Multiferroic properties of nanostructured BiFeO₃ tailored by milling and sintering by SPS

We report the magnetic and dielectric behavior of multiferroic bismuth ferrites synthesized using milling-annealing processes and tailored through the crystallite size reduction and the microstrain generated by mechanical milling. For synthesis, stoichiometric mixtures of Bi₂O₃ and Fe₂O₃ were mixed and milled for 5 h using a ball to powder weight ratio of 10:1 by high-energy ball milling and annealed at 650 °C. Subsequently, was carried out the Crystallite Size Reduction by Milling-Process at different milling times, in a range of 0–60 min, using a ball to powder weight ratio of 50:1 adding 5% wt. of methanol. X-ray diffraction and Rietveld analysis were used to evaluate the crystallite size and microstrain of the crystal structure. Morphology was studied by scanning electron microscopy, and magnetic behavior was evaluated using vibrating sample magnetometry. The powders obtained from Crystal Size Reduction by Milling-Process were sintered using spark plasma sintering in a 10 mm diameter graphite die at 750 °C. The effect of spark plasma sintering was evaluated by X-ray diffraction, Rietveld analysis, vibrating sample magnetometry and broadband dielectric spectroscopy. The results show a transformation from BiFeO₃ phase to Bi₂Fe₄O₉ phase for longest Crystallite Size Reduction by Milling Process time. Relationships between microstructural transformation and oxidation-reduction reactions, as well as antiferromagnetic-ferromagnetic changes, are discussed. Dielectric properties were found to decrease with high Crystallite Size Reduction by Milling-Process. Also, an unusual ratio was observed in magnetodielectric coupling which increases with Crystallite Size Reduction by Milling-Process time.