Magnetic domain observations on magnetite and titanomaghemite grains (0.5-10 μm)

By using the Bitter method, we have observed the magnetic domain structures of natural titanomaghemite grains and magnetite crystals (0.5–10 μm) that were grown in a glass ceramic matrix. The observations were carried out in a range of applied magnetic fields and were correlated to the hysteresis properties of the bulk samples. Our observations show that simple, lamellar structures cannot explain the high-field hysteresis behaviour in either the titanomaghemite or the magnetite sample. This high-field behaviour is probably dominated by complicated, stress-influenced domain structures and rotation processes. A plot of the number of lamellar domains observed in magnetite grains versus their grain size yields a single-domain to two-domain transition size of 0.25 μm.