Magnetic Hysteresis in Magnetite: Are Synthetic Samples Stressed?

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Newell EAR-0208446 Measurements of the magnetic properties of rocks can provide information on properties of magnetic minerals that reflect environmental changes and affect the reliability of paleointensity methods. Recent technology allows us to measure magnetic hysteresis rapidly and represent the full complexity of it using phenomenological models such as Preisach or FORC diagrams. To interpret these diagrams, we need a good physical model for the magnetic particles. The goals in this proposal are to develop a quantitative model for hysteresis in some well characterized synthetic magnetite samples, and to test it rigorously using hysteresis measurements at several temperatures combined with detailed particle size distributions. Recent micromagnetic models for the size dependence of hysteresis are much closer than previous models, but their predictions are still somewhat low. The investigator will test the hypothesis that stress anisotropy has an important effect on hysteresis even in the smallest particles. A numerical micromagnetic model will be used to calculate the magnetic hysteresis. Because magnetic properties depend strongly on the method of synthesis, the PI will concentrate on two sets of samples, aqueous precipitates and glass ceramics, with sizes below 0.4 microns. These samples have been intensively studied, are small enough for accurate micromagnetic modeling, and are analogues of natural systems. This work should either produce a realistic model for the magnetic properties or eliminate some models from consideration. A successful model could be used to interpret Preisach and FORC diagrams. It would also be the basis for future work on other magnetic phenomena such as thermoremanent magnetization.