Mechanism of spin ordering in Fe$_{3}$O$_{4}$ nanoparticles by surface coating with organic acids

TL;DR

This study reveals how organic acid coatings influence spin ordering in Fe3O4 nanoparticles by preventing surface spin-flip phenomena and promoting ferromagnetic interactions, explaining the near-bulk magnetization values.

Contribution

It introduces a new mechanism based on electronic structure calculations showing organic acids hinder surface spin-flip and enhance ferromagnetic superexchange in coated Fe3O4 nanoparticles.

Findings

Organic acids limit surface reconstruction and spin-flip at Fe sites.

Coated nanoparticles exhibit increased ferromagnetic superexchange.

The mechanism is supported by a simplified Fe(III) dimer model.

Abstract

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.