Abstract
The crystal and magnetic structure of brownmillerite Ca2Fe2-xAlxO5; x = 0.8 has been refined against the powder neutron diffraction data collected as a function of temperature from 3 K to 350 K. Ca2Fe2-xAlxO5 is a G-type antiferromagnet with Ibm2 symmetry, the magnetic ion Fe occupies both tetrahedral and octahedral sites and both the sites are antiferromagnetically coupled individually. In this report, the mechanism of spin reorientation transition shown by these compounds is elucidated. The moment direction is found to be directed preferably along the crystallographic c-axis at low temperatures along with a small fraction of domains that are aligned along the a-axis. As the temperature is increased the domain with a-axis aligned moment inflates at the expense of domains with the moment directed along c-axis. However, at all temperatures both types of domains are found to be coexisting. The results suggest phase coexistence of the two magnetic structures with temperature dependent dynamics.
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