HR: 10:50h
AN: GP12A-03    [Abstracts]
TI: Defect Moment and Magnetic Memory in Hematite
AU: * Ozdemir, O
EM: ozdemir@physics.utoronto.ca
AF: Ozden Ozdemir, Chemical and Physical Sciences, University of Toronto, 3359 Mississauga Road North, Mississauga, ON L5L 1C6 Canada
AB: Low-temperature SIRM cooling and warming curves for an MD crystal of hematite are similar to those of SD hematites. They have similar remanence losses at the Morin transition (Tm) (97-98 percent of the original SIRM), similar moments below Tm (2-3 percent of initial SIRM), and similar memories (30-37 percent of initial SIRM). These facts suggest that in SD and MD hematites alike, magnetic memory is controlled by a small fraction of the spins which are unusually strongly pinned by crystal defects. The existence of a weakly ferromagnetic (WF) moment in the antiferromagnetic (AF) phase below Tm is indicated by colloid patterns observed well below Tm and implies that some spins do not participate in the general rotation from the rhombohedral c-plane to the c-axis on cooling through the Morin transition of the bulk crystal. The "defect moment" of these few spins serves to restore preferred WF spin directions and domains in parts of the crystal during warming through the Morin transition and is thus responsible for the memory phenomenon. Weak ferromagnetism persists even at very low temperatures, much below Tm. Applying a saturating field to the initially demagnetized MD hematite crystal at 20 K produced a substantial SIRM. This SIRM remained unchanged in zero-field warming to Tm, then spontaneously increased by a factor of 25. Thus "memory" does not require cycling through Tm. The WF defect moment produced below Tm is just as effective in nucleating domain structure above Tm as WF moments produced above Tm and cooled below the transition. We propose that the mechanism of memory is spins pinned magnetoelastically by lattice defects such as dislocations. These spins rotate only partially out of the basal plane during cooling through Tm. Some basal-plane anisotropy, also magnetoelastic in origin, must remain below Tm in order to guide the spin nuclei into the preferred orientations they originally had above Tm on rewarming through the transition.
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005