HR: 09:35h
AN: GP11B-07 INVITED     [Abstracts]
TI: Thermoremanent magnetization (TRM) and Thellier laws in single-domain particles with mixed anisotropy
AU: * Newell, A J
EM: andrew_newell@ncsu.edu
AF: North Carolina State University, Department of Marine, Earth and Atmospheric Sciences Box 8208, Raleigh, NC 27695-8208 United States
AB: Thermoremanent magnetization (TRM) and viscous remanent magnetization (VRM) are two of the main components of remanence in rocks, and an understanding of their origins is essential for paleomagnetism. David Dunlop has made important experimental and theoretical contributions to almost every aspect of these phenomena. On the theoretical side he and his co-authors have applied Néel's single-domain (SD) theory to thermoviscous overprints and thermal fluctuation analysis; made numerous extensions to multidomain theories of TRM; and done micromagnetic modeling of TRM. Micromagnetic models of TRM face several challenges because of the inherent complexity of nonuniform magnetization. A lot of attention has been focussed on finding the height of the energy barrier between states, and significant progress has been made in this area. However, two other problems have mostly been overlooked. First, in Néel theory the energy barrier is a one-dimensional maximum, but in general it is a multidimensional saddle point. The shape of this saddle point affects relaxation rates between states. Second, there are several states and each state is connected to more than one other state. The combined effects of all these transitions must be determined somehow. These challenges are explored in a comparatively simple system, an SD particle with both cubic and uniaxial anisotropy. In such a particle there are up to eight remanent states and each state is connected to as many as four other states. Relaxation rates are calculated for saddle points using Kramers theory for high damping. The multiple connections are represented by a master equation. This equation is very difficult to solve for nonzero field. For small fields the master equation is reformulated using projection matrices and linear perturbation theory is used. The following results were obtained. In zero field there are two or three blocking temperatures depending on the geometry. A large field splits these blocking temperatures into as many as eight. Fortunately, to first order a small field has no effect on the blocking temperature. An important consequence is that the particle satisfies the Thellier laws for paleointensity.
UR: http://www4.ncsu.edu/~ajnewell/TRM_mixed_anisotropy.html
DE: 1521 Paleointensity
DE: 1533 Remagnetization
DE: 1540 Rock and mineral magnetism
DE: 1599 General or miscellaneous
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005