HR: 1340h
AN: GP13A-0043    [Abstracts]
TI: Configurational Anisotropy in Equidimensional Grains
AU: * Williams, W
EM: wyn.williams@ed.ac.uk
AF: Edinburgh University, School of GeoSciences University of Edinburgh, Edinburgh, EH17 8UD United Kingdom
AU: Muxworthy, A R
EM: adrian.muxworthy@gmail.com
AF: University of Southampton, School of Ocean and Earth Sciences,European Way, Southampton, SO14 3ZH United Kingdom
AU: Paterson, G
EM: gap1@noc.soton.ac.uk
AF: University of Southampton, School of Ocean and Earth Sciences,European Way, Southampton, SO14 3ZH United Kingdom
AB: The stability of small magnetic particles has often been characterised in terms of their grain size and shape. For uniformly magnetized, equidimensional grains it is easily shown that the grain's coercivity is determined solely by its magnetocrystalline anisotropy (K) and intrinsic magnetic intensity (Ms). For magnetite this yields a micro coercivity of 37.5 mT, but this can be substantially increased by elongating the grain, producing a value of 140 mT for grains with an axial ratio of 2:1. These results are true only for uniformly magnetized grains that switch by coherent rotation. Numerical micromagnetic modelling, however, has demonstrated that there is a measurable degree of non-homogeneity even in grains small enough to approach the superparamagnetic grain size. The non-uniform magnetization produces anisotropy independent of crystalline structure or grain elongation, and due entirely to the relationship between the domain configuration and the grain shape. This configurational anisotropy has long been acknowledged, but has been difficult to quantify until the advent of three-dimensional micromagnetic modelling. In this presentation we will examine the effect of domain state and grain shape on the switching field of equidimensional grains, where the magnetocrystalline anisotropy has been removed. Three grain shapes are considered: a cube, an octahedron and a tetrahedron. The effect of grain shape, domain structure and domain switching path will be examined as a function of grain size. The evolution of domain state will be shown, demonstrating that pseudo-single-domain grains can have significantly higher stability than that of uniform (single-domain) grains. The switching path and domain stability will be examined using a combination of constrained optimisation and nudged elastic band integration of the Landau-Lifshitz Gilbert equation.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005