Physics of the Earth and Planetary Interiors 129 (2002) 173–179 Grain size limit for SD hematite Günther Kletetschka a,∗ , Peter J. Wasilewski b a Code 691, GSFC/NASA, Greenbelt, MD 20771, USA b Code 921, GSFC/NASA, Greenbelt, MD 20771, USA Received 27 February 2001; accepted 21 August 2001 Abstract Grain sizes in the range (10 -4 to 10 -1 mm) are common in some rocks. Because thermal and/or chemical remanent magnetization of hematite in this range approaches intensities of single domain (SD) magnetite, careful exploration of this transition, may serve to develop new applications in rock magnetism that relate to magnetic anomaly source identification, and various paleomagnetic and grain size-dependent investigations. Grain size-dependent magnetic behavior of hematite reveals a SD–multidomain (MD) transition at 0.1 mm. This transition is recognized by variation in magnetic coercivity and susceptibility and is related to an anomaly in remanence recovery when cycling through the Morin transition. The coercivity decrease with increasing grain size occurs much more gradually above 0.1 mm than below this value. Magnetic susceptibility of the grains smaller than 0.1 mm has negligible dependence on the amplitude of the applied alternating magnetic field. For the larger grains a new amplitude-dependent susceptibility component is observed. The grain size of 0.1 mm is also associated with loss of most of the remanence when cycling through the Morin transition. This behavior is ascribed to a transition from the metastable SD to the MD magnetic state. The increase in magnetized volume causes the demagnetizing energy to destabilize the SD state, resulting in a transition where the demagnetizing energy is reduced by nucleation of the domain wall for grains larger than 0.1 mm. The 0.1 mm transition has no significant effect on shape of the temperature-dependent coercivity and saturation magnetization. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Hematite; Grain size; Domains; Magnetic properties; Morin transition; Susceptibility; Hysteresis 1. Introduction Recent work on pure natural hematite (Kletetschka et al., 2000a) examined acquisition of thermorema- nent magnetization (TRM) in weak external magnetic fields. TRM is acquired when a magnetic mineral (e.g. hematite) is demagnetized by heating above its Curie temperature and subsequently cooled down in the presence of a weak magnetic field (usually geo- magnetic). The intensity of TRM of hematite (acquired ∗ Corresponding author. Tel.: +1-301-286-3804; fax: +1-301-286-0212. E-mail address: gunther.kletetschka@gsfc.nasa.gov (G. Kletetschka). at 0.05 mT) systematically increases across a fairly large interval of grain sizes (10 -4 to 10 -1 mm). This increase appears to plateau at the 10 -1 mm grain size beyond which the TRM intensity stays more or less constant (Fig. 1 in Kletetschka et al., 2000a). The grain sizes in the range 10 -4 to 10 -1 mm are com- mon in natural settings and thus careful exploration of this transition region, characterized by the onset of large intensities of TRM, may serve to stimulate new applications in rock magnetism that utilize grain size-dependent properties. Hematite contains both a spin canted (Dzyaloshin- sky, 1958) and a defect moment (Gallon, 1968). The defect moment is generally attributed to foreign cations, but it also can occur from other defects, such 0031-9201/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0031-9201(01)00271-0