Viscous magnetization of 0.04–100 μm magnetites

Summary Acquisition and decay of small‐field viscous magnetization have been measured as a function of temperature (9‐500°C) for four synthetic magnetite dispersions ranging in mean particle size from 0.04 to 0.22 pand at room temperature only for an additional six samples (2‐100 μm). Room‐temperature viscous magnetization is pronounced in the single‐domain range, slight between 0.1 and 5 μm, and again substantial in the intermediate multidomain range (10‐15 μm). In all experiments, the acquisition rate exceeded the decay rate, resulting in residual undecayed viscous remanent magnetization (VRM) after a zero‐field decay time equal to the time of exposure to a field. Viscous magnetization is enhanced at elevated temperatures but not in direct proportion to absolute temperature. Instead, there appears to be a low‐temperature (< 20°C) threshold below which viscous effects are negligible and an approach to saturation at high temperature. VRM produced isothermally at a given temperature is significantly more difficult to erase by heating to a higher temperature than most thermal activation theories predict. Furthermore, even in quite brief experiments, the viscosity coefficient S = |σ J |σlog t | increased with time, the increase being more pronounced at high temperature. The implications of these observations are that viscous overprinting of natural remanent magnetization in magnetite‐bearing rocks, particularly during burial or intrusive reheating, may be more extensive and more difficult to erase than previously believed.