Temperature-dependent Raman spectral evidence of local structural changes in BiFeO₃ thin films: Influence of substrate and oxygen vacancies

Temperature-dependent Raman spectral studies of BiFeO3 (BFO) films coated on three different substrates, viz., conducting Si (BFO-Si), sapphire (BFO-SAP), and fluorine-doped tin oxide (BFO-FTO), are reported between 123 and 773 K. The activity of Bi-O and Fe-O modes in these samples as a function of temperature shows different spectral features despite having synthesized from the same precursor. To understand the source of these variations, the spectra obtained on the above films were compared with those of bulk BiFeO3 (BFO-bulk) prepared via spark-plasma sintering. As the temperature increases, modes corresponding to the Bi-O activity at low frequency (120-180 cm(-1)) exhibit a redshift in their positions in all the samples. Between 350 and 550 K, BFO-SAP and BFO-Si samples show discernible anomalies in the positions of modes corresponding to the Fe-O activity (200-500 cm(-1)), which is not observed in the BFO-bulk and BFO-FTO samples. These anomalies are more pronounced for the modes between 350 and 500 cm(-1), suggesting alterations in the N & eacute;el transition temperature (similar to 643 K for BiFeO3). Concurrently, another composite film of BiFeO3-CoFe2O4 coated on the Si substrate is explored. Raman studies on the composite film are used to compare and verify the influence of the substrate and defects on the magnetic ordering as a function of temperature. Our study highlights the significance and relevance of using Raman spectroscopy as a tool to discern various factors leading to local structural and magnetic variation in a given compound.
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