Low-temperature magnetic structure of Ca₂Fe₂O₅ determined by single crystal neutron diffraction

Full text: Ca₂Fe₂O₅ is a canted antiferromagnet (T_N = 720 K) which displays an anomalous elevation in its magnetic susceptibility for 60 K < T < 140 K. [1] Based on susceptibility measurements performed on oriented single crystals, Zhou et al. [2] proposed a reorientation of the antiferromagnetic (AFM) easy-axis from the crystallographic a axis to the c axis below 40 K, proceeding via a region of minimal magnetocrystalline anisotropy in the anomalous temperature interval. In order to test this proposition, we have refined the atomic and magnetic structure of Ca₂Fe₂O₅ against high-quality neutron Laue diffraction data collected on floating-zone-grown single crystals between 10 K and 300 K. An ad hoc sample mount was designed to apply a small (~35 Oe) magnetic field to the sample, ensuring perfect compatibility with the magnetic susceptibility data, which were also collected in a small field. Our refinements against both zero-field and in-field diffraction data reproduce the G-type AFM structure of Ca₂Fe₂O₅ excellently at room temperature, including the known ferromagnetic canting. Careful examination of the refinement results reveals that the material is in fact best described by the room-temperature magnetic structure at all measured temperatures, though in the intermediate temperature interval (measured at T = 100 K) the spins may be less well-ordered due to competing sublattice interactions. (author)