Effect of annealing on coercive force and remanent magnetizations in magnetite

A series of experiments is described in which single crystals of magnetite were annealed under controlled conditions. Structural changes in the magnetite were followed by changes in the magnitudes of coercive force and saturation isothermal remanent magnetization. The structural changes were identified by comparing their activation energies as estimated from the annealing curves with those of known physical processes. It seems likely that relaxation of an induced anisotropy by vacancy-assisted cation migration (with activation energy 1–1.5 ev) took place at temperatures well below the Curie temperature. This unstable feature poses a serious problem for certain methods of estimating the paleointensity of the geomagnetic field. A further structural change, which appeared to be thermally activated self-diffusion of dislocations (with activation energy around 6 ev) was apparent at higher temperatures, well above the Curie temperature. Associated with this structural change were pronounced changes in the alternating-field demagnetization curves of both isothermal and thermoremanent magnetizations in the magnetite. Changes in the Curie temperature were observed to accompany both high- and low-temperature effects.