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