Crystal structure, magnetic and transport properties, and electronic band structure of A2Mn2O7 pyrochlores (A = Y, In, Lu, and Tl)

Crystal structures, magnetic and transport properties, and electronic band structures for four compounds with the pyrochlore structure, Tl2Mn2O7, which exhibits colossal magnetoresistive behavior, and insulating A(2)Mn(2)O(7) (A = Y, In and Lu), are discussed. The cubic unit cell dimensions increase with increasing ionic size of the A(3+) ions. The ferromagnetic behavior is consistent with a superexchange interaction of Mn4+ spins with an intermediate Mn-O-Mn bond angle of about 130 degrees. In In2Mn2O7 and Tl2Mn2O7, strong hybridization of the In(5s)/Tl(6s) orbital with O(2p) and Mn(3d) may enhance the ferromagnetic interaction, leading to high transition temperatures over 120 K. The metallic behavior of Tl2Mn2O7 also originates from the hybridization among Tl(6s), O(2p), and Mn(3d). The free-electron-like minority down-spin band crosses the Fermi energy, leading to spin-polarized conduction. At the ferromagnetic Curie temperature, where majority local spins align, spin-polarized conduction electrons are produced: Thus, a ferromagnetic-and-metallic state appears in the Tl2Mn2O7 pyrochlore. [S0163-1829(99)00602-5].