Nanocrystalline multiferroic BiFeO3 thin films made by room temperature sputtering and thermal annealing, and formation of an iron oxide-induced exchange bias

Multiferroic nanocrystalline BiFeO ₃ films have been successfully made by room temperature sputtering and thermal annealing in oxygen at 500 °C. Nanocrystalline Bi was seen before annealing as well as β-Bi ₂O ₃ and the iron oxide phases, magnetite, maghemite, and FeO. Superparamagnetism was observed that can be attributed to magnetite and maghemite nanoparticles. The thermally annealed film contained BiFeO ₃ nanoparticles and magnetite, maghemite, and hematite as well as unidentified BiFe _xO _y phases. Superparamagnetism was also seen after annealing and the magnetic properties are predominately due to magnetite and maghemite nanoparticles rather than from multiferroic BiFeO ₃ . The saturation magnetic moment was 60% lower after annealing, which was due to some of the Fe in the iron oxide nanoparticles being incorporated into the BiFeO ₃ nanoparticles. An exchange bias was observed before and after annealing that cannot be attributed to a structure that includes BiFeO ₃. It is likely to arise from magnetite and maghemite cores with spin-disordered shells.