Some theoretical aspects of rock-magnetism

Abstract

The memoir is devoted to a brief theoretical study of the most typical magnetic properties of rocks. In particular §§ 3–16 are on ferrimagnetism, §§ 17–35 on single domain particles and §§ 36–57 on large multi-domain particles. Theoretical studies are made of the following aspects of the subject and compared with the experimental results: remanent magnetization (§ 38), initial susceptibility (§ 39), variation with applied field of thermoremanent magnetization (abbreviated to T.R.M.) (§§ 40, 41, 57), the ratio Q_k of T.R.M. acquired in a given field to the induced magnetization in the same field (§ 42), the additivity of partial T.R.M.'s in the case both of small grains (§ 28) and large grains (§ 57). Considerable space is devoted to the magnetic ‘viscosity’ due to thermal agitation in small grains (§§ 24–27) and in larger ones (§§ 49–56). Expressions are given for magnetic ‘viscosity’ in the range of Rayleigh's relations (§ 51) particularly with a demagnetizing field present (§ 54). The theoretical and experimental results on the irreversible decrease in isothermal remanent magnetization are briefly quoted both for small (§ 30) and large (§ 55) grains. Different reversing mechanisms are reviewed which could cause a negative T.R.M., that is one directed in the opposite sense to the field applied during cooling. Some are related to negative Weiss-Heisenberg exchange forces: reversal by diffusion involving ionic exchange between the two sub-lattices in a ferrimagnetic (§ 7), reversal by anomalous thermal variation is spontaneous magnetization (§§ 11, 12), reversal by diffusion with complete change of composition (§ 16). The others are effects of the demagnetizing field: reversal in mixtures of two constituents with different Curie Points (§§ 31–34), reversal by segregation, allotropy and chemical alteration (§ 35). The actual examples so far known are recalled.