Unusual Field Dependence of the Resistivity and Magnetoresistance in $Nd_{0.5}Ca{0.5}MnO_3$

An unusually large positive magnetoresistance (MR) is found in the compound $Nd_{0.5}Ca_{0.5}MnO_3$ at low temperatures (T< 50K). The positive MR is seen when the magneticfield is first applied and it collapses at higher fields leading to a large negative MR as observed recently in other manganates showing charge-ordering transition. These magnetic field induced effects (negative MR) show large hysteresis even up to temperatures as high as 200 K. We explain the positive MR as originating from antiferromagnetic ordering in the presence of a ferromagnetic interaction.