HR: 15:05h
AN: GP43B-05 INVITED     [Abstracts]
TI: Temperature- and Frequency-Dependent Susceptibility: A Useful Tool for Characterizing Nanoparticle Populations
AU: * Jackson, M
EM: irm@umn.edu
AF: Institute for Rock Magnetism, University of Minnesota, 291 Shepherd Labs 100 Union St SE, Minneapolis, MN 55455 United States
AB: In thermally-stable magnetic particles, low-field susceptibility is due to small reversible rotations of magnetic moments away from easy axes toward the applied field, and/or slight reversible displacements of domain walls from their zero-field minimum-energy locations. Below and at the boundary between superparamagnetic (SP) and stable single-domain (SSD) grain sizes, thermal fluctuations allow moments to rotate (irreversibly) over much larger angles to align with an external field, hopping over intervening maxima in anisotropy energy. This thermally-activated irreversible susceptibility component may be many times larger than the reversible susceptibility of thermally-stable grains. The SP-SSD boundary depends on various factors, and therefore the AC susceptibility of magnetic nanoparticles varies dramatically with grain characteristics (size and shape), measurement conditions (temperature and frequency), and intrinsic mineral properties (spontaneous magnetization, magnetocrystalline anisotropy and their temperature dependences). When all of these quantities are known or controlled, Néel-Brown theory provides a basis for forward calculation of in-phase and quadrature susceptibilities. In studies of natural materials, an inverse problem is usually of greater interest: quantifying the grain-size distribution using the measured frequency- and temperature dependence of susceptibility. Unfortunately this inverse problem cannot be solved without additional data or assumptions about systematic relationships between particle size and anisotropy (due to shape, crystallography and/or stress), which together determine the energy barrier for thermally-activated coherent reversal of magnetic moments. Nevertheless it is often possible to make reasonable assumptions and to draw important conclusions about the distribution of grain sizes in a sample by analyzing k(f,T) data.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005