THERMOMAGNETIC BEHAVIOR OF HEMATITE AND GOETHITE AS A FUNCTION OF GRAIN-SIZE IN VARIOUS NONSATURATING MAGNETIC-FIELDS

When interpreting thermomagnetic curves of non-saturated magnetic mine rals, irreversible heating and cooling curves need not necessarily imp ly chemical or structural changes. Increased aligning of magnetic mome nts on heating in an applied magnetic field can also induce an irrever sible cooling curve. The two processes can be distinguished by stirrin g the sample between subsequent thermomagnetic runs. Sample redispersi on considerably enhances the interpretative value of thermomagnetic an alysis and is therefore strongly recommended, in particular when analy sing 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 t he thermomagnetic heating curves of haematite is shown to be dependent on the competitive interplay between the temperature dependence of th e exchange energy and that of the coercive force with respect to the a pplied field. On heating, pure defect-poor haematite, which is magneti cally dominated by the canted moment, has an initially increasing ther momagnetic 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 irreversib le block-shaped thermomagnetic cooling curve lies above the heating cu rve, and shows hardly any dependence on applied held and grain size. T n contrast to the heating curve, the shape of the cooling curve depend s only on the temperature variation of the exchange energy. Our data s eem to indicate that for defect-poor haematites the domain configurati on acquired at the maximum heating temperature is retained on cooling to room temperature. More defect-rich haematite has a gently decreasin g thermomagnetic heating curve. On heating to increasingly elevated te mperatures (800 degrees C) the defects are annealed out off the lattic e, because the thermomagnetic curves approach those of defect-poor hae matite. The defect moment due to lattice defects seems to be additive to, but softer than, the canted moment. The canted and defect moment a ppear to have the same Neel (or Curie) temperature (approximate to 690 degrees C), because no change in temperature was observed, whilst the relative contributions did change, The thermomagnetic behaviour of go ethite is shown to be dependent on its coercivity and the amount of su bstituted ions.