Chemical Control of the Néel Temperature and Magnetic Anisotropy in the Brownmillerites Ca₂Fe_2–<i>x</i>Mn_<i>x</i>O₅, <i>x</i> = 0.10–0.25

Brownmillerite oxides with the composition Ca₂Fe_2-<i>x</i>Mn_<i>x</i>O₅ had been investigated previously for the limited range <i>x</i> = 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, <i>T</i>_N, is suppressed from 730 K for <i>x</i> = 0.0 to 465 K for <i>x</i> = 0.5 and the magnetic structure changes from G_<i>x</i> (<i>x</i> = 0.0) to G_<i>y</i> (<i>x</i> = 0.5-1.00). In this study, the compositions <i>x</i> = 0.10 and 0.25 are investigated using single-crystal X-ray and neutron diffraction methods. It is found that <i>T</i>_N decreases linearly from <i>x</i> = 0.0 to <i>x</i> = 0.5, with values of 584(2)K for <i>x</i> = 0.25 and 684(2)K for <i>x</i> = 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 <i>x</i> = 0.25 and <i>x</i> = 0.1, the magnetic structure is neither G_<i>x</i> or G_<i>y</i>, but the moments lie between the <i>a</i> and <i>b</i> 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.