Thermomagnetic behaviour of haematite and goethite as a function of grain size in various non-saturating magnetic fields

Abstract

When interpreting thermomagnetic curves of non-saturated magnetic minerals, irreversible heating and cooling curves need not necessarily imply chemical or structural changes. Increased aligning of magnetic moments on heating in an applied magnetic field can also induce an irreversible cooling curve. The two processes can be distinguished by stirring the sample between subsequent thermomagnetic runs. Sample redispersion considerably enhances the interpretative value of thermomagnetic analysis and is therefore strongly recommended, in particular when analysing non-saturated magnetic minerals. Stirring between subsequent runs was extensively used in the analysis of the thermomagnetic behaviour of haematite and goethite as a function of grain size (i.e. coercivity) in various non-saturating magnetic fields (10-350 mT). The shape of the thermomagnetic heating curves of haematite is shown to be dependent on the competitive interplay between the temperature dependence of the exchange energy and that of the coercive force with respect to the applied field. On heating, pure defectpoor haematite, which is magnetically dominated by the canted moment, has an initially increasing thermomagnetic heating curve. Further heating causes the magnetization to increase smoothly up to a certain temperature which depends critically on the applied field and the coercivity of the sample. The irreversible block-shaped thermomagnetic cooling curve lies above the heating curve, and shows hardly any dependence on applied field and grain size. In contrast to the heating curve, the shape of the cooling curve depends only on the temperature variation of the exchange energy. Our data seem to indicate that for defect-poor haematites the domain configuration acquired at the maximum heating temperature is retained on cooling to room temperature. More defect-rich haematite has a gently decreasing thermomagnetic heating curve. On heating to increasingly elevated temperatures (800°C) the defects are annealed out off the lattice, because the thermomagnetic curves approach those of defect-poor haematite. The defect moment due to lattice defects seems to be additive to, but softer than, the canted moment. The canted and defect moment appear to have the same Neel (or Curie) temperature (=680°C), because no change in temperature was observed, whilst the relative contributions did change. The thermomagnetic behaviour of goethite is shown to be dependent on its coercivity and the amount of substituted ions.