Investigation of spin phonon coupling in BiFeO3 based system by Fourier transform infrared spectroscopy

Abstract

In the present work, the low temperature infrared absorption spectra of BiFeO3 (BFO) are measured to explore the spin-phonon coupling in this compound. At 303 K, 4 weak transverse optic (TO) IR-active phonon modes E(TO6), E(TO7), E(TO8), and E(TO9) are observed. First two modes are corresponded to the Fe3+ cations caused by the internal vibration of FeO6 octahedra, E(TO8) is correlated to Fe-O bending vibration and E(TO9) is assigned to Fe-O stretching vibrations, respectively. At 213 K, two new modes E(TO5) and A1(TO3) are emerging out. Both are assigned to Fe3+ cations caused by the internal vibration of FeO6 octahedra. These modes get stronger and stronger with lowering the temperature due to the lattice contraction. When the temperatures decreases to T ≤ 213 K, an additional phonon mode is start appearing at around 638 cm−1 suggesting local lattice distortion of FeO6 octahedra. The temperature is corresponding with the FC and zero field cooled bifurcation temperature, which is related to the onset of spin glass behaviour. The occurrence of this additional phonon mode at this particular temperature suggests that there is strong spin-phonon coupling in BFO. This argument is further supported by the temperature dependence of this additional phonon peak. It shows anomaly around 124 K, which is related to spin reorientation of Fe3+ ions. This result clearly indicates that spin glass state and spin reorientation of Fe3+ is accompanied with the local structure distortion of FeO6 octahedra, providing evidence for the strong spin-phonon coupling in the BFO.