Thermo-remanent magnetization (TRM) of multidomain grains in igneous rocks

Abstract

The magnetic moment of a multidomain grain at its blocking temperature is determined by the domain arrangement which gives the minimum magnetic energy; for a large grain the energy minimum is produced by a self-demagnetizing field equal and opposite to the external field. Below the blocking temperature the domain configuration is frozen in and the moment increases with the increasing spontaneous magnetization. This results in a TRM proportional to small inducing fields, as in the theory of single domain grains and as required for agreement with experimental observations. Artificial TRM's of 17 rocks for which analyses of magnetic minerals are available show good agreement with calculated values. The theory also gives a formula for the maximum possible initial susceptibility of rocks which agrees well with experimental observations.