Ferrimagnetic/ferroelastic domain interactions in magnetite below the Verwey transition. Part I: electron holography and Lorentz microscopy

The crystallographic and magnetic microstructure of magnetite (Fe3O4) below the Verwey transition (∼120 K) is studied using transmission electron microscopy. The low temperature phase is found to have a monoclinic C-centered lattice with a c-glide plane perpendicular to the b-axis, which allows twin-related crystal orientations to be distinguished. Off-axis electron holography and Lorentz electron microscopy are used to show that magnetic domains present at room temperature become subdivided into sub-micron-sized magnetic domains below the Verwey transition, with the magnetization direction in each magnetic domain oriented along the monoclinic [001] axis. The nature of the interactions between the magnetic domain walls and the ferroelastic twin walls is investigated. Cooling and warming cycles through the transition temperature are used to show that a memory effect is likely to exist between the magnetic states that form above and below the transition. Our results suggest that ferroelastic twin walls have a strong influence on the low temperature magnetic properties of magnetite.