Phonon anomalies and phonon-spin coupling in oriented PbFe 0.5Nb0.5O3 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 ABO3 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 (TN), the diffuse ferroelectric-phase-transition temperature (Tm), 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 TN, Tm, and TB, respectively. The increase in frequency with increasing temperature for the lowest-energy F2g phonon mode is particularly unexpected. These changes suggest the transition of the crystal structure from an ordered phase to a disordered phase near TB. The Raman study revealed phonon anomalies in the vicinity of Tm and TN 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 TN. 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 TN owing to phonon modulation of the superexchange integral; however, the shifts in frequency with temperature are small (<3 cm⊃-1). © 2011 American Physical Society.