The Effects of Dislocations on the Verwey Transition as Observed by Transmission Electron Microscopy and Low Temperature Magnetic Measurements

Abstract:

Pure magnetite experiences a first order phase transition, called the Verwey transition, at ~120K whereby the mineral's crystal structure changes from cubic to monoclinic. This transformation has a profound effect on the magnetic properties of magnetite. Internal and external stresses have been shown to affect the onset of the Verwey transition, but the processes by which this occurs have not been observed. To further investigate this behavior, we used transmission electron microscopy on deformed magnetite samples to simultaneously image dislocations, magnetic domain walls, and low-temperature twins while cooling through the Verwey transition. To relate the observed changes to more readily-measurable bulk sample magnetic behavior, we made low-temperature magnetic measurements using SQUID magnetometry. According to these low temperature measurements, the temperature of the phase transition is depressed by as much as 6°C in the deformed samples. Combining these two techniques allows us, for the first time, to observe the Verwey transition in a defect-rich area and to observe the manner in which dislocations, and their associated stress fields, influence and impede the growth of twin structures as magnetite is cooled through the Verwey transition.