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
Is data on this page outdated, violates copyrights or anything else? Report the problem now and we will take corresponding actions after reviewing your request.