HR: 08:50h
AN: GP11B-04    [Abstracts]
TI: Modes of Demagnetization in Magnetite: Particles versus Populations
AU: * Halgedahl, S L
EM: shalg@mines.utah.edu
AF: Dept. Geology and Geophysics, University of Utah, 135 South 1460 East, Salt Lake City, UT 84112
AB: Bitter patterns have been studied on glass-ceramic magnetite particles after stepwise demagnetization of saturation remanence in both alternating fields (AFs) and in back-fields (BFs). Particles respond to both AFs and BFs through two mechanisms: (1) nucleation of new domain walls and new domains, and (2) wall motion, as in classical models of multidomain behavior. When preexisting walls are very strongly pinned in a state of saturation remanence, the first response to an AF or BF often is nucleation of a fresh wall. In these same particles, stronger AFs or BFs then trigger reorganization of walls through wall motion. In particles whose walls are less strongly pinned in a state of saturation remanence, the sole response to AFs and BFs is wall motion. However, rarely does the AF "demagnetization" process result in a particle containing domains of equal width, even after several treatments in a strong AF (e.g., 2 kOe peak amplitude). Furthermore, Bitter patterns suggest that AF demagnetization drives many particles into local energy minimum domain states with substantial associated moments. This result is surprising, in view of magnetite's strong tendency to self-demagnetize. If the polarities of these individual moments are random across a large population, then AF demagnetization would reduce the net moment of a bulk sample to zero. But while an alternating field may demagnetize the magnetic ensemble as a whole, it may not result in the constituent particles occupying demagnetized states.
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005