ROLE OF GRAIN-BOUNDARIES IN DOUBLE EXCHANGE MANGANITE OXIDES LA(1-X)A(X)MNO(3) (A=BA, CA)

We report the study of magnetotransport and magnetic properties of man ganite oxides La(1-x)A(x)MnO(3) (A = Ba, Ca). We have observed that po lycrystalline manganite shows a different resistivity behavior from th at of single crystalline material in that the former shows a small rho (P)/rho(5 K) (rho (P): peak resistivity, rho(5 K): resistivity at 5 K) ratio and a large magnetoresistance (MR) effect at T much less than T -c, whereas the latter shows a large rho(P)/rho(5 K) ratio and no sign ificant MR at low temperature, due to grain boundary contribution to t he resistivity. A La0.67Ba0.33MnO2.99 sample with a polycrystal size o f 5 mu m showed the grain boundary resistivity 150 times as large as t he grain resistivity at 5 K. A possible origin of low-field MR in poly crystalline manganites has been discussed. (C) 1997 Elsevier Science L td.