Editors : M. Kawasaki, A.J. Millis, S.von Molnr, M. Rajeswari, M. Rzchowski
a1 Center for Superconductivity Research and Department of Physics, University of Maryland, College Park, MD 20472, USA
a2 Current address: Laboratoire CRISMAT-ISMRA, CNRS UMR 6508, 6 Bd. du Maréchal Juin, 14050 Caen, France
a3 Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India
We have synthesized ceramics of A2FeReO6 double-perovskites A2FeReO6 (A=Ba, Ca). Structural characterizations indicate a cubic structure with a=8.0854(1) Å for Ba2FeReO6 and a distorted monoclinic symmetry with a=5.396(1) Å, b=5.522(1) Å, c=7.688(2) Å and β=90.4° for Ca2FeReO6. The barium compound is metallic from 5K to 385K, i.e. no metal-insulator transition has been seen up to 385K, and the calcium compound is semiconducting from 5K to 385K. Magnetization measurements show a ferrimagnetic behavior for both materials, with Tc =315 K for Ba2FeReO6 and above 385K for Ca2FeReO6. At 5K we observed, only for Ba2FeReO6, a negative magnetoresistance of 10% in a magnetic field of 5T. Electrical, magnetic and thermal properties are discussed and compared to those of the analogous compounds Sr2Fe(Mo,Re)O6 recently studied.