HR: 09:10h
AN: GP31A-05 INVITED     [PDF]
TI: Bitter Patterns on Glass-Ceramic Magnetite: Links Among LEM States, Saturation Remanence, and Demagnetization Processes
AU: * Halgedahl, S L
EM: shalg@mines.utah.edu
AF: University of Utah, Dept. Geology and Geophysics, 135 South 1460 East, Rm 719, Salt Lake City, UT 84112
AU: Bollin, B
EM: bbollin@mines.utah.edu
AF: University of Utah, Dept. Geology and Geophysics, 135 South 1460 East, Rm 719, Salt Lake City, UT 84112
AB: We have observed Bitter patterns on glass-ceramic magnetite particles in several states of magnetization induced at room temperature. Particle sizes range from 5 to 30 micrometers and most grains contain but a few (2-6) domains, placing them in the uppermost pseudosingle-domain to small multidomain ranges of size and behavior. Bitter patterns have been studied after the following treatments: (1) repeat demagnetizations in an alternating field (AF) with a peak amplitude of 500 Oe, (2) exposure to 25 kOe, resulting in saturation remanent magnetization (Mrs), (3) stepwise demagnetization of Mrs in back-fields of opposite polarity to the inducing field, and (4) stepwise AF demagnetization of Mrs. Similar to titanomagnetite and pyrrhotite, magnetite particles can occupy any one of a range of local energy minimum (LEM) domain states after each repeat AF treatment. However, the range of LEM states for most grains is quite narrow; for example, some grains fluctuate between only two states. Surprisingly, after AF demagnetization in a peak field of 500 Oe, some particles appear either to be saturated (no Bitter lines visible) or to contain only small, residual edge domains. Similar domain states also are observed when certain grains carry Mrs, and such saturated (or near-saturated) states can persist until demagnetization in hundreds of oersteds finally triggers nucleation of walls. The majority of particles do contain walls in states of Mrs. In these latter grains, both AF and back-field demagnetization drive three processes: wall motion, wall nucleation, and wall denucleation. Thus, remanence and demagnetization in small multidomain magnetite grains are not due to wall motion alone. Instead, these experiments suggest that, in magnetite, both the intensity of Mrs and the demagnetization of Mrs are crucially linked to LEM states and LEM-LEM transitions.
DE: 1540 Rock and mineral magnetism
DE: 3900 MINERAL PHYSICS
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting