INTERNAL FIELD-MAPPING IN SINGLE-DOMAIN AND MULTIDOMAIN GRAINS

Paleomagnetism is the science of recovering the direction, and in favourable situations the intensity, of an external field H0 applied at some time in the past to magnetic grains in rocks. The fact that H0 remains encoded in the net magnetization M of these grains is remarkable when one considers their microscopic domain structure. Each domain is magnetized to saturation and its spontaneous magnetization MS may be one or two orders of magnitude greater than M, which merely reflects a slight bias in favor of certain domains or domain orientations. The internal demagnetizing field Hi due to the magnetization distribution within the grain outweighs H0 by a factor Ms/M approximately, and so the net field to which atomic spins respond at any point in the crystal is essentially Hi. The internal field within ellipsoidal particles of uniform magnetization (single-domain or SD particles) is well studied, but we have very little information about the spatial distribution of Hi within SD crystals of more realistic form or inside multidomain (MD) grains. In this paper we tackle the problem of internal field mapping within cubic crystals and show that Hi is extremely inhomogeneous, even for the SD structure. -from Authors