Effect of cation ratio and order on magnetic circular dichroism in the double perovskite Sr2Fe1+xRe1-xO6

Superexchange-based magnetic coupling of the two B-site cations in rock-salt-ordered double perovskite oxides is extremely sensitive to the cation ratio and degree of order. However, as a result of the limited spatial resolution of most magnetic characterization techniques, it is challenging to establish a direct relationship between magnetic properties and structure in these materials, including the effects of elemental segregation and cation disorder. Here, we use electron energy-loss magnetic chiral dichroism together with aberration-corrected electron microscopy and spectroscopy to record magnetic circular dichroism (MCD) spectra at the nm scale, in combination with structural and chemical information at the atomic scale from the very same region. We study nanoscale phases in ordered Sr-2[Fe][Re]O-6, ordered Sr-2[Fe][Fe1/5Re4/5]O-6 and disordered Sr [Fe4/5Re1/5]O-3 individually, in order to understand the role of cation ratio and order on local magnetic coupling. When compared with ordered Sr-2[Fe][Re]O-6, we find that antiferromagnetic Fe3+-O2--Fe3+ superexchange interactions arising from an excess of Fe suppress the MCD signal from Fe cations in ordered Sr-2[Fe][Fe1/5Re4/5]O-6, while dominant Fe3+-O2--Fe3+ antiferromagnetic coupling in disordered Sr[Fe4/5Re1/5]O-3 leads to a decrease in MCD signal down to the noise level. Our work demonstrates a protocol that can be used to correlate crystallographic, electronic and magnetic information in materials such as Sr2Fe1+xRe1-xO6, in order to provide insight into structure-property relationships in double perovskite oxides at the atomic scale.