Chemical remanent magnetization in synthetic magnetite

Abstract

As a magnetic grain produced by a chemical process below its blocking temperature grows through a critical volume in the presence of a magnetic field, its moment becomes blocked and it acquires a chemical remanent magnetization, CRM. Despite its importance to paleomagnetism, the properties of CRM and the controls on its behavior are still poorly understood. We have modified techniques developed by Taylor et al. [1987] to synthesize magnetite at room temperature and atmospheric pressure, whereby magnetite is produced by alteration of “green rust” under reducing conditions. The period of synthesis is twenty‐four hours and a viscous component acquired over this amount of time can be demagnetized with alternating fields of 10 mT. The average direction of the CRM parallels the direction of the applied field and the intensity increases with increasing intensity of the applied field. The intensity is linearly related to the applied field up to approximately 1 mT, consistent with existing CRM theory developed by analogy to the acquisition of thermal remanent magnetization (TRM). Above about 1 mT, however, the CRM intensity approaches saturation at a rate substantially lower than that predicted by the TRM analog theory. Furthermore, the ratio of the CRM to an anhysteretic remanence acquired in the same field strongly depends on the intensity of the growth field. Analogy to TRM predicts this ratio to be independent of growth field. Both, the principal eigenvector of the anisotropy of isothermal remanence (AIR) matrix becomes more aligned with growth field and the percent anisotropy increases as the intensity of the growth field increases. The above observations suggest that the model of CRM acquisition based on analogy to TRM acquisition is incomplete. We therefore propose a revised model which incorporates the increased alignment of easy axes with growth field. Finally, the alignment of magnetic anisotropy ellipsoids of our synthetic samples with the growth field suggests a means to distinguish CRM from TRM in natural remanence.