HR: 0800h
AN: GP21A-0102    [Abstracts]
TI: Modelling the Hysteresis of Interacting Pseudo-Single-Domain Magnetite Particles
AU: * Krása, D
EM: david.krasa@ed.ac.uk
AF: University of Edinburgh, School of GeoSciences, Grant Institute, King's Buildings, Edinburgh, EH7 5HT, United Kingdom
AU: Williams, W
EM: wyn.williams@ed.ac.uk
AF: University of Edinburgh, School of GeoSciences, Grant Institute, King's Buildings, Edinburgh, EH7 5HT, United Kingdom
AB: Recent studies have shown the importance of fine magnetite exsolution structures as remanence carriers in igneous rocks. These structures often form arrays of tightly spaced individual grains straddling the single domain (SD) to pseudo-single-domain (PSD) threshold. Due to the proximity of neighbouring particles, magnetostatic interactions are expected to play an important role for the magnetic properties of these particle assemblages. We have used an unconstrained, fully three dimensional finite element (FE) micromagnetic model to calculate hysteresis curves of 3x3 arrays of magnetite particles with aligned easy axes with grain sizes r=50 … 200nm and grain spacings d=0 … 3· r . The calculations show that the domain state of individual particles is not only dependent on their grain size but also on the grain separation. Closer d generally leads to an increased SD-PSD size threshold. Associated with that is a characteristic change in magnetic stability: For d<50nm M_RS/M_S decreases sharply for all modelled grain sizes. This decrease is related to the appearance of supervortex structures at zero external field. Our modelling approach allowed us to observe the formation of these supervortices in the course of a hysteresis cycle. As expected, SD sized particles retain a uniform particle magnetisation throughout the whole hysteresis cycle. The particle arrays, however, form intermediate supervortex structures even for relatively large d because the individual particles' magnetisation cannot collapse into a vortex state to reduce its magnetic stray field. Only when the spacing increases to 150nm (i. e. d=3· r), does the magnetisation of all particles in the array rotate coherently. In contrast, PSD particle arrays generally reverse their magnetisation by spin curling and the formation of individual vortex states. For almost touching particles, though, an SD like magnetisation structure of individual grains is maintained throughout the hysteresis cycle by forming supervortex states. The individual vortex state, however, is energetically more favourable as soon as d is only slightly increased. The paper will discuss the particle size and spacing dependence of the average interaction field and its influence on magnetic grain size determinations.
DE: 0545 Modeling (4255)
DE: 0560 Numerical solutions (4255)
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting