A consideration of the origin of magnetism at the atomic level is an essential step in arriving at a physical picture of magnetic materials. A satisfactory description of the macroscopic behaviour of a magnetic body is only conceivable if it is acknowledged that the body is constituted of microscopic atomic magnets having the characteristics of the inhabitants of the atomic world. In the discussions of the previous chapter, this world was considered to be infinitely large, this simplification not generally invalidating the picture so obtained. The finite size of crystals and the resultant free surface is, however, a crucial element in controlling the way in which a magnetic body, such as a mineral grain in a rock, acquires or loses magnetization in response to the presence or absence of a magnetic field, in response to rising or falling temperature and behaves, magnetically, in general. It is the free surface which results in a magnetized body producing a magnetic field and thus allows rocks to contribute to anomalies in the geomagnetic field.
KeywordsDomain Wall Coercive Force Magnetization Vector Easy Axis Mineral Magnetism
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