Multidomain susceptibility and anomalously strong low field dependence of induced magnetization in pyrrhotite

Abstract

Abstract Large multidomain grains with high intrinsic susceptibilities ki theoretically have (total) susceptibilities k = 1/N where N is the geometric demagnetizing factor of the grain. Therefore, k should be nearly field-independent and also independent of the saturation magnetization of the magnetic material as long as ki is large. For magnetite and titanomagnetites the 1/N approximation appears to be reasonable, while for hematite ki is very small so that k ≈ ki. Monoclinic pyrrhotite has susceptibilities well below the maximum possible 1/N limit, even for a millimeter-sized single crystal. Unlike magnetite the susceptibility of pyrrhotite is strongly field-dependent in the field range 8–800 A m−1; k increases with field by as much as 70% per decade, the increase being larger in fields above 80 A m−1 than below. The field dependence is also more pronounced for the single crystal and a massive ore sample than for a rock sample containing pyrrhotite grains in the size range 20–300 μm, thereby indicating a grain size dependence. The anisotropy of magnetic susceptibility (AMS) also increases with the inducing magnetic field which may complicate quantitative correlations between strain and AMS. However, the orientation of the AMS ellipsoid remains unaffected. These results are important for the determinatiob of induced magnetization for anomaly modelling. Q- factor calculations and AMS studies when pyrrhotite bearing rocks are involved.