Metallic conduction and low-field giant magnetoresistance in the highly Mn4+-doped compound La1/3Ba2/3MnO3

Transport measurements are performed for the highly Mn4+-doped compounds La 1/3Bu2/3MnO3 (B = Ca, Sr, Ba). Different from La-1/3(Ca/Sr)(2/3)MnO3 in whi ch a transition to a charge-ordered state occurs at low temperatures, La1/3 Ba2/3MnO3 undergoes a semiconductor-metal transition at T-p (2) over tilde 30 K upon cooling and the low-temperature metallic resistivity is dominated by a quadratic contribution. The application of relatively low magnetic fi elds causes a significant reduction in resistivity and hence a giant magnet oresistance which increases smoothly with lowering temperature. These obser vations suggest a fundamentally different conduction mechanism from that in the double-exchange perovskite oxides.