First-principles insights into structure and magnetism in ultra-small tetrahedral iron oxide nanoparticles.

Structural and magnetic properties of ultra-small tetrahedron-shaped iron oxide nanoparticles were investigated using density functional theory. Tetrahedral and truncated tetrahedral models were considered in both non-functionalized form and with surfaces passivated by pseudo-hydrogen atoms. The focus on these two morphologies reflects their experimental relevance at this size scale and the feasibility of performing fully relaxed, atomistically resolved first-principles simulations. Moreover, a novel application of pseudo-hydrogen passivation to magnetic iron oxide nanoparticles is introduced as a practical strategy to probe intrinsic surface effects on magnetism while reducing artefacts from dangling bonds. Although these terminations are simplified representations, they were found to capture essential aspects affecting nanoparticle behavior. In non-functionalized models, significant distortions due to the undercoordination were observed, including Fe-O bond shortening by up to 0.46 Å and enhanced magnetic moments on oxygen atoms. These changes disrupted ferrimagnetic ordering, with spin-flipping in both tetrahedral and octahedral sublattices leading to an almost 90% reduction in total magnetization. Upon passivation, these effects were largely mitigated: Fe-O bond lengths became more uniform and ferrimagnetic alignment was stabilized as the energetically preferred state. Averaged spin-flip energies were computed to be 58 meV and 72 meV for both geometries, which is markedly lower than values for bulk γ-FeO (407-534 meV), suggesting that magnetic disorder may emerge during synthesis at typical growth temperatures. Charge transfer analysis further showed that surface coordination strongly affects electron distribution, with surface capping restoring near bulk-like charge states.