Surface ferromagnetism of complex defects lead-free ferroelectric Bi0.5Na0.5TiO3 materials

The complex surface defects induced magnetic properties of lead-free ferroelectric Bi0.5Na0.5TiO3 (110) materials system are investigated by using the density-functional theory. Various types of surface defects were considered for calculation, including antisite, vacancies and interstitial defects. The calculated results indicated that a perfect Bi0.5Na0.5TiO3 (110) surface produced zero magnetic moments. The conduction band was mostly contributed from Bi-6p and Ti-3d levels, where the valence band was built from O-2p. The antisite defects where Na replaced for Bi-site induced nonzero magnetic moments because of unsymmetrical spin O-2p up and O-2p down. While, the observation in nonzero magnetic moment of antisite defects Bi substitute for Na-site were originated from Ti-3d and Na-2s. The Na and Bi vacancies on the surface were tuned to the ferromagnetic because of the unsymmetrical contribution of O-2p, while the Ti and O vacancies were not. The Na and Bi surface interstitials induced the magnetic moments because of the contribution of the difference between Ti-3d and Bi-6p, respectively, while the Ti and O surface interstitials not induced the magnetic moments. We expected that our results could help to control chemical defects in new green multiferroic materials for smart electronic devices.