Strain modulated optical properties in BiFeO₃ thin films

Spectroscopic ellipsometry was used to investigate the strain-dependent optical properties of BiFeO₃ thin films. At room temperature, the compressively strained BiFeO₃/LaAlO₃ thin films show the largest band gap of about 3.12 eV. It redshifts to 2.75 eV for the tensile strained BiFeO₃/NdScO₃ thin films. With increasing temperature, observable anomalies in the band gap for all strained thin films near 640 K indicate that antiferromagnetic transition temperature is independent of strain and close to its bulk value, which are in good agreement with the first-principles calculations. These results further suggest a complex nature of charge-spin coupling in multiferroic BiFeO₃ thin films.