LOW-TEMPERATURE AND HIGH-TEMPERATURE HYSTERESIS OF SMALL MULTIDOMAIN MAGNETITES (215-540 NM)

Approximately equidimensional magnetite crystals, with mean sizes of 215, 390 and 540 nm, respectively, have been produced by reducing hematite crystals. Isothermal magnetic hysteresis properties show a clear progression toward multidomain-like behaviour as the mean grain size increases. A thermal fluctuation analysis of high-temperature coercive force data suggests that regions 200-250 nm in size are thermally activated as a unit in grains of all sizes; these are likely domain walls. Apparent demagnetizing factors calculated from both low- and high-temperature data are consistent with a mixture of two-domain (2D) and three-domain (3D) grains in all samples. However, theories of remanence in conventional 2D and 3D grains or in mixtures of 2D, 3D and metastable single domain grains do not explain the data in a satisfying way. -from Authors