Abstract

This paper on small uniaxial stress changing the remanent magnetization of rock is a companion to my previous paper on stress changing susceptibility, both phenomena being of current interest in attempts at earthquake forecasting.Theoretical expressions are derived (using rigorous energy-minimization but ignoring thermal activation) for reversible change in remanence parallel to the stress axis for samples containing single-domain grains of a ferromagnet with cubic magnetocrystalline anisotropy (K1 positive or negative) and anisotropic magnetostriction. The grains are assumed to be non-interacting and randomly oriented spheres or ellipsoids of revolution elongated along graphic, graphic, or graphic. Also, approximate expressions are given for samples containing multidomain grains with very strongly pinned walls. Thermal (or chemical), anhysteretic, and saturation remanence are discussed.For remanence change perpendicular to the stress axis, one expects −1/2 the above expressions for change parallel to the stress axis, which is easily proven for thermal remanence.The expressions predict that for magnetite-bearing rock the decrease in thermal remanence along a 100 bar (1 × 104 kPa) compression axis should be 0.76% for spherical single-domain grains, 0.27% for 1.4 to 1 elongation along graphic, and 0.09% for great elongation along graphic. The decrease for equidimensional multidomain grains with strongly-pinned walls should be ~0.38%. These are all much smaller than the corresponding estimates for susceptibility, but both remanence and susceptibility decreases should become larger and more comparable as titanium content increases.

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