The magnetic hysteresis properties of ball-milled monodomain titanomagnetite, Fe2.4Ti0.6O4

Synthetic titanomagnetite, Fe2.4Ti0.6O4 (TM60) was ground in a tungsten carbide ball mill for times up to 80 hours. Initially the material becomes ''harder'' (coercive force, H-c, and ratio of remanence to saturation, M(r)/M(s), rise), which is explicable in terms of reduced particle size and reduced domain wall multiplicity. On further grinding the material becomes ''softer'', as expected for monodomain particles with reducing volume. The ''optical particle size'' of typically 0.4 mu m, determined from direct observation by SEM, is, however, ten times bigger than the ''magnetic crystal size'' inferred from an analysis of the hysteresis loop parameters. The small magnetic crystal size is consistent with the observed X-ray line broadening. The two observations can be reconciled by inspection of Transmission Electron Micrographs which show an internal microstructure which can be interpreted as the presence of ''nanocrystals'' within the particle envelope. The production of such nanocrystals by the grinding of particles is described in the materials science literature. It may be that a review of the rock magnetism literature describing the magnetic properties of ''crushed grains'' will discover features explicable in terms of nanocrystals.