Chemical Control of the Néel Temperature and Magnetic Anisotropy in the Brownmillerites Ca2Fe2-xMnxO5, x = 0.10-0.25.
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abstract
Brownmillerite oxides with the composition Ca2Fe2-xMnxO5 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, TN, is suppressed from 730 K for x = 0.0 to 465 K for x = 0.5 and the magnetic structure changes from Gx (x = 0.0) to Gy (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 TN 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 Gx or Gy, 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.
