Energy barriers in threedimensional micromagnetic models and the physics of thermoviscous magnetization
Abstract
A firstprinciple micromagnetic and statistical calculation of viscous remanent magnetization (VRM) in an ensemble of cubic magnetite pseudosingle domain (PSD) particles is presented. The theoretical methods developed apply to all magnetic particle sizes, from single domain to multidomain. The numerical implementation is based on a fast relaxation algorithm for finding optimal transition paths between micromagnetic local energy minima. The algorithm combines a nudgedelasticband technique with action minimization. Initial paths are obtained by repetitive minimizations of modified energy functions. For a cubic PSD particle, 60 different local energy minima are identified, and all optimal energy barriers between them are numerically calculated for the case of zero external field. These results are used to estimate the energy barriers in weak external fields. Based on these, timedependent transition matrices are constructed, which fully describe the continuous homogeneous Markov processes of VRM acquisition and decay. By spherical averaging, the acquisition of remanent magnetization in an isotropic PSD ensemble is calculated from laboratory to geological timescales. The modelled particle ensemble shows a physically meaningful overshooting of magnetization during VRM acquisition. The results also clarify why VRM acquisition in PSD particles can occur much faster than VRM decay, and therefore explain occurrence of extremely stable VRM as found in some palaeomagnetic studies.
 Publication:

Geophysical Journal International
 Pub Date:
 October 2018
 DOI:
 10.1093/gji/ggy285
 arXiv:
 arXiv:1702.00070
 Bibcode:
 2018GeoJI.215..314F
 Keywords:

 Rock and mineral magnetism;
 Numerical modelling;
 Magnetic properties;
 Physics  Geophysics
 EPrint:
 20 pages, 5 figures, manuscript