UNUSUAL FIELD-DEPENDENCE OF THE RESISTIVITY AND MAGNETORESISTANCE IN ND0.5CA0.5MNO3

An unusually large positive magnetoresistance (MR) is found in the com pound Nd0.5Ca0.5MnO3 at low temperatures (T < 50 K). The positive MR i s seen when the magnetic held is first applied and it collapses at hig her fields leading to a large negative MR as observed recently in othe r manganates showing charge-ordering transition. These magnetic field induced effects (negative MR) show large hysteresis even up to tempera tures as high as 200 K. We explain the positive MR as originating from antiferromagnetic ordering in the presence of a ferromagnetic interac tion.