HR: 11:20h
AN: GP12A-05    [Abstracts]
TI: Isolating Viscous Overprints with Microwaves
AU: * Walton, D
EM: waltond@mcmaster.ca
AF: Physics Dept.McMaster Un. University, 1280 Main St. W., Dundas, Ont L8S 4M1 Canada
AB: During cooling in the kiln (or lava flow, etc.) a magnetization (NRM) is produced by the local magnetic field. It can be overprinted if the field changes afterwards. In the time elapsed before their retrieval, magnetic grains with relaxation times less than the time elapsed are reoriented by the environmental field, acquiring a viscous remanent magnetization (VRM) different from the NRM. The relaxation times of single-domain grains are exponentially dependent on their volume divided by the temperature, thus, during burial the magnetizations of the smallest grains are reset first, then as time elapses those of successively larger grains. But, the magnitude of the VRM is usually much less than that of the NRM, making the identification of its contribution to the resultant magnetization vector very difficult to identify. The VRM, and the time and temperature required to remove it have been the subject of theoretical interest for half a century. Not surprisingly, divergences in the theories have led to controversies whose experimental resolution, using thermal methods, has been complicated by the smallness of the VRM: by choosing the point at which the VRM has been removed it has been possible to confirm virtually any theoretical approach. Microwaves potentially provide a superior way of identifying the VRM: A microwave ferromagnetic resonance procedure is capable of removing the NRM first, leaving the VRM isolated, and easily measured, because it is possible to selectively demagnetize the larger grains before the smaller grains. This is because, under resonance conditions for a 2d order process, virtually all the energy of the microwave field is absorbed by the magnetic grains. However spin waves whose wave-length is larger than twice the scale of the particle are forbidden, hence there is a size dependent cut-off frequency. Thus, starting at low frequencies (and long wavelengths), and demagnetizing with increasing frequency steps it is possible to selectively remove the NRM that is carried by the larger grains, leaving the VRM, that requires higher frequencies, unaffected. The ability to isolate the VRM suggests that it can be used as an accurate method of dating the material.
DE: 1503 Archeomagnetism
DE: 1560 Time variations: secular and longer
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005