Effects of the core‐shell structure on the magnetic properties of partially oxidized magnetite grains: Experimental and micromagnetic investigations

Abstract

AbstractThe relationship between magnetic hysteresis parameters and the degree of oxidation of ultrafine magnetite particles is examined by both experimental measurements (distributed particle assemblage with median grain size of ∼80 nm and standard deviation 0.43) and micromagnetic simulations (single particles from 40 nm to 140 nm). Experimental results show that both coercivity (Bc) and the ratio of saturation remanence to saturation magnetization (Mrs/Ms) increase slowly, as the oxidation parameter z increases from 0 to ∼0.9. Thereafter, both parameters decrease sharply as magnetite becomes completely oxidized to maghemite. Numerical simulations of hysteresis loop and microstructure using a micromagnetic model with a core‐shell geometry (a stoichiometric core surrounded by an oxidized shell) show three categories of behavior for magnetic grains during oxidation. First, the coercivity of SD particles decreases as oxidation proceeds, but their remanence magnetization remains in a uniform state. Second, for PSD sized particles near the critical SD boundary (80 nm to 100 nm), the initial vortex domain structure changes to a SD as oxidation occurs and returns to a vortex state upon complete maghemitization, resulting in an initial rise and then fall of Bc and Mrs. Finally, larger PSD grains remain a vortex state throughout the maghemitization, with less variations of Bc and Mrs. The predicted magnetic properties exhibit good agreement with experimental observations and suggest that the domain arrangement is likely to be dominated by a core‐shell structure with strong exchange coupling at their interface. Overall, the partially oxidized magnetite in SD‐PSD range can reliably record palaeomagnetic signals.