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) system were investigated. The samples were prepared by a solid reaction route and were polycrystalline in nature. After sintering at 1420 degrees C for 24 h, they were cut into rectangular sheets for resistance measurement. 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 ferromagnetic transition was also recognized. The Curie temperature was higher than that of the resistance maximum. Below the-Curie temperature, a giant magnetoresistance (GMR) was observed for La0.7Ca0.3MnO3 and La0.7Ca0.2Sr0.1MnO3. 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 ordering of the Mn site but also the Jahn-Teller effect may play an important role in the GMR.