Magnetic domains and domain walls in pseudo-single-domain magnetite studied with magnetic force microscopy

Magnetic domain and domain wall structures in pseudo-single-domain grains (5-20 μm) of magnetite (Fe₃O₄) were studied using magnetic force microscopy. Many of the observed micromagnetic features can be explained by the magnetostatic effects of surfaces and grain edges and interactions within and between walls. Domain walls were frequently subdivided into 1-3 opposite polarity segments separated by Bloch lines, although some walls contained no Bloch lines. Subdivided walls display a characteristic zigzag structure along the easy axis direction, where zigzag angles can be as high as 20°-40°. The zigzagging structure, in addition to wall segmentation, further minimizes the magnetostatic energy of the walls. Bloch lines can be (de)nucleated during wall displacement or after repeated alternating field (AF) demagnetization. Within individual walls, the number of Bloch lines and their pjnning locations were found to vary after repeated AF demagnetization demonstrating that walls, like individual grains, can exist in several different local energy minima. The number of Bloch lines appears to be independent of domain state, but frequently the polarity of the wall was coupled with the direction of magnetization in the adjoining domains, such that wall polarity alternates in sign between adjacent walls across an entire grain. Even after the domain magnetization is reversed, the same sense of wall chirality is maintained across the grain producing unique grain chiralities. For one particular grain it was possible to reconfigure a likely three-dimensional (3-D) domain structure. The body and surface structures result primarily from a combined volume magnetostatic interaction between all grain surfaces and magnetocrystalline anisotropy. Finally, commonly observed open-flux features within the interior of grains or along grain edges terminating planar domains are inconsistent with the prediction of edge closure domain formation based on recent 2-D micromagnetic models. Our observations suggest that 3-D micromagnetic models are required to model results even for grains larger than 1 μn.