Superparamagnetic and single‐domain threshold sizes in magnetite

Particle size distributions have been obtained by grain counts on electron micrographs of four samples containing submicroscopic equant magnetite particles. Blocking temperature data indicate thermally unstable (superparamagnetic, SPM) magnetic behavior in one sample whose grains range from 100 to 650 A in size. The SPM is confirmed by a large increase of saturation remanence when the hysteresis of the sample is measured at 77°K. Quantitative estimates of the SPM, single-domain (SD), and multidomain (MD) magnetization fractions in this sample are made by comparing theoretical and observed values of the reduced saturation remanence at 77° and 300°K. The particle size distribution then yields a critical size ds = (290 − 360) ± 50 A for the onset of SPM in magnetite at room temperature, in close agreement with Neel's SD theory. The critical size d0 at which domain structure develops in equidimensional grains of magnetite is less well defined. It is certainly less than 480 ± 50 A and may even be less than ds; that is, a direct MD-SPM transition in magnetite is a distinct possibility. The extreme narrowness of the stable SD range in equidimensional magnetite particles suggests that either elongated SD grains or else small MD grains with pseudo-SD characteristics must be the major carriers of the high-stability natural remanence of igneous rocks.