High-resolution Transmission Electron Microscopy (HRTEM) of Dislocations in Magnetite

Magnetite is the primary mineral used for paleomagnetic and rock magnetic studies. Dislocations have been shown to significantly affect the remanent magnetization and coercivity of magnetite, and most naturally-occurring magnetite contains dislocations at a density of 10⁸ cm^-2. However our understanding of the orientation and style of dislocations in magnetite and of how these dislocations affect magnetic stability and remagnetization is limited. To better understand these effects, we must first characterize the dislocations. In a face-centered cubic mineral, dislocations are expected to form most easily in a {111} plane; however, we have observed dislocations in a variety of crystallographic orientations using high-resolution transmission electron microscopy (HRTEM). HRTEM images are sensitive to dislocations parallel to the electron beam, so it is important to image samples along multiple zone axes. The magnetite used in this study was cut from 1 cm diameter octahedra of nearly stoichiometric magnetite. Although our samples have likely experienced very little stress, we observe a variety of dislocations. The geometry of these dislocations will be incorporated in a dislocation simulator developed at Lawrence Livermore Laboratories and the resulting stress fields in a micromagnetic model.