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We report a systematic study on the structural, electronic, magnetic, and ferroelectric properties of [111]-oriented BiFeO3/BiAlO3 (BFO/BAO) superlattice using density-functional calculations. It is found that the Fe-O-Fe superexchange interactions in BFO/BAO superlattice are greatly suppressed by the inserted BAO layers, with the antiferromagnetic-ferromagnetic transition energy decreasing from around 280 meV per BFO formula unit (five atoms) to 11.6 meV per BFO/BAO formula unit (ten atoms). The tensile strain can further decrease this energy, making the magnetic transition more plausible. In addition, we find that BFO/BAO superlattice preserves the large ferroelectric polarization as well as energy gap of bulk BFO. Therefore, BAO may be a good candidate for constructing the BFO-based superlattice with improved multiferroicity. [ABSTRACT FROM AUTHOR]

The tensile strain and oxygen vacancy effects on the electronic and magnetic properties of multiferroic material BiCoO3 have been studied by density functional theory + U calculations. It is found that the oxygen vacancy at the top of the pyramid structure along the <001> direction (denoted as O1) can be stabilized at 1+ and 2+ charge states, which significantly changes the local magnetic moment of Co ions and possibly provides net magnetic moments to the BiCoO3 system. While the tensile strain degrades the polarization of BiCoO3 about 20 μC/cm2, the formation of oxygen vacancies becomes easier as the strain increases up to 5%. It indicates that the polarization and magnetic property of BiCoO3 could be tuned by the strain and oxygen vacancies, though the polarization of BiCoO3 is slightly degraded by the oxygen vacancies. Meanwhile, the change of electronic structure and magnetic property introduced by oxygen vacancy is illustrated according to the crystal field theory. [ABSTRACT FROM AUTHOR]