ABSTRACT
Recent extensive studies on the manganese oxides with perovskite or layeredperovskite structures have shown that a close interplay among spin, charge and or bital (crystal lattice) degrees of freedom is quite important for understanding many unconventional physical properties of these oxides. The perovskite manganites are in general formulated as R E \-X AZ^MnOs, where RE and A E are a trivalent rare earth and an alkaline earth element, respectively. In REMnOs (x = 0), the electron configuration of (3d4; is realized for the Mn3+ site, and due to ordering of eg-orbital (or cooperative Jahn-Teller distortion [1-4]) the substance is a layered (or A-type) antiferromagnetic insulator. In R E \-xAExMn0 3 , Mn4+ (3d3; t^e®) is created by the substitution of R E with A E and holes are accordingly introduced in the eg-band. In Lai-^Sr^MnOs and Lai-jCa^MnC^, which have long been known as conducting ferromagnets [5-7], the mobile ^-electrons mediate the ferromagnetic interaction between the neighboring Mn3+ and Mn4+ sites, which is termed double exchange interaction [8-10].
