Properties of the ferrimagnetic double perovskites $A_2FeReO_6$ (A = Ba and Ca)

Ceramics of $A_2 FeReO_6$double perovskites have been prepared and studied for A = Ba and Ca. $Ba_2 FeReO_6$has a cubic structure (Fm3m) with $a \approx 8.0854(1)$ \AA whereas $Ca_2FeReO_6$has a monoclinic symmetry with $a \approx 5.396(1) \AA$, $b \approx 5.522(1)\AA$, $c \approx 7.688(2) \AA$ and $\beta = 90.4^o$ (P21/n) . The barium compound is metallic from 5 K to 385 K, i.e. no metal - insulator transition has been seen up to 385 K, and the calcium compound is semiconducting from 5 K to 385 K. At 5 K, we observed a negative magnetoresistance of 10% in a magnetic field of 50 kOe for $Ba_2FeReO_6$. Magnetization measurements show a ferrimagnetic behaviour for both materials, with $T_c \approx$ 315 K for $Ba_2FeReO_6$ and above 385 K for $Ca_2FeReO_6$. A specific heat measurement on the barium compound gave an electron density of states at the Fermi level, $N(E_F)$, equal to $5.9 \times 10^{24}eV^{-1}mol^{-1}$. Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds $Sr_2Fe(Mo, Re)O_6$