Thermoremanent magnetization of multidomain hematite

Abstract

We have studied thermoremanent magnetization (TRM) produced by fields of 10–140 μT in the (0001) basal plane of a 10 × 6 × 2 mm natural single crystal of hematite, both before and after zero‐field cycling through the Morin transition at TM = 260 K. Stepwise thermal demagnetization of TRM indicated high‐unblocking temperatures between 680°C and the Curie‐Néel temperature TN = 690°C. In contrast, TRM was easily demagnetized by alternating fields, TRM intensity decreasing exponentially with increasing field in typical multidomain fashion. The observed 100‐μT MTRM is 1.1 kA/m. This strong TRM, almost equal to the saturation remanence, results from hematite's weak internal demagnetizing field. Domain walls move almost unhindered to their limiting positions, and TRM intensity approaches saturation. On cooling through TM, spins rotate to the antiferromagnetic c axis, and hematite's weak ferromagnetism is largely lost. However, on reheating in zero field through TM, as the spins rotate back into the basal plane, a “memory” remanence is regenerated in the original TRM direction. This TRM memory was about 25% of MTRM for our crystal and was even more resistant to thermal demagnetization than the original TRM. The 25% memory of TRM is similar to that of 0.12‐ to 0.42‐μm single‐domain hematites. High‐unblocking‐temperature TRM and TRM memory must be due to magnetoelastic pinning of spins in the basal plane by lattice defects, because both TRM and memory decrease with high‐temperature treatment, which anneals out defects. The memory phenomenon seems to be in essence an amplification of residual magnetism that survives below the Morin transition. Remanence produced in a demagnetized sample below TM and room temperature remanence that has been cooled through TM increase in identical ways on warming through the transition. We propose that small regions of canted spins, pinned by crystal defects, remain below TM when the bulk of spins have aligned with the antiferromagnetic c axis. These nuclei serve to regenerate room temperature domain structure and remanence in warming through TM.