Role of disorder and doping in ferromagnetic double perovskites*

We use an effective classical spin model, which we have recently derived and validated, to examine the effect of disorder on the magnetic properties of double perovskites like Sr$_{2}$FeMoO$_{6}$ (SFMO). We compute the effects of excess Fe, excess Mo, and anti-site disorder on the magnetization $M(T)$ and the ferromagnetic transition temperature $T_c$, and show that our results are quantitatively consistent with available experiments. We then use these results to propose routes to increase the ferromagnetic $T_c$ above that of pure SFMO (420K). We predict that, by adding excess Fe on Mo sites and compensating for the change in conduction electron density by La substitution on the Sr sites, we can raise $T_c$ by as much as 40\%, without substantially sacrificing the conduction electron polarization $P$ at the chemical potential.