How do magnetic interactions in nanoscale intergrowths affect palaeomagnetic interpretations?

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We are all familiar with the experiment where we place a piece of paper on top of a bar magnet and image the magnetic field by sprinkling iron filings on to the paper. The iron filings align with the magnetic field of the magnet, tracing out the well-known dipole field pattern. In this type of experiment the magnetic particles are free to move toward the high field regions. If the particles had been glued to the paper so that they could not move, then the magnetisation within the particles would have moved instead. In fact the magnetisation within the particles would rotate to align with the direction field of the magnet, but of course this rotation would not have been visible to the eye. The ability of the magnetisation within a particle to align with a magnetic field and remain aligned even if we remove the original magnetising field, means the particles are able to produce a permanent record of the field direction. This recording capability of small magnetic particles has been exploited by man for over 50 years, and used, for example, in magnetic tape recorders and for hard drive computer storage. In both tapes and drives the recording media consists of a thin film coated with a fine power of magnetic particles. Such technology allows computer drives to store huge amounts of data which can be deleted and recorded time and time again, but this high reliability can only be achieved by ensuring that all the magnetic particles are of a consistent grain size and chemical purity. Magnetic particles are not just man-made, but also occur naturally in most rocks and soils. Unlike man-made recording media, however, these naturally occurring magnetic minerals can have a wide variety of chemical compositions, grain sizes, and particle concentrations. In some cases these magnetic minerals will contain zones within a single grain with each zone having a slightly different chemical structure. These distinct magnetic minerals will develop in response to environmental conditions at the time when the rocks or soils are formed. For example, lavas thrown out from the same volcano can produce different magnetic minerals depending on how slowly the lavas cool, and soils will contain different magnetic minerals depending on the amount of organic material present. Being able to correctly identify the type of magnetic mineralogy, therefore, can tell us something about the environment in which that sample was formed. Accurate identification of the magnetic minerals is important for another reason. Natural magnetic minerals will record the direction of the Earth's magnetic field as the minerals are formed. However, the reliability of the recording will greatly depend upon the types of magnetic minerals that are present. It is possible that the direction of magnetisation in some minerals is more influenced by the crystalline structure of the mineral itself rather than the geomagnetic field. This is thought to be likely in the case when a magnetic grain is chemically zoned, as mentioned earlier. It is therefore important to understand how different multi-zoned grains are able to make a magnetic recording, and develop simple tests to distinguish which mineral systems are reliable recorders of the geomagnetic field, and which are not.