Abstract
Single domain (SD) particles have long been suggested as a potentially strong and stable source for the paleomagnetic signal, but their actual occurrence in rocks has been much questioned. Two possible modes of occurrence of SD Fe-Ti oxides can be distinguished. These are as magnetite rods bounded by ilmenite lamellae in intergrown grains (analogous to Alnico permanent magnet alloys), and as isolated ultrafine particles, perhaps representing part of a much broader grain-size distribution (analogous to the oxide coatings used in tape recording). The evidence for the presence in rocks of these two types of magnetic particle is reviewed. Where intergrown grains are present the SD ‘Alnico model’ seems to be valid, and where isolated titanomagnetite grains are involved the SD ‘tape-recorder model’ is satisfactory. Where pure Fe3O4 particles are involved the situation is less clear, and current thinking stresses the role of the so-called pseudo-single domain (PSD) moments.
