Temperature-Dependent Raman and Dielectric Spectroscopy of BiFeO3 Nanoparticles: Signatures of Spin-Phonon and Magnetoelectric Coupling

We report the temperature-dependent Raman and dielectric spectroscopy of chemically synthesized BiFeO 3 nanoparticles (average size ∼50−60 nm). The Raman spectra (90−700 K) show two sets of transitions in the lowest Raman E mode, associated with Bi−O bond motion situated in close proximity to the spin reorientation transitions reported for BiFeO 3, thereby indicating the existence of possible coupling between magnons and phonons for particle size below the helical order parameter (62 nm). These transitions are slightly shifted in temperature in comparison to the bulk single crystals. We also observe a step-like behavior in Raman peak position around the Nèel temperature, suggesting that the phonons are influenced by the magnetic ordering in nanosized BiFeO 3 . The heat-flow measurements show two sharp endothermic peaks at 1094 and 1223 K representing rhombohedral to orthorhombic or monoclinic transition followed by transition into the cubic phase above 1200 K. The low temperature (20−325 K), frequency-dependent (1−10 6 Hz) dielectric constant and loss tangent measurements show that the loss tangent (∼10 −3 ) and ac conductivity values (∼10 −8 Ohm −1 -cm −1 ) are orders of magnitude lower than the reported values for BiFeO 3 ceramics, indicating high levels of ionic purity of our samples. The real part of the permittivity shows a slight reduction in its value (∼30) in comparison to the bulk single crystals. Similar to the Stokes Raman shift, its temperature-dependent dielectric constant also shows four weak anomalies at ∼85, 168, 205, and 230 K situated in close proximity to the spin reorientation transitions, indicating magnetoelectric coupling.