THE MAGNETIC HYSTERESIS PROPERTIES OF BALL-MILLED MONODOMAIN TITANOMAGNETITE, FE2.4TI0.6O4

Synthetic titanomagnetite, Fe2.4Ti0.6O4 (TM60) was ground in a tungste n carbide ball mill for times up to 80 hours. Initially the material b ecomes ''harder'' (coercive force, H-c, and ratio of remanence to satu ration, M(r)/M(s), rise), which is explicable in terms of reduced part icle size and reduced domain wall multiplicity. On further grinding th e material becomes ''softer'', as expected for monodomain particles wi th reducing volume. The ''optical particle size'' of typically 0.4 mu m, determined from direct observation by SEM, is, however, ten times b igger than the ''magnetic crystal size'' inferred from an analysis of the hysteresis loop parameters. The small magnetic crystal size is con sistent with the observed X-ray line broadening. The two observations can be reconciled by inspection of Transmission Electron Micrographs w hich show an internal microstructure which can be interpreted as the p resence of ''nanocrystals'' within the particle envelope. The producti on of such nanocrystals by the grinding of particles is described in t he materials science literature. It may be that a review of the rock m agnetism literature describing the magnetic properties of ''crushed gr ains'' will discover features explicable in terms of nanocrystals.