Direct demonstration of microwave demagnetization of a whole rock sample with minimal heating

Microwave demagnetization has been employed in a number of palaeointensity determinations and has been shown to produce equivalent results to thermal methods, yet the degree of demagnetization caused by the magnetic component of the microwave field, rather than dielectric heating, has never been fully established. Here we present results that show unequivocally that a sample of basalt may be demagnetized by the microwave magnetic field with minimal heating. The fields within a resonant cavity are described quantitatively and the possibility of perturbation due to the sample is accounted for. This allows direct comparison of the demagnetizing effect of the electric and magnetic field components. While the magnetic field can demagnetize the sample it is shown that without careful control of the sample's shape and placement, the effects of the electric field become dominant. (C) 2010 Elsevier B.V. All rights reserved.