FIELD-INDUCED INSULATOR-METAL TRANSITION IN PR1-XCAXMNO3

We present a detailed study of the properties of Pr1-xCaxMnO3 which sh ows that for a range of compositions (0.3 less than or equal to x < 0. 5) there is a first-order magnetic field induced insulator-metal trans ition. In zero field the resistivity rho of this material shows an act ivated behaviour and rho exceeds 10(8) Omega cm below 50 K. At 4 K, ap plication of magnetic fields of up to 80 kOe, allows rho to be reduced by at least 8 orders of magnitude depending on the value of the appli ed field. For the x = 0.4 composition the magnetic and charge ordering (CO) effects observed in this material are decoupled. The antiferroma gnetic (AFM) ordering at T-N = 170 K develops at considerably lower te mperatures than the CO state which is observed at T-CO = 250 K. Below T-CO, application of a magnetic field produces metamagnetic transition s. The field transforms the magnetic correlations from either paramagn etic or AFM to ferromagnetic (FM). At temperatures below 25 K the magn etic field produces irreversible changes in the magnetic order of this material. This in turn leads to the formation of long lived conductin g states in zero field.