Properties of the ferrimagnetic double perovskites A2FeReO6(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 8.0854(1) Å whereas Ca 2 FeReO 6 has a monoclinic symmetry with a 5.396(1) Å, b 5.522(1) Å, c 7.688(2) Å and = 90.4° (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 2 FeReO 6 . Magnetization measurements show a ferrimagnetic behaviour for both materials, with T c 315 K for Ba 2 FeReO 6 and above 385 K for Ca 2 FeReO 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 × 10 24 eV -1 mol -1 . Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr 2 Fe(Mo,Re)O 6 .