First-order reversal curve (FORC) diagrams of magnetic mixtures: Micromagnetic models and measurements

First-order reversal curve (FORC) diagrams were modelled and experimentally measured for different assemblages of single-domain (SD) magnetite particles with bimodal distributions of coercivities, and varying concentrations. When fitting end-member FORC diagrams to the FORC diagrams of mixtures, linear additivity is obeyed for the non-interacting mixtures, with predicted mixing ratios within the errorbars of the actual mixing ratios. Interacting mixtures are not always linearly additive. Experimental bimodal mixtures of magnetite particles (SD + pseudo-single-domain, PSD) have FORC diagrams that are usually linearly additive, regardless of the magnetite concentration of the sample (10 or 1%). Measured FORC diagrams of mixtures of SD magnetite and SD hematite again show that linear additivity of the two end-members holds. However, the very small M s of hematite compared to that of magnetite makes it difficult to detect a contribution on a FORC diagram from hematite, unless it comprises at least 88% of the mixture.