Domain Observations in Geological Materials

Abstract

The stability and origin of magnetic remanence in rocks and meteorites carried by ferromagnetic grains with nonuniform magnetization are not completely understood. This research aims to provide experimental data at the micromagnetic level through domain observations with submicron resolution of magnetic microstructures in small particles of synthetic intermediate titanomagnetite (Chapters 4 and 5) and in the Chelyabinsk meteorite (Chapter 6) using Magnetic Force Microscopy (MFM). To better interpret magnetic microstructures of three-dimensional particles from two dimensional MFM-images, crystallographic data from electron backscatter diffraction and numerical micromagnetic modeling were obtained. MFM-imaging was also combined with electron diffraction x-ray spectroscopy (using electron microscopy or electron microprobe) to identify the composition of magnetic phases investigated in this work. This research provided evidence for the existence of magnetic vortex states in micron-sized intermediate titanomagnetite particles exhibiting a range of available local energy minimum states at room temperature such as single vortex and multivortex states. The experimental results agree well with micromagnetic modeling conducted in this study. Moreover, this research demonstrated a high temperature stability of single vortex state and transdomain transitions between multivortex states and single vortex state with increasing temperatures up to the Curie temperature. Our findings suggest that intermediate titanomagnetite grains in the single vortex state can be a source of stable and reliable paleomagnetic records. In addition, high temperature results obtained in this study provide constraints for micromagnetic models and theory of thermoremanent magnetization in vortex states. MFM investigation of Chelyabinsk meteorite revealed the presence of submicron inclusions of magnetite and Cr-magnetite that are potentially stable carriers of paleomagnetic record in this meteorite but have not been identified by bulk magnetic characterization. This work has demonstrated that the integration of MFM-imaging with high-resolution, non-magnetic characterization techniques helps to provide meaningful conclusions about domain state and magnetic mineralogy of the main carriers of paleomagnetic records in rocks and meteorites.