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Superparamagnetism, the archetypal small particle phenomenon, was first described by Louis Néel (1949a,b) in the context of rock magnetism. Key features of the underlying micromagnetism had been described by Stoner and Wohlfarth (1948), in the context of industrial magnetic alloys. The physical theory of superparamagnetism was further developed by William Fuller Brown (1963, 1979; see Rubens 1979). Many recent and ongoing additional advances are motivated by the potential for improved nanotechnological applications. One of my main aims in writing this chapter is to help bridge the gap that presently exists between the still largely phenomenological methods of rock and mineral magnetism and rapidly developing fundamental advances in the theory of micromagnetism, the theory of magnetic measurements applied to nanomagnets, and the study of synthetic model systems of ensembles of interacting magnetic nanoparticles. It is hoped that the reader will acquire appreciations of both the underlying atomistic theory of small particle magnetism and the wealth of phenomena involving the magnetism of natural materials, that often have nanoscale structural or chemical features, including their sizes. The reward for taking a more fundamental approach is that magnetism becomes one of the most sensitive probes of such structural and chemical features of complex natural solids and composites.
Magnetism in the Earth sciences
Magnetism is many things in the Earth. Three large areas can be distinguished: (1) geomagnetism, where one is primarily concerned with paleorecords of geomagnetic fields (inclination, intensity, reversals), models of the geodynamo and geomagnetic field generation, or the use of geomagnetic fields for prospecting or probing planetary interiors, (2) magnetic geology (magnetic petrology, magnetic fabric, environmental magnetism), where one is primarily concerned with magnetic rock and mineral records that are used to study petrogenesis, geotectonic activity, weathering, diagenesis, sediment transport, etc., and (3) mineral magnetism, where one is primarily concerned with the …