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We report the magnetotransport properties of the heterojunction composed of La0.82Ca0.18MnO3 (LCMO) and 0.5 wt % Nb-doped SrTiO3 (SNTO). At temperature below 120 K, the heterojunction only exhibits a negative magnetoresistance (MR) independent of bias current. At temperatures above 120 K, with increasing bias current a crossover of MR from negative to positive is observed. This intriguing crossover of MR can be ascribed to the dominant spin character changing from the majority spin carriers to the minority spin carriers at the LCMO-SNTO interface due to their occupation in the complicated bands of La0.82Ca0.18MnO3. (c) 2006 American Institute of Physics.