Anomalous magnetic ordering induced spin–phonon coupling in BiFeO 3 thin films

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Low-temperature magnetic properties and Raman spectra of epitaxial BiFeO 3 (BFO) thin films, grown on (111) SrTiO 3 substrates, have been studied. For all three prominent A 1 -symmetry Raman modes (138, 170, and 214 cm −1), there was a good linear correlation between the mode-frequency softening and the square of the in-plane magnetization in the temperature range between 80 and 300 K. These observations were ascribed to the spin–phonon coupling below the Néel temperature ( T N = 643 K ). The observed anomalous enhancements in Raman intensity below 200 K reveal spin reorientation transition due to change in the magnetic spin ordering.