Neutron Laue diffraction study of the complex low-temperature magnetic behaviour of brownmillerite-type Ca₂Fe₂O₅

The atomic and magnetic structure of brownmillerite Ca₂Fe₂O₅ has been refined against single-crystal neutron Laue diffraction data collected at 300, 100 and 10 K under zero-field and low-magnetic field (35 Oe = 35 × 10³/4π A m⁻¹) conditions. Ca₂Fe₂O₅ is a canted G-type antiferromagnet with Pcm′n′ symmetry, the magnetic moments on Fe being directed approximately along the crystallographic c axis at room temperature. The refinement results show clearly that this magnetic structure persists down to T = 10 K, despite a previous suggestion that an anomalous magnetic susceptibility enhancement observed in Ca₂Fe₂O₅ single crystals between 40 and 140 K might signify a reorientation of the antiferromagnetic easy axis from c to a below 40 K. Alternative explanations for this susceptibility anomaly are considered in terms of the evidence for partial or short-range loss of order in the anomalous regime, possibly due to the presence of multiple competing sublattice interactions.