First principles understanding of single domain magnetizations—Part II: Non-ideal magnetic behaviour in ideal single domain titanomagnetite

SUMMARY Louis Néel’s theory of magnetism, which describes a rock’s magnetization as being carried by ‘ideal’ uniformly magnetized or single domain (SD) particles, has been a cornerstone of paleomagnetic studies for over seven decades and has enabled paleomagnetists to make plate tectonic reconstructions, date geological and archaeological materials and recover the history of Earth’s magnetic field. Unfortunately, many geological samples produce experimental results that disagree with the predictions made by Néel theory. This ‘non-ideal’ behaviour is often associated with larger particles that are non-uniformly magnetized. In this paper, we use simulations based on Néel theory to demonstrate that non-ideal behaviour is also expected in assemblages of SD particles with a range of sizes and shapes previously assumed to be ideal. This effect occurs over a relatively small range of shapes and sizes for magnetite, but a much wider range for titanomagnetites. Our results call into question the typical interpretation of SD particles as ideal magnetic carriers. Instead, we suggest that the geological stability of a rock’s magnetization is influenced not only by the domain state of the magnetic carriers, but also by their shape and composition. This has important implications for paleointensities (and cooling rate corrections thereof), paleodirections and the dating of viscous magnetizations.