Time-Asymmetric FORC Diagrams: A New Protocol for Visualizing Thermal Fluctuations and Distinguishing Magnetic Mineral Mixtures

First-order reversal curves (FORCs) are nowadays routinely used to assess domain states and magnetostatic interactions of magnetic minerals. While a huge step forward from bulk magnetic measurements in terms of sample characterization, there is a missing link between the FORC diagrams and remanence behavior: FORC diagrams mainly reveal domain states, while remanence behavior is largely controlled by thermal activations. We present a new tool to visualize thermal fluctuations in so-called time-asymmetric (TA) FORC diagrams. TA-FORCs differ from traditional FORCs in that they maintain the reversal field H-a for a longer time (minutes) than the FORC measurement field H-b (milliseconds). During this extended hold time, thermal activations cause some magnetic grains to change their magnetization, giving rise to an upward shift in the FORC diagram. The magnitude of this shift gives insight into the thermoviscous stability of the mineral and its remanence acquisition behavior. This not only allows to distinguish thermoviscous effects in FORC diagrams from magnetostatic (i.e., interactions/domain state related) effects but also provides a way to separate mixtures of magnetic minerals: two minerals with similar coercivity spectra that would totally overlap in traditional FORC diagrams show different upward shifts in TA-FORC diagrams, which in some cases enable complete separation of the minerals visually. This effectively provides two independent FORC signatures for two magnetic constituents in a sample such as two grain populations of different grain sizes, grain shapes, and/or mineral.