Structural, magnetic, and optical properties of BiFeO 3 and Bi 2 FeMnO 6 epitaxial thin films: An experimental and first-principles study

${\text{BiFeO}}_{3}$ and ${\text{Bi}}_{2}{\text{FeMnO}}_{6}$ films were epitaxially grown on ${\text{SrTiO}}_{3}$ (001) substrates by pulsed-laser deposition, and their structural, magnetic, magneto-optical and optical properties were measured. In ${\text{Bi}}_{2}{\text{FeMnO}}_{6}$, Fe is mainly present in the $3+$ valence state, while Mn shows multivalence states. ${\text{Bi}}_{2}{\text{FeMnO}}_{6}$ exhibits low magnetization at room temperature and at 5 K indicating there is no significant $B$-site ordering. The ${\text{BiFeO}}_{3}$ film shows high optical transparency, while ${\text{Bi}}_{2}{\text{FeMnO}}_{6}$ shows high absorption loss in the infrared. Density-functional theory modeling of ${\text{BiFeO}}_{3}$, ${\text{BiMnO}}_{3}$ and ${\text{Bi}}_{2}{\text{FeMnO}}_{6}$ was carried out by applying the generalized gradient approximation (GGA) and $\text{GGA}+U$ methods. The formation enthalpy of ordered ${\text{Bi}}_{2}{\text{FeMnO}}_{6}$ is positive for several crystal symmetries and for ferromagnetic (FM) or antiferromagnetic (AFM) spin structures at 0 K temperature, indicating $B$-site ordering is not favored. The electronic structure calculations are consistent with the electronic and optical properties of these films.