Tuning between negative and positive magnetoresistance in (La0.7Sr0.3MnO3)1-x(La1.85Sr0.15CuO4)x composites

(La0.7Sr0.3MnO3)(1-x)(La1.85Sr0.15CuO4)(x) [(LSMO)(1-x)(LSCO)(x), x is the weight fraction] composites were prepared by mixing the sol-gel-derived LSMO and LSCO powders. Negative magnetoresistance (MR) is observed for all samples at temperatures above the superconductive T-c of LSCO and for the samples with x<0.5 within the entire measured temperature range. With increasing magnetic field, a positive MR is detected in the samples with x>0.5, which is ascribed to the magnetic suppression of the superconductive coupling within and/or between LSCO grains. This field and composition sensitive competition between positive and negative MR demonstrates a coexistence of superconductive and ferromagnetic orders, and favors a potential application in magnetic-field-sensitive devices. (C) 2002 American Institute of Physics.