Properties of the ferrimagnetic double perovskites A₂FeReO₆ (A= Ba and Ca)

Abstract

Ceramics of A₂ FeReO₆ double perovskites have been prepared and studied for A = Ba and Ca. Ba₂FeReO₆ has a cubic structure (Fm3m) with a ≈ 8.0854(1) Å whereas Ca₂ FeReO₆ has a monoclinic symmetry with a ≈ 5.396(1) Å, b ≈5.522(1) Å, c ≈ 7.688(2) Å and β= 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 Ba₂ FeReO₆. Magnetization measurements show a ferrimagnetic behaviour for both materials, with T_c= 315 K for Ba₂ FeReO₆ and above 385 K for Ca₂ FeReO₆. 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 × 10²⁴ eV^-1 mol^-1. Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr₂ Fe(Mo,Re)O₆.