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

Practical utilization of room temperature multiferroic BiFeO 3 is intrinsically limited by the absence of ferromagnetism. In this backdrop, development of weak ferromagnetism in 20 nm-sized BiFeO 3 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 6s 2 lone pair activity of Bi atom and which leads to greater susceptibility of Bi-sublattice to modification during size reduction. Thus, local (BiO 6 , FeO 6 ) units are observed to undergo large distortion and rotation respectively. We demonstrate with calculations that FeO 6 rotation is geometric consequence of BiO 6 distortion. In the case of our BiFeO 3 nanoparticles, experimental BiO 6 disorder induces FeO 6 rotation that drives Fe–O–Fe into ferromagnetic configuration. These local ferromagnetic units give rise to weak magnetism. This structural route to magnetism in BiFeO 3 can be generalized to encourage A-site disorder controlled magnetism or any functional octahedral rotation in ABO 3 perovsites. The results additionally propagate the effectiveness of particle size-dependence in generating A-site strain rather than chemical doping or external pressure.