HR: 15:00h
AN: GP33A-04 INVITED     [Abstracts]
TI: A quantitative model of dipolar interactions and their effect of first order reversal curves (FORC) of thermally activated, single-domain particles
AU: * Egli, R
EM: eglix007@umn.edu
AF: Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN 55455, United States
AB: Recently, the study of interacting particles was driven by the modeling effort undertaken to interpret first-order reversal curve (FORC) diagrams of natural rocks and sediments. Understanding the effect of magnetostatic interactions is of primary importance in rock magnetism and paleomagnetism, and FORC can provide useful information for this purpose. However, fully quantitative theories of FORC measurements have not been formulated yet. A quantitative model of dipolar interactions and their effects on FORC is presented here for the case of thermally activated, single-domain (SD) particles. This model is based on the statistic treatment of the interaction field (IF) produced by a random assemblage of magnetic moments. An exact solution of the model for the case of weak interactions shows that a rigorous analysis of the FORC function and its relationship with the intrinsic properties of the magnetic particles and their geometric arrangement is possible. Within the validity range of this solution, a random assemblage of uniaxial SD particles is characterized by following properties: (1) The statistical distribution of IF at any point in the assemblage - but not the IF itself - is independent of the magnetization state. (2) The IF at any point during a FORC measurement can be effectively modeled by two contributions related to the constant and the switching component of each magnetic moment. (3) The FORC distribution is the sum of two functions, P and Q. P is symmetric and represents the intrinsic effect of dipolar interactions. Its contour lines have the characteristic tear-drop shape observed experimentally in highly dispersed magnetic particles. Q has a characteristic boomerang shape and represents the contribution of the reversible part of the hysteresis loop of individual particles. Its relative contribution is negligible in the upper half of the FORC plane. (4) The intrinsic distribution of coercivities coincides with the first marginal distribution of the FORC function. (5) The distribution of IF is obtained from a vertical profile of the FORC function through the origin, and not through the central peak, as commonly assumed. Commonly used FORC data processing softwares may introduce additional artifacts in this region of the FORC diagram - such as a "reversible ridge" - which adversely affect the evaluation of the IF distribution. The effective volume concentration of the magnetic particles can be estimated from the IF distribution, and is thus directly provided by the FORC diagram. (6) Thermal activations produce an additional vertical spread of the FORC function that explains FORC diagrams of weakly magnetic, high-coercivity minerals such as hematite and goethite.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly