To shed light on the oxidation state of the Mn cations in the samples under study, we analyzed the photoelectron spectral region corresponding to the Mn 3s region. The fitting procedure for the ferrite nanoparticles with empirical x values of 0.37, 0.47, and 1.40 are shown in Figure -E and summarized in Table in the S.I. All spectra were successfully fitted with two sets of doublets and an extra contribution at higher binding energy (BE). A detailed explanation of the signals found in this region and for Fe 3s signal is provided in Section 1 in the S.I. In brief, the doublet with the lower BE corresponds to Mn cations, and the one with higher BE corresponds to Fe cations; note also that the relative intensities of both sets are qualitatively in agreement with the expected stoichiometry of the ferrite samples, confirming that the Mn/Fe atomic fraction is proportional to x (Figure in the S.I.).