Thermoremanence and stable memory of single‐domain hematites

Abstract

We report thermoremanent magnetization (TRM) intensities and thermal demagnetization behavior of seven samples of single‐domain hematite (α Fe2O3) with grain sizes between 0.12 and 0.42 μm, before and after zero‐field cycling through the low‐temperature Morin transition (TM ≈ 240 K). TRM was unaffected by 100 mT alternating field demagnetization and by 600°C thermal demagnetization. Most demagnetization occurred between 625°C and the Néel temperature of 680–690°C. The TRM memory recovered after low‐temperature cycling was parallel to the original TRM and equally resistant to thermal demagnetization. TRM and TRM memory of single‐domain hematites are mainly due to the hard spin‐canted magnetism intrinsic to the crystal structure above the Morin transition, and not to the small and softer defect magnetism that survives below TM. However, the defect magnetism may play a role in renucleating the spin‐canted magnetism in a preferred direction during warming through TM. TRM intensities are well predicted by Neél single‐domain theory and increase in almost exact proportion to grain size. Although smaller than TRM intensities of multidomain hematites, single‐domain TRMs are potent sources of remanent magnetic anomalies, particularly for larger grains (10–15 μm), and are likely to be more stable over geological time than multidomain hematite TRMs.