A Monte Carlo investigation of the representation of thermally activated single-domain particles within the Day plot

Abstract

Hysteresis based measurements are a standard part of many rock magnetic investigations. Whilst they provide information on the various magnetic minerals present in a natural sample they can also offer an insight into the domain state of the grains. The most commonly employed method to derive such structural information from hysteresis measurements is the Day plot, which employs ratios of different parameters to demarcate the possible domain states. Numerous experimental and modelling investigations have demonstrated that interpretation of the Day plot is not a simple task because the position at which an assemblage of magnetic grains plots within the space of the diagram is not solely a function of domain state. Here two mechanisms that appear to influence the Day plot are investigated. It is well known that thermal activation of an assemblage has the effect of randomising the magnetisation of grains until at sufficiently high temperatures they become superparamagnetic. A collection of Monte Carlo simulations for uniaxial, single-domain, Stoner-Wohlfarth (SW) grains with atomic moments that rotate coherently (Stoner and Wohlfarth, 1948), are utilised to demonstrate that the expression of this process could be easily misinterpreted as the signature of a different domain state. Secondly, the different published methods for determining the coercivity of remanent magnetisation are assessed and it is shown that the same sample can plot in a variety of different locations within the space of the Day plot depending on which method is employed.