Phonon anomalies and phonon-spin coupling in oriented PbFe _0.5Nb_0.5O₃ thin films

We present Raman data on both single-crystal and thin-film samples of the multiferroic PbFe_0.5Nb_0.5O₃ (PFN). We show first that the number and selection rules of Raman lines are compatible with a face-centered-cubic Fm-3m structure, as is known in other ABO₃ relaxors, such as PbSc_1/2Ta_1/2O₃. We then compare Raman data with anomalies in magnetization and the dielectric constant near the magnetic-phase-transition temperature (T_N), the diffuse ferroelectric-phase-transition temperature (T_m), and the pseudostructural-phase transition temperature (Burns temperature ∼T _B). The temperature evolution of the Raman spectra for the PFN film shows measurable changes in phonon positions, intensities, and full width at half maxima near 200, 410, and 650 K-temperatures that match well with experimentally observed T_N, T_m, and T_B, respectively. The increase in frequency with increasing temperature for the lowest-energy F_2g phonon mode is particularly unexpected. These changes suggest the transition of the crystal structure from an ordered phase to a disordered phase near T_B. The Raman study revealed phonon anomalies in the vicinity of T_m and T_N that are attributed to the dynamical behavior of polar nanoregions and spin-phonon coupling owing to its relaxor and multiferroic nature, respectively, which is well supported by dielectric and magnetic properties of the PFN thin film. Softening of the Fe-O mode was observed near the T_N. We correlate the anomalous shift of the Fe-O mode frequency with the normalized square of the magnetization sublattice; agreement with the experimental results suggests strong spin-phonon coupling near T_N owing to phonon modulation of the superexchange integral; however, the shifts in frequency with temperature are small (<3 cm⊃-1).