Domain observations of titanomagnetites from room temperature to Curie point and the nature of thermoremanent magnetism in fine particles

Abstract

Single-domain theory¹ has been remarkably successful as a theoretical basis for rock magnetism and hence palaeomagnetism. However, the grain size of much of the magnetic material present in rocks is too large to be single-domain according to classical theory². The resolution of this paradox may lie in the recognition that difficulties in domain wall nucleation^3–10 can preclude the multi-domain state even though it is energetically favoured. Thus, multi-domain particles are observed in a single-domain state in rocks carrying saturation remanence magnetization (IRMs). Inhibition of domain wall nucleation has also been shown to produce high-coercivity materials suitable for permanent magnets^11–13. Thermo-remanent magnetization (TRM) is one of the most important mechanisms of remanent magnetization for the study of palaeomagnetism, being acquired for example by lavas as they cool on the Earth's surface. We present here domain studies of titanomagnetites showing the increasing importance of the magnetostatic energy in the external field as temperature is increased, and that rocks carrying weak-field TRM, like IRMs, contain multi-domain grains in a single-domain state. These particles contribute very large moments to TRM; indeed, particles of this nature may carry a substantial part of the palaeomagnetic signal. The results also suggest that weak-field cooling of dispersions of suitable fine particles might leave them in a particularly hard magnetic state of some utility.