KERR MICROSCOPY OF SMALL SYNTHETIC TI-RICH TITANOMAGNETITE GRAINS

Magnetic domain structure observations have been carried out on small synthetic Ti-rich titanomagnetite (TM) grains (ulvospinel content 50% - 70%) employing the magneto-optical Kerr effect (MOKE). On randomly o riented grains simple domain structures with planar domains or complex domain structures with curved domains and reverse spike domains have been observed. In addition to the description of the domain pattern, a qualitative determination of the magnetization direction within the d omains is presented. Planes with a magnetization parallel to the surfa ce show broad domains with nearly straight domain walls while planes w ith a magnetization normal to the surface show narrow domains and more complex patterns. With the help of the observed domain patterns on th e randomly oriented grains and the knowledge about the magnetization d irection within the domains a three-dimensional (3-D) domain structure model for Ti-rich TM is proposed. These Ti-rich titanomagnetites have only one easy direction of the magnetization and show areal unaxial c haracter, although the magnetic mineral is cubic. By analysing the dom ain structure of more than a hundred TM particles in a range of size b etween 1.7 and 20 microns the transition from the single-domain (SD) t o the multi-domain (MD) state is determined. The fit of the data in th is study gives a critical SD size of approximate to 1.1 mu m for Ti-ri ch TM.