The measurement of anisotropy of magnetic susceptibility using a cryogenic (SQUID) magnetometer and a comparison with results obtained from a torsion-fiber magnetometer

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We present a new set of orientations for use in the measurement of anisotropy of magnetic susceptibility (AMS) on a cryogenic magnetometer that provides a number of advantages over previously described methods. These include simplicity of analysis, a means of providing immediate internal consistency checks for detection of sample misorientations and other problems, a fixed sample holder that minimizes effects external to the sample, and ease of measurement for both cubic and cylindrical samples. Seven samples were measured five times each for AMS using this method. For comparison, the same replications for each sample were also carried out on a torsion-fiber magnetometer. The cryogenic magnetometer yielded determinations of the principal susceptibility axes that were in excellent agreement with the torsion-fiber measurements, except when the combination of two conditions occurred: (a) per cent anisotropy was low, and (b) the ratio of remanence to induced moment was ≥ 1.0. Scatter increased with either of these two conditions and, in addition, the directions of the apparent susceptibility axes were significantly affected when the remanence exceeded the induced moment. We also found significant discrepancies in the magnitude of susceptibility determined using the two methods, though these are dependent on measurements not inherent to either method. We conclude that measurement of AMS using a cryogenic magnetometer is feasible for many samples if a torsion-fiber instrument is not available, but that the latter remains the instrument of choice. © 1988.

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Physics of the Earth and Planetary Interiors

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