Observation of antiphase boundaries in Sr₂FeMoO₆

A high-purity Sr₂FeMoO₆ sample was synthesized. According to Rietveld analysis of x-ray diffraction (XRD) data the occupancy of Fe at its regular B site was 95.6% with the rest occupying the Mo site, i.e., the antisite of Fe. Despite this, according to ⁵⁷Fe Mössbauer spectra 22% of the Fe atoms resided in a trivalent state having an extremely low internal field of ∼2.8 T even at 5 K, whereas a typical 46-T field was seen for the main component, which is known to arise from mixed-valence Fe^II/III. Since the low-field Fe^III species were totally invisible to probing by XRD measurements, it is suggested that they are atoms located on antiphase boundaries (APB’s). Magnetization measurements revealed a slightly lowered saturation magnetization (M_s) for the sample exhibiting the highest concentration of APB’s, as compared to samples with low concentration of APB atoms. The two-Fe-atom-thick APB layer consists of Fe atoms antiferromagnetically coupled to each other. The spins of the APB layer atoms turned perpendicularly to an applied magnetic field, as observed by Mössbauer spectroscopy.