TOWARD A BETTER UNDERSTANDING OF THE LOWRIE-FULLER TEST

We develop a theory of acquisition and alternating field (AF) demagnet ization of anhysteretic remanence (ARM) and saturation isothermal rema nence (SIRM) in multidomain (MD) grains in order to better understand the Lowrie-Fuller test. Our theory shows that the relative stabilities of low-field ARM and high-field SIRM against AF demagnetization are d etermined by the distribution f(h(c)) of microcoercivity h(c) in a sam ple, as found earlier by Bailey and Dunlop. When f(h(c)) is nearly con stant, weak-field ARM is more resistant to AF demagnetization than SIR M. In contrast, when f(h(c)) varies exponentially or is a Gaussian dis tribution, SIRM is more AF resistant than ARM. These contrasting stabi lity trends are conventionally called single-domain (SD)-type and MD-t ype Lowrie-Fuller results, respectively, but in reality, both types oc cur in the MD size range. We propose instead the descriptive terms L-t ype result (low-field remanence, i.e., ARM, more stable) and II-type r esult (high-field remanence, i.e., SIRM, more stable). The Lowrie-Full er test does not distinguish one type of domain structure from another , but it does depend indirectly on grain size. We show that the distri bution f(h(c)) in a given sample is determined primarily by the grain size d and the dislocation density rho. A nearly constant f(h(c)) occu rs in grains with small d and/or rho, but a Gaussian f(h(c)) is approa ched with increasing d and/or rho. The transition from L-type to II-ty pe behavior in the Lowrie-Fuller test occurs at a critical grain size d(t) approximate to 2/(rho w), where w is the domain-wall width. The l ower the dislocation density, the larger the transition size in the Lo wrie-Fuller test. This simple relationship explains the increase in th e transition size from about 5-10 mu m observed for crushed magnetite grains to approximate to 100 mu m for hydrothermally grown magnetites, which have lower dislocation densities than crushed grains.