Startlingly strong shape anisotropy of AC magnetic susceptibility due to eddy currents

Abstract

SUMMARY In order to better understand the anisotropy of AC magnetic susceptibility due to eddy currents (EC susceptibility) in rocks and minerals, we investigated the anisotropy of both in-phase (ipAMS) and out-of-phase (opAMS) magnetic susceptibility. Copper cylinders were chosen as experimental material because copper is diamagnetic, internally isotropic magnetically, and its out-of-phase magnetic susceptibility is no doubt due to eddy currents. Results of laboratory measurements were successfully compared to theoretical models. Both the in-phase and out-of-phase EC susceptibility components are strongly frequency dependent. The principal directions of both ipAMS and opAMS are closely related to the shapes of the copper bodies. While the degree of AMS of elongated cylinders asymptotically approaches 2, which is the value for infinite cylinder, it can be two orders higher in strongly flattened cylinders of the same volume. Analysis of directional susceptibilities has shown that EC susceptibility is well described by the tensor of second rank. These results are useful for the structural interpretation of the opAMS of rocks and ores whose opAMS is due to eddy currents (in electrically conductive minerals, such as graphite, and galena).