TRANSPORT PROPERTY OF (LA(1-X)A(X))MNO3 (A=CA,SR,BA)

The transport properties of the (La(1-x)A(x))MnOs (A = Ca, Ba, Sr) sys tem were investigated. The samples were prepared by a solid reaction r oute and were polycrystalline in nature. After sintering at 1420 degre es C for 24 h, they were cut into rectangular sheets for resistance me asurement. At first, the resistance of the (La(1-x)A(x))MnOs increased with decreasing temperature, and after reaching a maximum, decreased rapidly. The resistance maximum is probably related to the first-order phase transition, since it showed a hysteresis behavior. A ferromagne tic transition was also recognized. The Curie temperature was higher t han that of the resistance maximum. Below the-Curie temperature, a gia nt magnetoresistance (GMR) was observed for La0.7Ca0.3MnO3 and La0.7Ca 0.2Sr0.1MnO3. However, La0.7Ca0.2Ba0.1MnO3, which had the same crystal structure, ferromagnetic transition and metallic behavior as La0.7Ca0 .2Sr0.1MnO3, did not show GMR. This suggests that not only the spin or dering of the Mn site but also the Jahn-Teller effect may play an impo rtant role in the GMR.