Nanostructured BiMnO3+δ Obtained at Ambient Pressure: Analysis of Its Multiferroicity

The BiMnO3+delta phase has been prepared by mechanosynthesis, at ambient pressure and room temperature, as nanocrystalline powders. This oxide exhibits a metastable character, similar to those prepared by high-pressure and high-temperature methods, remaining stable up to 673 K. Nanostructured ceramic materials (92% densification), maintaining the BiMnO3+delta composition, have been successfully processed by the spark plasma sintering technique, at the very low temperature of 673 K, in a time as short as 6 minutes. Conditions of synthesis and sintering are the key factors that determine the actual delta value, or Mn oxidation state. This parameter plays an important role in their magnetic properties, which vary from ferromagnetic for lower delta values (the case of the mechanosynthesized and SPS processed samples) to spin-glass behavior for higher delta values (thermally annealed samples). Electrical measurements, carried out on the dense BiMnO3+delta ceramic material, show no evidence of ferroelectricity down to 77 K, in good accordance with previous structural studies that indicated their centrosymmetric structures.