Trapdoor Viscous Remanent Magnetization

Viscous remanent magnetization (VRM) in multidomain (MD) particles has many puzzling properties that deviate from the current VRM theory, based on N & eacute;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 based on this 'trapdoor' VRM (tdVRM) naturally explains the experimental observations of initial-state dependence, time-lag variation, non-linear logt $\log \,t$ dependence, and acquisition-decay asymmetry for MD VRM. It is discussed how tdVRM can be experimentally distinguished from single-domain VRM and how the new theory can help to improve age determination by VRM analysis. The magnetization of rocks is an important property used to determine the history of the Earth surface and variations of Earth's magnetic field. Magnetite particles in rocks are abundant recorders of this field, and particles larger than about 500 nm in diameter are called multidomain (MD) magnetites. If a rock, after its formation, lies at Earth's surface for a long time, its magnetization slowly changes-the newly acquired remanent magnetization is called a viscous remanent magnetization (VRM). This VRM is well understood for magnetite particles below about 150 nm in diameter, but no comprehensive theory so far explains all its experimentally observed properties. Here a statistical model is developed that can explain the experimental facts and helps to understand their physical origin. The new theory allows theoretical investigation of different methods to determine the amount of time a rock was exposed to Earth's present-day magnetic field. This leads to a better understanding of time-temperature relations for VRM, and improves the determination of relocation ages based on VRM. Multidomain viscous remanent magnetization (MD VRM) is dominated by decay into trapdoor potential minima Previously inexplicable properties of MD VRM are explained by a quantitative trapdoor VRM theory Trapdoor VRM suggests an improved method for VRM age determination