First principles study of magnetism and defect energetics in non-stoichiometric Sr$_{2}$FeMoO$_{6}$

The influence of disorder and stoichiometry-breaking point defects on the structural and magnetic properties of Sr$_{2}$FeMoO$_{6}$ have been investigated with the help of electronic structure calculations within the GGA+U approach. Defining the chemical potentials of the constituent elements from constitutional defects, we calculate the energetics of the possible point defects in non-stoichiometric Sr$_{2}$FeMoO$_{6}$ and find transition metal-ion antisites and oxygen vacancies to be the dominant point defects. In non-stoichiometric Sr$_{2}$Fe$_{1+x}$Mo$_{1-x}$O$_{6}$, both Fe$_{Mo}$ antisites and Mo$_{Fe}$ antisites lead to a systematic decrease in saturation magnetization ($M_{s})$. Only Mo$_{Fe}$ antisites destroy the half metallic character of the electronic structure. Oxygen vacancies also reduce the $M_{s}$, but the half-metallicity is retained. The optimized unit cell lattice parameters stay within a relatively narrow range despite large changes in composition. In stoichiometric Sr$_{2}$FeMoO$_{6}$, the $M_{s}$ decreases linearly with increasing Fe/Mo antisite disorder with loss in half-metallicity. The calculated results are in excellent quantitative agreement with experimental values.