Transport and magnetoresistance properties of nanocrystalline La0.7Ca0.3MnO3

Transport, magnetic and magnetoresistance properties of nanocrystalline samples of La0.7Ca0.3MnO3 prepared using a method of coprecipitation from solutions have been systematically studied. Temperature (T) dependencies of both electric resistivity and magnetoresistance (MR) exhibit peaks near Curie temperature TC ≅ 266 K, the value of which was obtained from AC magnetic susceptibility (χ) measurements. In addition, broad resistivity peak, which is not accompanied with any anomaly on χ(T) and MR(T) curves, is observed at Tm ≅ 230 K. Relatively low value of the saturation magnetization at 5 K suggests that ferromagnetic metallic phase does not extend to the whole sample. Current-voltage I(V) characteristics are linear over more than two orders of V at T < TC, in rather contrast to the picture when intergrain tunneling is the dominant conduction mechanism. The results are discussed in terms of an inhomogeneous transport through a mixture of high resistive paramagnetic regions separated by more conducting ferromagnetic regions, where increased amount of the paramagnetic phase is provided by the expanded intergrain area. (orig.)