The demagnetization temperature necessary to remove viscous remanent magnetization

Recent experimental studies have shown that the temperature necessary to demagnetize viscous remanent magnetization (VRM) in magnetite‐bearing rocks is significantly higher than predicted by standard Néel single‐domain theory. The Walton time‐temperature equation, which has been invoked by some to explain the enhanced thermal stability, actually relates intensities of VRMs acquired at different temperatures rather than describing the unblocking of a previously acquired VRM. We show that the low‐field approximation (LFA) used in all previous formulations, which predicts equal VRM acquisition and decay rates, is not valid for a magnetite assemblage in the earth's field. A medium‐field approximation (MFA) predicts significantly faster VRM acquisition, and therefore greater thermal stability in decay, for the larger particles in the assemblage. Conventional theory does apply to red‐bed acquisition/decay experiments, since the LFA is valid for hematite.