Relationship between Magnetic Domain Structures and Shear Stress in Magnetic Minerals of Geological Materials

Abstract:

The magnetic properties of rocks are affected by geomagnetic fields, heat and shearing stresses after formation. The shear stress associated with tectonic stress, for example, from a fault, can cause magnetic domain­-wall displacement. The wall displacement is thought to reflect the magnetic domain structure. Therefore, the hysteresis of strain, which rocks are subjected to in fault movements, can be used to estimate the structural disturbances of the magnetic domain. Previous studies have shown that the stress effect on magnetic domains can be observed by using industrial and synthetic materials, such as Si–Fe alloys (Chikazumi & Suzuki, 1995). However, few studies have used natural geological materials to study the effect of stress on magnetic domains. This study’s objective is to verify the shear stress history of rocks by observing the magnetic domain structures via the Bitter method and magnetic force microscopy (MFM). Here, the Bitter method is the powder pattern method, whereby a colloidal suspension of the magnetic powder is prepared from magnetic materials, and the patterns of the domain wall produced by ferromagnetic particles are observed under a microscope. On observing the domain structures of industrial and synthetic materials, the remnant stress on the surface layer is removed after mechanical polishing (Hoffmann et al., 1987) because the remnant stress influences the domain structures. There are several technical problems during the observation of rocks compared with those during observation of industrial and synthetic materials. Electrolytical polishing does not work because the electrical conductivity is very low (e.g., Hoffmann et al., 1987). In previous studies of the observation of magnetic domains of rock minerals, the surface strain layer was removed by polishing with colloidal silica (Ozdemir, 1995,etc). In this study, we will attempt to conduct a verification experiment and observations of the magnetic domain structures in natural minerals of rocks, and would like to discuss the relationship between the shear stress and magnetic domain structures. Here, the polishing method was according to Hoffmann et al. (1987), and the magnetic domain was observed by the Bitter method and MFM. As a result, we can observe the magnetic domain walls, and each domain wall has a spacing of 3-5 μm.