Micromagnetic structure of oxidized magnetite nanoparticles: sharp structural versus diffuse magnetic interface

The oxidation of magnetite to maghemite is a naturally occurring process that leads to the degradation of the magnetic properties of magnetite nanoparticles. Despite being systematically observed with traditional macroscopic magnetization measurement techniques, a detailed understanding of this process at the microscale is still missing. In this study, we track the evolution of the magnetic structure of magnetite nanoparticles during their oxidation to maghemite through numerical micromagnetic simulations. To capture realistic interparticle effects, we incorporate dipolar interactions by modeling the nanoparticles arranged in chains. Our computational results are benchmarked against experimental data from magnetotactic bacteria, studied over a time scale of years. To resolve the magnetization at the interface between both oxide phases, we propose spin-polarized small-angle neutron scattering (SANS), an experimental technique capable of probing magnetization textures at nanometer length scales. By analyzing the pair-distance distribution function extracted from numerically-computed SANS cross sections, we identify distinct signatures of magnetic disorder. Specifically, our findings suggest that the magnetization from the non-oxidized core region varies smoothly across the (structurally sharp) interface into the oxidized shell. The existence of such a diffuse magnetic interface may account for the superior magnetic properties of partially oxidized magnetite nanoparticles compared to fully converted maghemite samples.