Rock magnetism

Paleomagnetic techniques make use of the phenomenon that certain minerals are capable of retaining a record of the past direction of the Earth's magnetic field. These minerals are all paramagnetic, that is, they contain atoms which possess an odd number of elec trons. Magnetic fields are generated by the spin and orbital motions of the electrons. In shells with paired electrons, their magnetic fields essentially cancel each other. The unpaired electrons present in paramagnetic substances cause the atoms to act as small magnets or dipoles.

When a paramagnetic substance is placed in a weak external magnetic field, such as the Earth's field, the atomic dipoles rotate so as to become parallel to the external field direction. This induced magnetization is lost when the substance is removed from the field as the dipoles return to their original orientations.

Certain paramagnetic substances which contain a large number of unpaired electrons are termed ferromagnetic. The magnetic structure of these substances tends to devolve into a number of magnetic domains, within which the atoms are coupled by the interaction of the magnetic fields of the unpaired electrons. This interaction is only possible at temperatures below the Curie temperature, as above this temperature the energy level is such as to prohibit interatomic magnetic bonding and the substance then behaves in an ordinary paramagnetic manner.

Within each domain the internal alignment of linked atomic dipoles causes the domain to possess a net magnetic direction. When placed in a magnetic field the domains whose magnetic directions are in the same sense as the external field grow in size at the expense of domains aligned in other directions. After removal from the external field a preferred direction resulting from the growth and shrinkage of the domains is retained so that the substance exhibits an overall magnetic directionality. This retained magnetization is known as permanent or remanent magnetism.

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