Frequency dependence of magnetic susceptibility for populations of single-domain grains

Abstract

SUMMARY A recent paper (Dearing et al. 1996) on the use of frequency dependence of magnetic susceptibility (xfd) in magnetic grain-size investigations of environmental materials has proposed a fundamentally new model for the magnetic susceptibility of singledomain (SD) grains measured at different frequencies, and is divergent from the now well-established theory of SD behaviour. This new model appears to be founded on a confusion about the behaviour of SD grains, as predicted by theory, and magnetic measurements of real materials, which naturally have a distribution of magnetic grain sizes. Here I try to clear up this confusion by showing how different log-normal distributions of magnetic grain sizes affect the frequency dependence of magnetic susceptibility measurement. This analysis highlights that the limiting maximum xfd value of -15 per cent obtained for environmental materials can be explained simply as a constraint imposed by the magnetic grain-size distribution having a finite minimum width (-1.0 on a natural log scale). There is no reason to suppose that if the grain-size dispersion was narrower higher Xfd values could not be observed. Furthermore, the xfd value is non-unique and so cannot be used quantitatively to determine the amount or grain-size distribution of SD grains in a sample