Magnetotransport of CaCu3Mn4O12 complex perovskite derivatives

Neutron powder diffraction, magnetic and magnetotransport studies were carried out on new derivatives of the CaCu3Mn4O12 (AʹA3B4O12) complex perovskite. The samples were prepared in polycrystalline form under moderate pressure conditions. Substitutions at A and Aʹ sites of CaCu3Mn4O12, with only Mn^4+ and insulating behavior, imply electron doping that affects the magnetic and transport properties. X-ray Absorption Spectroscopy showed that Mn^3+/Mn^4+ valence mixing occurs only at B site, progressively filling the eg band and providing the metallic character in these compounds, as we observe in most of these samples. A semiconducting behavior is observed in samples with 50% Mn^3+ at B site. This can be understood by the opening of a gap in the conduction band corresponding to the half filling of the eg states. This is the case of the tetravalent rare earth doped samples (Ce and Th at Aʹ site) and of the appropriate A site doped Ca(CuMn2) Mn4O12 sample. At the strongly distorted A positions, Mn^3+, with localized eg electrons, act as magnetic impurities at very low temperatures (<40 K) giving rise to the observed upturn in the resistivity. The magnetic origin of this scattering is evidenced by its drastic reduction under a magnetic field.