Correlation between ferromagnetism and the concentration of interfacial defects in multiferroic Bi7Fe2.75Co0.25Ti3O21 studied by positron annihilation

This paper investigated the effect of the annealing temperature on the interfacial defects and the magnetization of a single-phase multiferroic Bi7Fe2.75Co0.25Ti3O21. With the increase of annealing temperature, the average thickness of the nonaplates increased from 80 to 180 nm. But the magnetic property measurement shows that the saturation magnetization gradually decreases with the increase of the annealing temperature correspondingly. Positron annihilation measurements reveal that the interfacial defects disappear obviously when the annealing temperature increased, which is found to agree well with the variation of saturation magnetization. The results suggest that with the higher concentration of interfacial defects may bring about higher saturation magnetization for the Aurivillius phase material, opening a window to improve the magnetic performance through controlling the concentration of interfacial defects.