LOW-TEMPERATURE MAGNETORESISTANCE AND MAGNETIC-ORDERING IN PR1-XCAXMNO3

In zero field, Pr1-xCaxMnO3 shows thermally activated behaviour for al l values of x. For x greater than or equal to 0.3 charge ordering occu rs at 250 K, with long-range antiferromagnetic ordering at lower tempe ratures. For a range of compositions (0.30 less than or equal to x les s than or equal to 0.45) a first-order magnetic-field-induced insulato r-metal transition produces changes in resistivity of up to 12 orders of magnitude at low temperatures. These conducting states are metastab le, resulting in a large hysteresis in resistivity with changes in bot h magnetic field and temperature. There is a marked relationship betwe en the resistivity and the magnetic state of the material. Below the c harge-ordering temperature, metamagnetic transitions occur which trans form the magnetic correlations from either paramagnetic or antiferroma gnetic to ferromagnetic. The range of compositions for which these met amagnetic transitions occur, the fields required to induce the ferroma gnetic state, and the irreversible changes in the nature of the magnet ic ordering all correlate with the field-induced changes in resistivit y.