Low-temperature behaviour of haematite: susceptibility and magnetization increase on cycling through the Morin transition

This is the authors' abstract. We don't add key points for this paper.

It has been realized previously (e.g. Borradaile 1994) that cycling through the Morin (Tm, occurring in ideal a-Fe2O3 at -10 °C) may have implications for the of some haematite-bearing rocks. We investigate the behaviour of the low-field (xlf), several magnetizations (in fields of 5, 25, 100 and 1600 mT) and on cycling through Tm of several well-characterized haematite types of varying and particle shape. Before low-temperature treatment, xlf of the haematites between ~40 and ~235*10^(-8) m^3 kg^(-1). Below Tm, where only haematite's moment resides, xlf was much more uniform at ~19 to ~28*10^(-8) m^3 kg^(-1). return to room temperature, increases in xlf of up to ~50 per cent were observed (when cycling in the Earth's magnetic field as well as in a field-free space), inferred to a function of the domain state of the haematite. This was shown for one of the (LH2 which occurs as small platelets and is particularly well crystalline) a relation y=(8.60±1.01) ln(x)-2.98 was obtained, where x is the grain size (mm) y is the percentage susceptibility increase.We suggest that transdomain changes induce change in xlf. The nucleation of (additional) domain walls in ‘metastable’ single-domain (SD) to pseudo-single-domain (PSD) grains is made possible by the low anisotropy at Morin transition. In view of this mechanism, small stable SD haematite particles not be affected and the grain size corresponding to y=0 (~1.5 mm for LH2) represent the ‘real’ SD threshold size. Thermal cycling to over the Curie (680 °C) is needed to return to the original domain state before the LT , as expressed by a return to the original xlf values. Measuring xlf between field (AF) demagnetization steps shows that AF demagnetization gradually the xlf increase, which appears to be soft; 30 mT is sufficient to erase 90 per . Thermal cycling in a 5 mT field between temperatures above Tm showed that changes in domain structure are noticeable before the isotropic point is . After cycling, magnetization is added to PSD and multidomain (MD) grains intriguingly appears to be remanence, probably induced by the broadening and irreversible displacement of loosely pinned domain walls. Complete cycling the isotropic point considerably enhances the new remanence component in ‘metastable’ SD to MD particles by an increase in the number of domains. If this can be extrapolated to the intensity of the Earth’s magnetic field, this would that large ‘metastable’ SD to MD specularite crystals with a well-developed Morin are susceptible to acquiring geologically irrelevant remanence components subjected to low ambient temperatures. Fine-grained haematite pigment, on the hand, would not be affected. Thermal demagnetization alone would not be able to these two remanences as the new domain structure persists up to close to the temperature. Our findings indicate that a cleaning procedure consisting of an AF step followed by stepwise thermal demagnetization is preferable in order to the original remanence component properly in haematite-bearing rocks.