Structural evolution from Bi$_{4.2}$K$_{0.8}$Fe$_{2}$O$_{9+\delta}$ nanobelts to BiFeO$_{3}$ nanochains and their multiferroic properties

In this study, we reported the structural evolution of Bi$_{4.2}$K$_{0.8}$Fe$_{2}$O$_{9+\delta}$ nanobelts to BiFeO$_{3}$ nanochains and the related variations of 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$_{9+\delta}$ 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$_{9+\delta}$ nanobelts into one-dimensional BiFeO$_{3}$ nanochains. Both the Bi$_{4.2}$K$_{0.8}$Fe$_{2}$O$_{9+\delta}$ nanobelts and the BiFeO$_{3}$ nanochains showed multiferroic behaviors, with their ferroelectric and ferromagnetic properties clearly established by piezoresponse and magnetic measurements, respectively. Interestingly, 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.