How is a FORC diagram different from a Preisach diagram?

First-order reversal curve (FORC) diagrams are a new method of characterizing magnetic minerals in natural samples by measuring the microcoercivity distribution f(H_c) and the interaction field distribution g(H_u). The equivalence between FORC and more traditional Preisach diagrams was tested by measuring both for samples of synthetic single-domain (SD) magnetite, synthetic elongated SD maghemite and natural submarine basalts. Even though the resolution of a Preisach diagram is about one order of magnitude less than that of a FORC diagram, there is good agreement in all cases between the two diagrams. The main results (coercivity and spreading of the distribution peak) are very consistent. However, patterns in the low coercivity region are problematic for Preisach diagrams. In examples where some important features of the FORC distribution were at low coercivities, the Preisach diagram failed to image these features and showed closed contours instead. Similarly, samples containing an important PSD or MD fraction are not suitable for a good result on Preisach diagrams, because their magnetization is dominated by induced rather than remanent magnetization. Therefore, FORC diagrams are preferable to Preisach diagrams.