Using the TPP-2 formula developed by Tanuma, Powell, and Penn, it is possible to estimate the inelastic mean free path (IMFP) of the photoelectrons coming from both spectral regions, which is a function of the photoelectron kinetic energy. For the Mn and Fe 3s photoelectrons with kinetic energy of around 1400 eV, the IMFP is roughly 2.1 nm; in the case of the Mn and Fe 2p photoelectrons with kinetic energy about 800 eV, the IMFP is roughly 1.4 nm. The comparison between both data sets is displayed graphically in Figure in the S.I; note that the dotted line with a slope equal to 1 represents the ideal case where the composition remains homogenous along the first 6 nm. However, it is apparent that for the three samples the average Mn/Fe atomic ratio is higher in the first 4 nm than if it is computed extending the probed depth for other 2 nm inside the nanoparticle core. In other words, the nanoparticle's surface is enriched in Mn content with respect to the deeper zones near the core. The deviation from compositional homogeneity is accentuated as the level of Mn substitution becomes lower; this fact could indicate that the incorporation of Mn into the ferrite structure starts in the most superficial layers and then extends toward the interior of the MNP.