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 considerahle proportion of remanence in multidomain (MD) magnetite grains is resistant to low-field a.f. or low-temperature demagnetization. The source of this high stability is still a matter of debate. A number of workers have suggested that MD grains of all sizes contain a remanence fraction with truly single domain (SD) character. We suggest that the critical diagnostic features which should be investigated to determine whether the high stability fraction is SD or MD in character are whether blocking (T b ) and unblocking (T ub ) temperatures are equivalent, and whether the intensity of remanence is affected by the thermal pre-history of the sample. We have carried out such experiments on samples containing crushed natural magnetites in 7 grain sizes from 5-10 μm to 100-150 μm. We show that T b and T ub are equivalent for pTRM 400 20 for grain sizes up to 15-20 μm, 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. Out experiments demonstrate that the stable fraction has magnetic properties which are truly MD in character for magnetite grains larger than 20 μm.