Chemical Control of the Néel Temperature and Magnetic Anisotropy in the Brownmillerites Ca2Fe2–x Mn x O5, x = 0.10–0.25

Brownmillerite oxides with the composition Ca₂Fe_2-xMn_xO₅ had been investigated previously for the limited range x = 0.5 to 1.0. Mn³⁺ preferentially occupies the octahedral site. This substitution has a strong impact on the magnetic properties as the magnetic ordering temperature, T_N, is suppressed from 730 K for x = 0.0 to 465 K for x = 0.5 and the magnetic structure changes from G_x (x = 0.0) to G_y (x = 0.5-1.00). In this study, the compositions x = 0.10 and 0.25 are investigated using single-crystal X-ray and neutron diffraction methods. It is found that T_N decreases linearly from x = 0.0 to x = 0.5, with values of 584(2)K for x = 0.25 and 684(2)K for x = 0.1. This behavior is rationalized in terms of a competition between antiferromagnetic and ferromagnetic exchange pathways in the perovskite layers involving Fe³⁺-Fe³⁺ and Fe³⁺-Mn³⁺ interactions. Profound changes in the magnetic anisotropy also occur as for both x = 0.25 and x = 0.1, the magnetic structure is neither G_x or G_y, but the moments lie between the a and b axes over nearly the entire temperature range. It has been shown that enhanced dielectric parameters are associated with another brownmillerite oxide, Ca₂FeCoO₅, in which nonaxial magnetic anisotropy is found within a narrow temperature range, 120-250 K. In the materials prepared in this study, this behavior should be evident at ambient temperature and over a wide temperature range.