We study the influence of the cation size in the magnetic properties of ${(AA}^{\prime }{)}_2{\mathrm{FeReO}}_6$ ${(AA}^{\prime }={\mathrm{Ba}}_2,{\mathrm{Ba}}_{1.5}{\mathrm{Sr}}_{0.5},{\mathrm{B}\mathrm{a}\mathrm{S}\mathrm{r},\mathrm{B}\mathrm{a}}_{0.5}{\mathrm{Sr}}_{1.5},{\mathrm{Sr}}_2,{\mathrm{Ca}}_{0.5}{\mathrm{Sr}}_{1.5},{\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{r},\mathrm{C}\mathrm{a}}_{1.5}{\mathrm{Sr}}_{0.5},{\mathrm{Ca}}_2)$ double perovskites. As the average cation size decreases, the crystallographic structure at room temperature evolves from cubic to tetragonal and monoclinic. The large lattice effects observed for the monoclinic compounds could be responsible for their anomalous behavior. The Curie temperature increases anomalously from ≈303 K for ${\mathrm{Ba}}_2$ to ≈522 K for ${\mathrm{Ca}}_2,$ which seems to need an additional ferromagnetic coupling to the usual $\left(\mathrm{Fe}{t}_{2g}-\mathrm{Re}{t}_{2g}\right)pdd-\pi$ coupling. At 5 K, the magnetization at 5 T is close to $3{\mu }_B/\mathrm{f}.\mathrm{u}.\mathrm{}$ for those compounds with average cation size between that of ${\mathrm{Ba}}_2$ and that of ${\mathrm{Ca}}_{0.5}{\mathrm{Sr}}_{1.5}$ and the coercivity is found to be large in all cases (in the KOe range). The remaining compounds $({\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{r},\mathrm{C}\mathrm{a}}_{1.5}{\mathrm{Sr}}_{0.5},{\mathrm{Ca}}_2)$ undergo a simultaneous structural and magnetic transition below 150 K which produces a huge increase in the coercivity and reduces the magnetization. Magnetotransport properties change accordingly. From our results, a novel strong magnetostructural coupling in these compounds is anticipated. These results are interpreted within a scenario where the Re orbital state plays a crucial role in the ground state.

• Received 6 June 2003

DOI:https://doi.org/10.1103/PhysRevB.69.144401