First‐order reversal curve (FORC) diagrams of elongated single‐domain grains at high and low temperatures

Abstract

First‐order reversal curve (FORC) diagrams are a new technique for identifying magnetic minerals and characterizing their domain structures and interactions. FORC diagrams have potentially important applications for determining magnetic mineral compositions and grain sizes in paleomagnetism and environmental magnetism. However, the interpretation of the FORC parameters Hc and Hu as being equivalent to microcoercivity and particle interaction field, respectively, needs to be thoroughly tested in order to validate the FORC method. In order to test this interpretation, we measured FORC diagrams on elongated single‐domain particles at temperatures ranging from 20 to 853 K (580°C). The FORC distribution contracts significantly along both axes above 430°C, and the bulk coercive force decreases rapidly. The main causes of these features are decreasing spontaneous magnetization Ms(T) and increasing thermal activation of single‐domain moments pinned by shape anisotropy. A profile through the FORC peak along the Hc axis can be integrated to give a synthetic alternating field demagnetization curve or isothermal remanence curve. The synthesized curves match measured curves quite well at a variety of temperatures, and the FORC median Hc varies approximately in proportion to the bulk coercive force. A profile through the FORC peak parallel to the Hu axis contracts with heating approximately as Ms(T), and the half width of the distribution is similar to independent measures of local interaction fields Hi. These properties support the interpretation of the FORC variables Hc and Hu as being equivalent to microcoercivity Hc and interaction field Hi, respectively.