Emergence of two-magnon modes below spin-reorientation transition and phonon-magnon coupling in bulk BiFeO3: An infrared spectroscopic study

We have studied the low-temperature magnetic and infrared data of the BiFeO3 ceramic sample to investigate the behavior of phonons and magnon-phonon coupling below 300 K. From the magnetic study, we noticed two magnetic transitions near 150 and 257 K due to the spin-reorientation. Besides, this sample is found to be a mean-field system rather than a glassy system. In our assignment of the infrared modes, all the 13 allowed phonon modes are accounted for rhombohedral BiFeO3 at room temperature itself. Almost all the modes show anomalous behavior in their frequency dependence, close to the observed magnetic transitions. In addition, the emergence of two-magnon modes at 106 and 111 cm−1 below 160 K is observed due to a modification in the noncollinear spin structure. These results indicate the presence of magnon-phonon coupling in multiferroic BiFeO3 below room temperature. •An infrared spectroscopic study on BiFeO3 across the magnetic transitions is done.•Anomalous behavior of optical phonons near the magnetic transitions is observed.•The appearance of two-magnons below spin-orientation transition is detected.•The presence of magnon-phonon and magnetoelectric coupling in BiFeO3 is confirmed.