Structural evolution from Bi4.2K0.8Fe2O9+δ nanobelts to BiFeO3 nanochains in vacuum and their multiferroic properties

Abstract

In this paper, we report the structural evolution of Bi_4.2K_0.8Fe₂O_9+δ nanobelts to BiFeO₃ nanochains and the related variations in multiferroic properties. By using in situ transmission electron microscopy with comprehensive characterization, it was found that the layered perovskite multiferroic Bi_4.2K_0.8Fe₂O_9+δ nanobelts were very unstable in a vacuum environment, with Bi being easily removed. Based on this finding, a simple vacuum annealing method was designed which successfully transformed the Bi_4.2K_0.8Fe₂O_9+δ nanobelts into one-dimensional BiFeO₃ nanochains. Both the Bi_4.2K_0.8Fe₂O_9+δ nanobelts and the BiFeO₃ nanochains showed multiferroic behavior, with their ferroelectric and ferromagnetic properties clearly established by piezoresponse and magnetic measurements, respectively. Interestingly, the BiFeO₃ nanochains had a larger magnetization than the Bi_4.2K_0.8Fe₂O_9+δ nanobelts. Moreover, the BiFeO₃ nanochains exhibited a surprisingly large exchange bias with small training effects. This one-dimensional BiFeO₃ multiferroic nanostructure characterized by a relatively stable exchange bias offers important functionalities that may be attractive for device applications.