| Summary: | 碩士 === 國立臺灣師範大學 === 物理學系 === 100 === We report the optical properties of 30 nm-thick multiferroic BiFeO3 (BFO) thin films. The x-ray diffraction data indicate that BiFeO3 thin film is rhombohedral phase grown on LSAT and DSO substrates while is tetragonal-like phase on a LAO substrate.
The absorption coefficient of these thin films determined from the transmission spectra shows the value of bandgap to be about 2.58 eV and 3.08 eV for BFO/LAST and BFO/LAO. Furthermore, the bandgap of BFO/LSAT deviates from the theoretical predictions in terms of Bose-Einstein model below 100 K that is likely associated with spin-glass phase observed in bulk BFO or misfit strain change between thin film and substrate at low temperatures. Additionally, the spectral weight of two oscillators in the energy range of 2.8 ~ 4.4 eV shows an extreme behavior at 200 K in BFO/LSAT and at 150 K in BFO/LAO. These anomalies could be related to spin-reorientation transition.
Raman-active phonon mode observed near 173 cm-1 in BFO/LSAT exhibits a discontinuous redshift at about 600 K, indicating a spin-phonon interaction with a coupling constant of 12.9 mRy/Å2. On the other hand, the peak position and linewidth of the 221 cm-1, 269 cm-1, 359 cm-1, 586 cm-1, and 685 cm-1 phonon modes observed in BFO/LAO show a step-like change at about 360 K, which could arise from either the structural transformation between tetragonal-like Mc- and MA-type or antiferromagnetic ordering. Finally, the 586 cm-1 and 685 cm-1 phonon modes observed in BFO/LAO also exhibits a discontinuous redshift at about 600 K driven by a spin-phonon coupling.
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