Electron microscopy, magnetoclimatology, archaeomagnetic secular variation

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For fifty years, the magnetic properties of geological and archaeological materials have proven to be a rich source of information concerning our planet's history and dynamics. The switching of the north and south magnetic poles (geomagnetic polarity reversals), the creation and destruction of the ocean floor and the resulting drift of the continents (plate tectonics), the waxing and waning of the great ice sheets (past global warming -- and cooling) are prime examples that spring to mind. And yet there remain serious shortcomings in our understanding of the basic physical mechanisms that lead to the capture of these signals. One of the knotty problems involved is that of interactions between the individual magnetic crystals in the various natural archives in question. These complicate the magnetic properties so much that in his classic papers Louis Néel (Nobel Prize in Physics 1970) wisely avoided this problem by considering a dilute assemblage of tiny non-interacting magnetic particles. Unfortunately, the minerals of interest (iron-titanium oxides) often occur as nanometre-scale intergrowths in which magnetic regions lie in close proximity to one another. Three recent developments make it possible to undertake a renewed attack on this problem. First, electron microscopy is now able to provide detailed images of the magnetic field pattern inside samples of interest. Second, vastly increased computing power now makes it possible to calculate the behaviour of arrays of interacting magnetic particles. Third, the development of new laboratory techniques provides a direct experimental measure of the strength of the interactions. In addition to these mineral physics studies, palaeoclimatic and archaeomagnetic investigations will also be carried out. The former will involve samples already collected from several sites around the world and will attempt to elucidate the mechanisms by which climatic information is magnetically encoded in natural sedimentary archives. The latter will involve new samples collected from archaeological structures in Iberia and Egypt in order to extend the geographic coverage of earlier campaigns in Greece and Italy.