Self‐reversal of thermoremanent magnetization in the ilmenite‐hematite System: Order‐disorder, symmetry, and spin alignment

Abstract

A model of the process by which certain members of the ilmenite‐hematite solid solution series acquire reverse thermoremanent magnetization (reverse TRM) is proposed. It is based on the observation that the cation order‐disorder transformation at work in this mineral system produces cation‐ordered regions, or domains, antiphase with respect to cation positions. Given the fundamental assumption that the so‐called “x phase”, the weakly magnetic phase known to control self‐reversal, is in fact Fe‐rich cation‐disordered boundary material between the two types of cation‐ordered domains, symmetry considerations help eliminate the possibility that the direction of magnetic remanence associated with the boundaries lies parallel to the line of Fe spin. Rather, the net magnetization of the boundary material need be essentially at right angles to the orientation of spin, as is the case for pure hematite. Results of thermomagnetic experiments further suggest that superexchange with the Fe‐rich boundary material produces a femmagnetic structure within each cation‐ordered domain such that its net moment lies along the direction of spin, that is, perpendicular to the net boundary moment. At this stage the moments of each pair of cation‐ordered domains (antiphase in composition) cancel. We propose that reverse TRM arises during cooling in an applied field through rotation of particular Fe spins within each cation‐ordered domain, causing the spin arrangement to become progressively noncollinear. Competing superexchange interactions within the cation‐ordered regions may be responsible for such a spin rotation. According to this model, each cation domain provides a component of magnetization opposite to the net magnetization associated with the boundary material. Provided that there exists in a given grain equal fractions of each type of cation domain, the grain will fully self‐reverse. Synthesized polycrystalline samples as well as rocks containing ferrian ilmenites having compositions in the self‐reversing range will acquire reverse TRM.