Phonon anomalies and phonon-spin coupling in oriented PbFe0.5Nb0.5O3 thin films

Abstract

We present Raman data on both single-crystal and thin-film samples of the multiferroic PbFe0.5Nb0.5O3 (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 ABO(3) relaxors, such as PbSc1/2Ta1/2O3. 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 similar to 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)).