) is higher than that in the oxidized SFMNR (Figure ). The distribution of valence states of both Fe and Mo from surface to bulk regions is illustrated schematically in Figure . The emergence of FNR@FeOx nanoparticles and oxygen vacancies on the surface of SFMNR significantly lowers the Fe valence, resulting in an elevation of Mo valence to maintain the electroneutrality of the system. This dynamic alteration of cation valences with depth is crucial for stabilizing the formation of FNR@FeOx nanoparticles on the SFMNR perovskite oxide.