Micromagnetic Modeling of First-Order Reversal Curves (FORC) Diagrams for Single-Domain and Pseudo-Single-Domain Magnetite

A micromagnetic model based on a conjugate gradient algorithm was used to calculate FORC diagrams for isolated grains of magnetite as well as for arrays of grains. In the case of individual elongated grains, we found that the FORC diagram consists of a single peak centered on the coercive force if the grain is single-domain. For a pseudo-single domain grain, we observe multiple peaks on the FORC diagram. The modelling of arrays of elongated single-domain particles reveals two distinct types of patterns on the FORC diagram depending on the spacing between particles. In a 2x2x2 array a secondary branch on the reversal curves appears if the spacing is less than two times the particle width. This feature translates into the appearance of one negative and two positive peaks on the FORC diagram. In the case of a 3x3x3 array we also observe several secondary branches when the spacing between grains is less than 2.5 times the particle width, leading to the appearance of several peaks on the FORC diagram. Arrays of cubic single-domain grains having random magnetocrystalline anisotropy axes show the same features, the only difference being a lower coercive force. Therefore the presence of symmetrical peaks on a FORC diagram can be an indicator of the presence of magnetic interactions in an ensemble of grains.