The additivity of partial thermal remanent magnetization in magnetite

Summary. Experiments were done to test the additivity of partial thermal remanent magnetizations (PTRMs) for prepared samples containing magnetite particles whose sizes range from SD (single domain) to MD (multidomain). The experiments compare the sum of two PTRMs with total-TRM, all produced by the same external field of 0.47 oe. The most significant conclusion of this paper is that, to first order, the additivity of PTRMs is obeyed for the magnetites of this study regardless of particle size. However, small, higher order deviations from additivity occur such that ΣPTRM > TRM by an average of about 1 per cent. Though small, these departures from additivity are significant at the 99 per cent confidence level, and they can be understood in terms of Néel's theory for SD particles. The small departures from additivity are intrinsic to the experimental procedure in which some particles acquire remanence twice, in each of the two PTRM steps. In the limit of small inducing fields additivity should be obeyed exactly for the magnetites of this study and for samples of interest in palaeomagnetism. The deviations from additivity should have no effect on palaeointensity determinations by the Thelliers' version of the Thellier palaeointensity method. For palaeointensity determinations by Coe's version of the Thellier method the effects of deviations from additivity would be very small, less than 4 per cent on the average for a worst-case experimental configuration, and these effects can be minimized by producing PTRMs parallel to the original NRM and by using weak laboratory fields.