Low-temperature first-order reversal curve (FORC) diagrams for synthetic and natural samples

[ 1] First-order reversal curve (FORC) diagrams were measured on a variety of synthetic and natural ( submarine basalts and potsherds) samples, as well as for a mixture, between room temperature and 10 K. Measuring FORC diagrams allowed us to examine the behavior of the coercivity field and interaction field distributions with decreasing temperature. Generally, as the temperature is decreased, FORC contours were observed to expand in all directions, e. g., the maximum coercivity peak migrates toward a higher coercivity. For synthetic magnetite samples, there were abrupt changes in the FORC distribution at the Verwey transition ( 120 K), while in the more complex natural magnetite samples the FORC distribution gradually progressed with temperature. As the temperature decreased, FORC diagrams were found to display different domain state characteristics. For example, samples with dominant superparamagnetic signals became single-domain (SD)-like, a SD-like sample stayed SD-like, and a PSD-like sample became more SD-like. In addition to these general features, we also observed some more specialized features. First, in both synthetic and natural magnetite samples, a secondary higher-coercivity peak is occasionally present below the Verwey transition, which we suggest is a twinning contribution. Second, the coercivity increases by a factor 15 between 300 K and 20 K in some of the seamount samples. Third, the effect of field cooling/ zero-field cooling on FORC diagrams is negligible, with the hysteresis parameters HC and HCR displaying a greater dependency. Finally, it is shown that in mixtures of magnetite and hematite, the hematite contribution disappears on cooling below the Morin transition, leaving a strong, well-defined magnetite signal.