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XAFS investigation of the correlation of Bi-sublattice disorder with ferromagnetism of multiferroic BiFeO3 nanoparticle

Practical utilization of room temperature multiferroic BiFeO3 is intrinsically limited by the absence of ferromagnetism. In this backdrop, development of weak ferromagnetism in 20 nm-sized BiFeO3 nanoparticles is very optimistic. The origin of ferromagnetism is curious and paradoxical from long-range-order perspective since average superexchange angle Fe–O–Fe of the nanoparticle is in antiferromagnetic configuration. In this work, we resolve this paradox by establishing the beneficial role of local disorder with x-ray absorption spectroscopy. We distinguish between the natures of (Bi, Fe)-sublattice disorder and establish their correlation that eventually leads to ferromagnetism. Our results reveal intrinsic large Bi positional disorder, which may be attributed to 6s2 lone pair activity of Bi atom and which leads to greater susceptibility of Bi-sublattice to modification during size reduction. Thus, local (BiO6, FeO6) units are observed to undergo large distortion and rotation respectively. We demonstrate with calculations that FeO6 rotation is geometric consequence of BiO6 distortion. In the case of our BiFeO3 nanoparticles, experimental BiO6 disorder induces FeO6 rotation that drives Fe–O–Fe into ferromagnetic configuration. These local ferromagnetic units give rise to weak magnetism. This structural route to magnetism in BiFeO3 can be generalized to encourage A-site disorder controlled magnetism or any functional octahedral rotation in ABO3 perovsites. The results additionally propagate the effectiveness of particle size-dependence in generating A-site strain rather than chemical doping or external pressure.