  Saturation magnetization values close to the bulk have been reported for
coated magnetite nanoparticles with organic acids. The mechanism of this effect
is not yet understood. Here we show that a previously proposed rationalization
in Nano Letters 12 (2021) 2499-2503 was based on electronic structure
properties that are not consistent with several existing density functional
theory studies. Our study is based on a wide set of Hubbard-corrected density
functional tight binding (DTFB+U) and hybrid density functional theory (HSE06)
calculations on Fe$_{3}$O$_{4}$ nanocubes of more than 400 atoms. We provide a
new explanation for the spin ordering in coated nanoparticles, through the
investigation of spin-flipping phenomena. In particular, we show that the
spin-flip of d electrons at octahedral Fe$^{3+}$ sites, which is confirmed to
be more favorable near the surface, especially where atomic reorganization can
take place such as at corner sites, can be hampered by the presence of adsorbed
organic acids because they do not only limit the surface reconstruction but
also allow for additional ferromagnetic superexchange interaction between
octahedral Fe sites as a consequence of the carboxylates bridging binding mode.
The proof-of-concept of this mechanism is given by a simplified model of the
Fe(III) tert-butoxide dimer.
