Trapdoor viscous remanent magnetization

Viscous remanent magnetization (VRM) in multidomain particles still exhibits many puzzling properties deviating from the current theory of VRM, based on Ne\’el’s single-domain model of magnetic particles with an almost symmetric double-well potential. In larger magnetic particles experimental evidence indicates that more complex magnetization structures preferentially change from high-energy states to low-energy states with large energy differences, such that VRM is preferentially acquired by directed magnetization changes in strongly asymmetric double-well potentials. Here a statistical model explains how this trapdoor VRM (tVRM) naturally explains the experimental observations of initial-state dependence, time-lag variation, non-linear log t dependence, and acquisition-decay asymmetry for multidomain VRM. It is discussed how tVRM can be experimentally distinguished from single-domain VRM and how the new theory can help to improve age determination by VRM analysis.