Magnetic and structural properties of the double-perovskite Ca_2FeReO_6
We suceeded in the preparation of polycrystalline Ca_2FeReO_6 which has a Curie temperature of 540 K, the highest value of all magnetic perovskites investigated up to now. This material has been characterised by X-ray and neutron powder diffraction. We found at 548 K a monoclinic unit cell (space group P2_1/n) with a=5.4366(5) A, b=5.5393(5) A, c=7.7344(5) A, and beta=90.044(4) deg. For low temperatures a phase separation in two monoclinic phases with identical cell volume is observed in neutron scattering. The two phases possess different magnetic structure and coercivity. 57-Fe-Moessbauer spectroscopy measurements show the presence of four different Fe(3+) positions indicating two different phases at room temperature, indistinguishable in the diffraction experiments. The conductivity is thermally activated for all temperatures and no significant magnetoresistivity is observed.