Effects of size and oxygen annealing on the multiferroic behavior of bismuth ferrite nanoparticles

We have compared the multiferroic behavior in different-sized BiFeO₃ nanoparticles (11–29 nm) prepared by a solution evaporation method. The effects of oxygen vacancies on the behavior of phase pure BiFeO₃ have been studied for particles prepared under two different annealing environments, air and oxygen. We find that BiFeO₃ nanoparticles show weak ferromagnetism at room temperature which increases with decreasing particle size. However, the effects are determined not to be related to the presence of more oxygen vacancies, since oxygen annealing increases the magnetic moment in this system. In dielectric studies, we observe clear evidence for magnetoelectric coupling with the imaginary part of dielectric constant exhibiting a clear anomaly around the magnetic transition temperature (T ~ 560 K). In high temperature resistivity measurements insulating and activated resistivity behavior is observed with the activation energies being lower for T < T _N. The increased ferromagnetism in smaller particles is explained in terms of suppression of the antiferromagnetic spin spiral whereas oxygen annealing actually leads in our case to an improvement of the crystallinity.