Magnetotransport properties of La1−xCaxMnO3 (0.52 ≤ x ≤ 0.75): Signature of phase coexistence

We report the temperature and magnetic field dependence of transport properties in epitaxial films of the manganite La1−xCaxMnO3 in the overdoped region of the phase diagram for x > 0.5, where a charge-ordered (CO) and an antiferromagnetic (AF) phase are present. Resistivity, magnetoresistance and angular dependence of magnetoresistance were measured in the temperature interval 4.2K < T < 300K, for three concentrations x = 0.52,0.58 and 0.75 and in magnetic fields up to 5 T. The semiconductor/insulator-like behavior in zero field was observed in the entire temperature range for all three concentrations x and the electric conduction, at lower temperatures, in the CO state obeys 3D Mott's variable-range hopping model. A huge negative magnetoresistance for x = 0.52 and x = 0.58, a metal-insulator transition for B > 3 T for x = 0.52 and the presence of anisotropy in magnetoresistance for x = 0.52 and x = 0.58 show the fingerprints of colossal magnetoresistance (CMR) behavior implying the existence of ferromagnetic (FM) clusters. The declining influence of the FM clusters in the CO/AF part of the phase diagram with increasing x contributes to a possible explanation that a phase coexistence is the origin of the CMR phenomenon.