Dependence of giant magnetoresistance on oxygen stoichiometry and magnetization in polycrystalline La_0.67Ba_0.33MnO_z

We have studied resistivity, magnetization, and magnetoresistance in polycrystalline La_0.67Ba_0.33MnO_z by reducing the oxygen stoichiometry from z=2.99 to 2.80. As the oxygen content decreases, the resistivity of La_0.67Ba_0.33 MnO_z increases and the magnetic transition temperature shifts to lower temperature. A large magnetoresistance effect was observed over a wide temperature range for all samples except the insulating z=2.80 sample. The similarity between our results on oxygen-deficient polycrystalline La_0.67 Ba_0.33MnO_z and films previously reported to have a very large intrinsic magnetoresistance is discussed. At low temperature the magnetoresistance was observed to be strongly dependent on the magnetization. A possible mechanism for this effect is discussed.