Spin-lattice coupling phase transition and phonon anomalies in bismuth ferrite BiFeO3

Abstract Phase transition, phonon vibration, and magnetization in (100) BiFeO 3 (BFO) multiferroic single crystals have been characterized as functions of temperature. The BFO single crystal exhibits a weak ferromagnetic hysteresis loop possibly due to appearance of ferromagnetic Fe O Fe orderings and a modulation of the Fe O Fe bond. Zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibilities suggest a weak spin-glass behavior below ∼200 K. High-resolution transmission electron microscopy (HRTEM) suggests a rhombohedral R 3 c symmetry (with tilted FeO 6 octahedra) and lattice distortion in the surface layer. The BFO crystal undergoes a structural transition sequence of rhombohedral−tetragonal−orthorhombic− cubic at ∼620–640, ∼1000, and ∼1100 K, respectively. The spin-lattice coupling accounts for the rhombohedral tetragonal transition upon approaching the Neel temperature (T N ). The low-frequency Raman-active modes ψ 4,II , E (1), A 1 (1), and A 1 (2) reveal clear softening anomalies in frequency shifts and broad peaks in full-width-at-half-maximum (FWHM) around 450 K, which is attributed to the Fe O Fe ordering transition without change of structural symmetry.