Role of grain boundaries in double exchange manganite oxides La_1-xA_xMnO₃ (A = Ba, Ca)

We report the study of magnetotransport and magnetic properties of manganite oxides La_1-xA_xMnO₃ (A = Ba, Ca). We have observed that polycrystalline manganite shows a different resistivity behavior from that of single crystalline material in that the former shows a small ρ _P/ρ _{5 K} (ρ _P: peak resistivity, ρ_{5 K}: resistivity at 5 K) ratio and a large magnetoresistance (MR) effect at T ≪ T_C, whereas the latter shows a large ρ _P/ρ _{5 K} ratio and no significant MR at low temperature, due to grain boundary contribution to the resistivity. A La_0.67Ba_0.33MnO_2.99 sample with a polycrystal size of 5 μm showed the grain boundary resistivity 150 times as large as the grain resistivity at 5 K. A possible origin of low-field MR in polycrystalline manganites has been discussed.