Structural evolution from Bi_4.2K_0.8Fe₂O_9+δnanobelts to BiFeO₃nanochains in vacuum and their multiferroic properties

In this paper, we report the structural evolution of Bi(4.2)K(0.8)Fe(2)O(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(4.2)K(0.8)Fe(2)O(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(4.2)K(0.8)Fe(2)O(9+δ) nanobelts into one-dimensional BiFeO(3) nanochains. Both the Bi(4.2)K(0.8)Fe(2)O(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 BiFeO(3) nanochains had a larger magnetization than the Bi(4.2)K(0.8)Fe(2)O(9+δ) nanobelts. Moreover, the BiFeO(3) nanochains exhibited a surprisingly large exchange bias with small training effects. This one-dimensional BiFeO(3) multiferroic nanostructure characterized by a relatively stable exchange bias offers important functionalities that may be attractive for device applications.