Direct Measurement of Magnetostriction in Rocks using Capacitance Micrometry

A capacitance micrometer, sensitive to displacements as small as 1 Å, has been used to measure directly the magnetostriction curves of several Canadian and Australian basalts both parallel (λ//) and perpendicular (λ⊥) to applied fields. They are found to be similar to magnetization curves in general shape, with λ⊥= −1/2 λ// within the precision of measurement (˜2 per cent) in high fields, but relatively stronger λ⊥ in low fields, indicating negative volume magnetostriction in low fields. Saturation magnetostrictions λs are found to correspond to the fractional contents of magnetic minerals, in confirmation of values inferred from stress-induced magnetic anisotropies. Hysteresis is not apparent and magnetostriction associated with saturation remanence is unmeasurably small (<0.005 λs) showing that negligible permanent domain rotation accompanies the temporary application of a high field and that the domain structures of individual grains are intrinsically isotropic.