FORC-Rayleigh: A new measurement protocol for investigating the origin of magnetization changes in first-order reversal curves

First-order reversal curves (FORC) are gaining increasing popularity as an effective tool for characterizing magnetic minerals in rocks and sediments. The associated two-dimensional representation of hysteretic magnetization processes, known as FORC diagrams, enables the identification of specific fingerprints associated with magnetic grain size and mineralogy, and, in certain cases, the separation of magnetic components with widely overlapping coercivities, thus reducing the ambiguity of other magnetic characterization techniques. Recent progress in micromagnetic calculations also enable to calculate FORC diagrams for given magnetic mineral assemblages with enough statistical relevance to enable direct comparisons with real counterparts. Yet, interpretation ambiguities cannot be totally excluded, due to the different magnetization processes underlying each point of the FORC diagram. The additional measurement of zero-field hysteresis measurements to the original measurement protocol (Zhao et al., 2017) enables the separation of reversible and irreversible magnetization processes on a non-local basis, yielding different types of diagrams for each contribution. A local solution, which works for every point along a magnetization curve, is proposed here. It consists in the repeated measurement of Rayleigh loops in the applied field of the classic FORC protocol, as if low-field susceptibility would be instantaneously measured on the top of magnetometric measurements. Adequate processing of this modified measurement protocols divides the slope of magnetization curves into four contributions originating from (1) irreversible, (2) reversible, (3) viscous, and (4) aftereffect magnetization processes. Selected examples show the additional information that can be extracted from these measurements, as well as the disambiguation of not yet explained FORC features associated with the pseudo-single-domain and multidomain signatures of magnetite particles.