Nanostructured BiMnO3+δ obtained at ambient pressure: analysis of its multiferroicity

The BiMnO3+δ 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+δ 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 δ value, or Mn oxidation state. This parameter plays an important role in their magnetic properties, which vary from ferromagnetic for lower δ values (the case of the mechanosynthesized and SPS processed samples) to spin-glass behavior for higher δ values (thermally annealed samples). Electrical measurements, carried out on the dense BiMnO3+δ ceramic material, show no evidence of ferroelectricity down to 77 K, in good accordance with previous structural studies that indicated their centrosymmetric structures.