Electronic structure near cationic defects in magnetite

Frenkel pair formation energy for cubic Fe3O4in DFT + U calculations

The cubic phase of magnetite is stabilized above the Verwey transition temperature of about 120 K via a complex electron-phonon interaction that is still not very well understood. In this work using the DFT + U method we describe our attempt to calculate point defect formation energies for this cubic phase in the static approximation. The electronic structure calculations and atomic relaxation peculiarities are discussed in this context. Only the cubic phase model with a small band gap and charge disproportionation (Fe²⁺/Fe³⁺) gives an adequate point defect formation energies, not the semi-metallic model. The relaxation of the local defect atomic structure and the relaxation of the surrounding crystal matrix are analyzed. Point defects cause only local perturbations of atomic positions and charge-orbital order. After analysis of the supercell size effects for up to 448 atoms, we justify the use of small supercells with 56 atoms to make calculations for the cubic phase. The extensive experimental results of Dieckmannet alon defects in magnetite at high temperature are deployed for comparison of our DFT + U results on Frenkel pair formation energies.

Role of Defects and Radiation Damage on He Diffusion in Magnetite: Implication for (U-Th)/He Thermochronology

The discovery of He retentivity in magnetite has opened up the use of the magnetite (U-Th)/He method as a thermochronometer to date the exhumation of mafic and ultramafic rocks, and also as a chronometer to date magnetite crystallization during serpentinization. However, published He diffusion data reveal more complex behavior than expected. To resolve this issue and generalize the understanding of He retention in magnetite, we conducted a multiscale theoretical study. We investigated the impact of natural point-defects (i.e., vacancies unrelated to radiation damage) and defects associated with radiation damage (i.e., vacancies and recoil damage that form amorphous zones) on He diffusion in magnetite. The theoretical results show that He diffusion is purely isotropic, and that defect-free magnetite is more He diffusive than indicated by experimental data on natural specimen. Interestingly, the obtained theoretical trapping energy of vacancies and recoil damage are very similar to those obtained from experimental diffusion data. These results suggest that He diffusion in magnetite is strongly controlled by the presence of vacancies and radiation damage, even at very low damage dose. We propose that, when using magnetite (U-Th)/He thermochronometry, the impact of vacancies and radiation damage on He retention behavior should be integrated.

Electronic phase transitions in ultrathin magnetite films

Magnetite (Fe3O4) shows singular electronic and magnetic properties, resulting from complex electron-electron and electron-phonon interactions that involve the interplay of charge, orbital and spin degrees of freedom. The Verwey transition is a manifestation of these interactions, with a puzzling connection between the low temperature charge ordered state and the dynamic charge fluctuations still present above the transition temperature. Here we explore how these rich physical phenomena are affected by thin film geometries, particularly focusing on the ultimate size limit defined by thicknesses below the minimum bulk unit cell. On one hand, we address the influence of extended defects, such as surfaces or antiphase domains, on the novel features exhibited by thin films. On the other, we try to isolate the effect of the reduced thickness on the electronic and magnetic properties. We will show that a distinct phase diagram and novel charge distributions emerge under reduced dimensions, while holding the local high magnetic moments. Altogether, thin film geometries offer unique possibilities to understand the complex interplay of short- and long-range orders in the Verwey transition. Furthermore, they arise as interesting candidates for the exploitation of the rich physics of magnetite in devices that demand nanoscale geometries, additionally offering novel functionalities based on their distinct properties with respect to the bulk form.