HR: 11:40h
AN: GP12A-05    [Abstracts]
TI: Multiple blocking temperatures and thermoremanent magnetization in single-domain particles
AU: * Newell, A J
EM: Andrew_Newell@ncsu.edu
AF: North Carolina State University, Center for Research in Scientific Computation Department of Marine, Earth and Atmospheric Sciences Box 8208, Raleigh, NC 27695-8208 United States
AB: The standard theory for thermoremanent magnetization (TRM) in single-domain particles, the Neel theory, assumes the particles have uniaxial anisotropy. However, some single-domain particles may have a mixture of uniaxial and cubic anisotropy. These include particles in ocean floor basalts and remagnetized carbonates. A more general theory for TRM is developed for such particles. In the Neel theory, a particle acquires TRM at a single blocking temperature. Above this temperature the magnetization vector varies randomly in response to thermal fluctuations. It spends most of its time near one of the two energy minima, but occasionally jumps from one minimum to the other. In the new theory for mixed cubic and uniaxial anisotropy there are up to eight minimum energy states, and jumps occur between neighboring states. The time dependence for the occupancy of states can be complicated, but the magnetization is only affected by two relaxation rates - one in the direction of the uniaxial axis and one perpendicular. This leads to two blocking temperatures. At the higher blocking temperature the states are divided into two groups and communication between these groups is cut off. At the lower blocking temperature all jumps cease. The probability distributions at the blocking temperatures are calculated in the small-field approximation to give the dependence of TRM on applied field. This dependence from the predictions of Neel theory, but these particles are still suitable for paleointensity measurements.
UR: http://www4.ncsu.edu/~ajnewell/TRM_mixed_anisotropy.html
DE: 1521 Paleointensity
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly