Frequency dependence and the electromagnetic susceptibility tensor in magnetic fabric studies

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A series of experiments has been performed on paleomagnetic sized samples where the anisotropy of magnetic susceptibility (AMS) has been measured at room temperature using alternating frequencies varying from 50 Hz to 5 kHz. The results of these experiments indicate that at frequencies of 1 kHz or higher a clear and systematic frequency dependence exists for some paramagnetic and diamagnetic minerals that readily conduct currents (e.g. copper, biotite, clays, and other substances). On the other hand, minerals that exhibit strong ferrimagnetic behavior, like magnetite, even though they are conductors, do not appear to exhibit any systematic frequency dependence over the range of frequencies used in these experiments. In single specimens, the electromagnetic susceptibility (EMS) tensor observed at higher frequencies (above 1 kHz) can be differentiated from the AMS tensor determined for ferrimagnetic materials in low magnetic fields at low frequencies (below 500 Hz) by simple matrix subtraction of the two tensors. Tests on the small sample sizes used in paleomagnetic studies indicate that skin effects are not important in samples this small, unless the sample contains very high conductivity minerals like pyrrhotite. Initial measurements on single quartz crystals at frequencies of 500 Hz and greater appear to yield information concerning the C crystal axis orientation and therefore may be useful in structural geology. © 1993.

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Physics of the Earth and Planetary Interiors

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