Energy barriers in three-dimensional micromagnetic models and the physics of thermo-viscous magnetization in multidomain particles

A first principle micromagnetic and statistical calculation of viscous remanent magnetization (VRM) in an ensemble of cubic magnetite pseudo single-domain particles is presented. This is achieved by developing a fast relaxation algorithm for finding optimal transition paths between micromagnetic local energy minima. It combines a nudged elastic band technique with action minimization. Initial paths are obtained by repetitive minimizations of modified energy functions. For a cubic pseudo-single domain particle, 60 different local energy minima are identified and all optimal energy barriers between them are numerically calculated for zero external field. The results allow to estimate also the energy barriers in in weak external fields which are necessary to construct the time dependent transition matrices which describe the continuous homogeneous Markov processes of VRM acquisition and decay. By spherical averaging the remanence acquisition in an isotropic PSD ensemble was calculated over all time scales. The modelled particle ensemble shows a physically meaningful overshooting during VRM acquisition. The results also explain why VRM acquisition in PSD particles can occur much faster than VRM decay and therefore can explain for findings of extremely stable VRM in some paleomagnetic studies.