Abstract: A La_0.67Ca_0.33MnO₃(LCMO)/MgO granular composite system was fabricated by a chemical route. The grain size of the parent LCMO powders can be altered by controlling the initial sintering temperature. It shows that electrical transport and magetoresistive properties of the composite system strongly depend on the MgO content and initial sintering temperatures. For the samples with T_s1=1100℃, the insulator-metal transition can be observed only in pure LCMO and the x=1mol% composite. However, For the samples with T_s1=900℃, the insulator-metal transition can still be observed in the x=7mol% composite in which the maximal low field magnetoresistance (H=0.3T) is enhanced from 5% in pure LCMO to 27%. SEM analysis shows that
linking between LCMO grains is weakened with increasing MgO content or elevating the initial sintering temperature. The experimental results were discussed in terms of the spin polarized tunneling mechanism.

Key words: grain boundary effect, low field magnetoresistance, spin polarized tunneling, weak-link