Ferrimagnetic Properties of Magnetite

Abstract

Magnetite (Fe3O4, ferrous--ferric oxide) is ubiquitous as the source of the magnetism of most biological magnetic systems. Although a cation-deficient form of it, maghemite ($γ$- Fe2O3), and impurity-substituted magnetite (titanomagnetite) have on occasion been identified in biomagnetic systems, magnetite continues to be the primary magnetic source in biology. It is of interest, therefore, to inquire into the origin of its magnetism, or more properly, the ferrimagnetism of magnetite. In this chapter we deal with the ferrimagnetism of magnetite single crystals first. We then occupy ourselves with the application of magnetic domain theory to the particle-size-dependent properties of magnetite and the various kinds of intrinsic remanent magnetization contributed to by magnetite. The only type of natural remanent magnetization which we have not discussed here is depositional remanent magnetization (DRM), as it is still rare to find examples where biogenic magnetite has been convincingly shown to be responsible for the DRM in a sediment. Future research may prove otherwise. Finally, we deal with some practical magnetic techniques for determining the magnetic domain state, hence the effective particle size, of magnetite.