Thermoremanent magnetization in submicroscopic magnetite

Abstract

Induction and demagnetization characteristics of thermoremanent magnetization (TRM) have been studied for four synthetic magnetites with mean particle sizes of 0.04–0.22 μm. The experiments are intended to simulate the acquisition and laboratory demagnetization of natural remanent magnetization carried by submicroscopic but non-single-domain titanomagnetite particles in igneous rocks. Although hysteresis properties indicate that the magnetites are above single-domain (SD) size, their weak-field TRM are SD-like. TRM intensity, for a 1-oe inducing field, ranges from 6 to 12 emu/cm3. Median alternating demagnetization fields are between 275 and 400 oe, while blocking temperatures are generally within 50°C of the Curie point. However, neither Neel's SD theory nor Stacey's pseudo-SD theory can account for the observed dependence of TRM on inducing field, since theoretically TRM should saturate in rather low fields. Neel's two-domain theory fits the strong-field data very well but is inadequate in the weak-field region. These failures indicate that theoretically assumed two- and four-domain structures are not appropriate in particles just above SD size. Blocking temperature data are consistent with well-developed multidomain (MD) structure (i.e., thermal activation volume associated with a domain wall notably less than particle volume) in 0.22-μm particles but a wavelike spin structure without recognizable domains in particles ≤0.1 μm in size. The dependence of weak-field TRM on grain size d also supports this interpretation: TRM varies approximately as d−1 between 0.1 μm and the pseudo-SD threshold at 15–20 μm, but below 0.1 μm there is no clear size dependence. Particles with wavelike spin structure have not been recognized previously but may well explain the MD-like hysteresis and SD-like TRM of many igneous rocks.