METASTABILITY OF DOMAIN STATE IN MULTIDOMAIN MAGNETITE - CONSEQUENCESFOR REMANENCE ACQUISITION

Experimental studies on isothermal remanent magnetization (IRM) acquis ition of synthetic specimens containing crushed natural magnetite in s even grain sizes from 5-10 mu m to 100-150 mu m show that the initial demagnetization state before IRM acquisition strongly affects tile sam ples' ability to acquire IRM. It is easier to acquire IRM from an init ially thermally demagnetized state than from an alternating field (AF) demagnetized state, and low-temperature demagnetization of the therma lly demagnetized state increases resistance to IRM acquisition. These data are interpreted as confirmation of the existence of a number of p ossible domain states (metastable states) which have variable capacity for remanence acquisition. Our results also demonstrate that activati on of domain structure is much more effective by AF than by direct (de ) fields. We suggest that the continually changing external field expe rienced during AF demagIletization allows the magnetic structure to re organize incrementally towards the demagnetized state, requiring a con siderably lower peak field (about 3 times lower) to achieve the same d emagnetization effect. Thermoremanent magnetization (TRM) in larger gr ain sizes is partially self-reversed. A difference in grain size depen dence is observed for a number of magnetic parameters. An parameters v ary with grain size below about 50 mu m. Above 50 mu m, parameters ass ociated with coercivity, AF demagnetization, or low-temperature treatm ent (H-cr; saturation IRM (SIRM) memory; TRM memory; median aquisition field (MAF) of IRM after AF; median destructive fields (MDF) of SIRM) are independent of grain size; parameters associated with TRM or ther mal demagnetization (MDF of TRM; MAF of IRM after thermal demagnetizat ion; MAF of IRM after thermal demagnetization followed by low temperat ure demagnetization; self-reversal as percentage of TRM) show grain si ze dependency throughout the grain size range. Our interpretation of t he data invokes the existence of subtle material inhomogeneities in bo th natural and synthetic magnetites, giving rise to a limited range of Curie temperatures below 580 degrees C (perhaps down to 560 degrees C ). We suggest that their control over the local energy minima states a vailable to the grain is more significant in the TRM or thermally dema gnetized state, while shielding by ''soft'' domain walls is dominant i n tile AF demagnetized state where walls are in the simplest domain co nfiguration, leading to differing grain size dependencies of these dif ferent states.