Experimental studies on isothermal remanent magnetization (IRM) acquisition of synthetic specimens containing crushed natural magnetite in seven grain sizes from 5--10 μm to 100--150 μm show that the initial demagnetization state before IRM acquisition strongly affects the sample's ability to acquire IRM. It is easier to acquire IRM from an initially thermally demagnetized state than from an alternating field (AF) demagnetized state, and low-temperature demagnetization of the thermally demagnetized state increases resistance to IRM acquisition. These data are interpreted as confirmation of the existence of a number of possible domain states (metastable states) which have variable capacity for remanence acquisition. Our results also demonstrate that activation of domain structure is much more effective by AF than by direct (dc) fields. We suggest that the continually changing external field experienced during AF demagnetization allows the magnetic structure to reorganize incrementally towards the demagnetized state, requiring a considerably lower peak field (about 3 times lower) to achieve the same demagnetization effect.

Thermoremanent magnetization (TRM) in larger grain sizes is partially self-reversed. A difference in grain size dependence is observed for a number of magnetic parameters. All parameters vary with grain size below about 50 μm. Above 50 μm, parameters associated with coercivity, AF demagnetization, or low-temperature treatment cr; saturation IRM (SIRM) memory; TRM memory; median acquisition field (MAF) of IRM after AF; median destructive fields (MDF) of SIRM> are independent of grain size; parameters associated with TRM or thermal demagnetization (MDF of TRM; MAF of IRM after thermal demagnetization; MAF of IRM after thermal demagnetization followed by low temperature demagnetization; self-reversal as percentage of TRM) show grain size dependency throughout the grain size range. Our interpretation of the data invokes the existence of subtle material inhomogeneities in both natural and synthetic magnetites, giving rise to a limited range of Curie temperatures below 580 ¿C (perhaps down to 560 ¿C). We suggest that their control over the local energy minima states available to the grain is more significant in the TRM or thermally demagnetized state, while shielding by ''soft'' domain walls is dominant in the AF demagnetized state where walls are in the simplest domain configuration, leading to differing grain size dependencies of these different states. ¿ American Geophysical Union 1995