Low pressure synthesis of BiMnO3 nanoparticles: anomalous structural and magnetic features

In the present work, low pressure synthesis of BiMnO3 (BMO) nanoparticles (NPs) by hydrothermal method is reported. Temperature-driven structural transitions of BMO are studied by X-ray diffraction (XRD) and exhibited that as-synthesized BMO is stabilized in monoclinic phase with C2 symmetry. However at 473 K, XRD pattern reveals abrupt changes in the intensity of peaks and lattice parameters. The origin of temperature-dependent symmetry transition is understood by sphenoid to prismatic transition by curtail of crystalline growth along (110) direction. The monoclinic structure with C2/c symmetry is found till 723 K. However, at 773 K and above XRD patterns are matched with Bi2Mn4O10 and Bi2O3 phases. The temperature profile of magnetic study exhibits antiferromagnetic behaviour. M-H curve provides evidence of weak ferromagnetism. The present results suggest that NPs of BMO exhibit coexistence of antiferromagnetism-ferromagnetism, however bulk BMO is ferromagnetic. X-ray photoelectron spectroscopy study reveals the presence of Mn in both +3 and +4 states in BMO NPs. Superexchange interaction between Mn4+-O-Mn3+ induces AFM along with weak FM in BMO. This study signifies the low pressure synthesis of BMO and its anomalous structural and magnetic behaviour.