  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.

Geophysics