Abstract
In this work, BiFeO3 powders were synthesized by a sol–gel method. The influence of annealing temperature on the structure and magnetic properties of the samples has been discussed. X-ray diffraction studies showed that the purest phase was formed in the temperature range of 400 °C to 550 °C and the samples annealed at a temperature below 550 °C were of nanocrystalline character. Mössbauer spectroscopy and magnetization measurements were used as complementary methods to investigate the magnetic state of the samples. In particular, the appearance of weak ferromagnetic properties, significant growth of magnetization, and spin-glass-like behavior were observed along with the drop of average grain size. Mössbauer spectra were fitted by the model assuming cycloidal modulation of spins arrangement and properties of the spin cycloid were determined and analyzed. Most importantly, it was proved that the spin cycloid does not disappear even in the case of the samples with a particle size well below the cycloid modulation period λ = 62 nm. Furthermore, the cycloid becomes more anharmonic as the grain size decreases. The possible origination of weak ferromagnetism of the nanocrystalline samples has also been discussed.
Highlights
DESPITE being studied for many years, BiFeO3(BFO) still attracts much attention from scientists worldwide
BiFeO3 samples were successfully fabricated by sol–gel wet chemical method followed by annealing in the temperature range 350 °C to 800 °C
The higher temperature of heat treatment (600 °C and 800 °C) promotes the formation of the Bi25FeO40 selenite phase. As it was proved by scanning electron microscopy (SEM) and X-ray diffraction (XRD) investigations lowering of temperature causes a systematic drop of the average particle size down to 21 nm for the sample annealed at 380 °C
Summary
DESPITE being studied for many years, BiFeO3(BFO) still attracts much attention from scientists worldwide. BFO belongs to the multiferroic class of materials. The crystal structure of BFO can be viewed as a rhombohedral distortion of the ideal perovskite cell. METALLURGICAL AND MATERIALS TRANSACTIONS A structure: (1) relative polar displacement of Bi3+ and Fe3+ ions along the [111] pseudocubic direction and (2) opposite rotation of the oxygen octahedra around the [111] axis in the adjacent cells.[1] It was proved that the first one reduces crystal symmetry from cubic (Pm3m) to the rhombohedral (R3m) and leads to the appearance of spontaneous polarization along the diagonals of the pseudocubic unit cell.[2] The second one causes further lowering of the symmetry to the R3c space group and, it is believed to determine the spin canting and weak ferromagnetism in this system.[3]
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