Theory of the magnetic properties of igneous rocks in alternating fields

Abstract

Each magnetic grain in a rock contains ferromagnetic domains with a range of sizes, the small ones being magnetically harder and responding less readily to demagnetizing fields than the larger ones. By exposing a grain which has a thermoremanent magnetic moment to an alternating field, domains larger than a critical size are demagnetized, leaving the smaller domains unaffected. Domain size distributions are calculated for spherical and cubical grains and used to derive characteristic curves of the fraction of an initial thermoremanent magnetization which remains after demagnetization in a progressively increasing field. Coercive force, saturation remanence and anhysteretic magnetization are also calculated from the domain size distributions. Irregularities of grain shape are shown to lead to a pseudosingle domain behaviour which is responsible both for very hard components of thermoremanent momenta and for the fact that small, randomly directed moments may be found to remain after rooks have been demagnetized in the highest fields.