Domain structures in epitaxial (110) Fe ₃O ₄ particles studied by magnetic force microscopy

Magnetic domain structures on single-crystalline magnetite (Fe ₃O ₄) particles, prepared by microfabrication techniques from molecular-beam epitaxial (110) magnetite films grown on MgO, were studied by magnetic force microscopy. The (110) magnetite film thickness was 250 nm and the patterned particles ranged in size from 2×2 to 10×10 μm. The patterned particles showed in-plane, stripe-like domain structures with ill-defined and fragmented walls mainly aligned along the in-plane [110] direction. In both the parent film and the patterned particles, an out-of-plane component of the stray field was observed within domain interiors as a fine-scale (100-300 nm) and spatially variable magnetic contrast present in both the remanent state and in applied fields. Individual wall sections were observed to be highly fragmented with variable widths (100-300 nm) and offsets and subdivided into opposite polarity segments of variable lengths. Remagnetization of a 10×10 μm particle in fields up to 500 Oe occurred by reverse spike domain nucleation at the edge of the particle followed by growth and propagation towards the interior of the particle similar to classical behavior of uniaxial materials. In contrast, the unusual domain wall structures are a consequence of the antiferromagnetically coupled, growth-induced, structural antiphase domains and antiphase boundaries (APB) know to form in epitaxial thin films of magnetite. Magnetically, the particles behave differently at the different length scales. A particle as a whole (micrometer length scale) behaves as a magnetically uniaxial object, but on a smaller length scale (submicron scale), the magnetic microstructure is strongly influenced by the antiphase structural domains. Analysis of the domain spacing as a function of particle size yields an estimate of the average exchange stiffness constant that is nearly 2 orders of magnitude lower than the value in bulk magnetite. This is consistent with the idea that exchange interactions across the APBs are severely suppressed due to spin frustration.