MAGNETIC-PROPERTIES OF THE STABLE FRACTION OF REMANENCE IN LARGE MULTIDOMAIN (MD) MAGNETITE GRAINS - SINGLE-DOMAIN OR MD

It has been recognized since the early work of Verhoogen (1959) that a considerable proportion of remanence in multidomain (MD) magnetite gr ains is resistant to low-field a.f. or low-temperature demagnetization . The source of this high stability is still a matter of debate. A num ber of workers have suggested that MD grains of all sizes contain a re manence fraction with truly single domain (SD) character. We suggest t hat the critical diagnostic features which should be investigated to d etermine whether the high stability fraction is SD or MD in character are whether blocking (T-b) and unblocking (T-ub) temperatures are equi valent, and whether the intensity of remanence is affected by the ther mal pre-history of the sample. We have carried out such experiments on samples containing crushed natural magnetites in 7 grain sizes from 5 -10 mu m to 100-150 mu m. We show that T-b and T-ub are equivalent for pTRM(20)(400) for grain sizes up to 15-20 mu m, but that T-ub extends up to the Curie temperature for larger grain sizes. We also show that the stable fraction of MD TRM and pTRM has the same dependence on pre -history as the total TRM. Our experiments demonstrate that the stable fraction has magnetic properties which are truly MD in character for magnetite grains larger than 20 mu m.