Stable remanence and memory of multi-domain materials with special reference to magnetite

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Abstract
The characteristics of memory of remanence across the so-called isotropic point (TK ) in single crystals of magnetite are demonstrated experimentally. Memory is shown to be related to the highest microscopic coercivity fraction in the crystal. In order to investigate the nature of the magnetically hard fraction, studies of memory as a function of state of internal stress were undertaken with nickel and cobalt. They revealed the sensitivity of the transition of remanence and of memory to internal stress. A simple model of memory is proposed which invokes magneto-elastic control of the domain configuration in the vicinity of TK . The nucleation of the domain configuration of memory is considered to be controlled by local fluctuations of the wall energy function due to internal stress centres. Features of the model are also used to interpret the role of internal stress in thermo-remanent magnetization of multi-domain materials. It is shown theoretically that the sources of pinning stress are unlikely to be the stress fields of single dislocations but could be the larger stress fields due to pile-ups and interacting pairs of pile-ups. The geophysical significance of the demonstrable importance of internal stress in stable remanence is assessed.