Oxygen deficiency-induced anomalous enhancement of Neel temperature and magnetic coupling for Bi_0.9Ca_0.1FeO_3-δ and Bi_0.9Pb_0.1FeO_3-δ

Temperature dependent neutron diffraction patterns of the Ca-doped BiFeO₃ and Pb-doped BiFeO₃ show that their Neel temperatures (T_N) increase to 710 K and 680 K, while pure BiFeO₃ has a T_N ∼ 643 K. This anomalous increase in T_N was investigated in detail by probing near edge X-ray absorption fine spectroscopy, X-ray photoemission spectroscopy techniques and structural refinement of the neutron diffraction pattern. Room temperature X-ray absorption spectroscopy clearly shows that there is no evidence of mixed valence states despite divalent cation doping in trivalent Bi-sites. A room temperature X-ray photoemission spectroscopy study revealed that instead of mixed valences, divalent doping has introduced oxygen vacancies in the system. Structural refinement of neutron diffraction patterns also supports the presence of oxygen vacancies in Bi_0.9Ca_0.1FeO_3-δ and Bi_0.9Pb_0.1FeO_3-δ. Oxygen deficiency plays a pivotal role in reducing Fe-O bond length in Fe-O₆ octahedra and hence increasing the Fe-O-Fe bond angle in Bi_0.9Ca_0.1FeO_3-δ and Bi_0.9Pb_0.1FeO_3-δ. This decreased Fe-O bond length and increased Fe-O-Fe bond angle favors the Goodenough-Kanamori-Anderson (GKA) coupling. The GKA coupling increases the magnetic interaction between the spins and hence increases the T_N. Additionally, doping of divalent cations (Ca²⁺ and Pb²⁺) results in the destruction of cycloidal spin structure and formation of a simple antiferromagnetic (AFM) structure. This structure can easily be canted near the heterogeneous interface with a ferromagnetic layer to induce the Dzyaloshinskii-Moriya (DM) interaction and enhance the magneto-electric (M-E) coupling.