Temperature dependence of electronic transitions and optical properties in multiferroic BiFeO₃ nanocrystalline film determined from transmittance spectra

The ultraviolet-infrared transmittance spectra of BiFeO₃ nanocrystalline film have been studied in the temperature range 5.3-300 K. A redshift trend of the absorption edge and optical constants with increasing the temperature can be observed. Four interband electronic transitions can be uniquely assigned and strongly depend on the temperature. Moreover, two magnetic transitions located at about 150 and 200 K have been observed and can be interpreted as spin-reorientation transitions. It was found that the optical band gap decreases from 2.69±0.01 to 2.65±0.01 eV with increasing the temperature due to the modification of the electron-phonon interactions.