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.