FIRST-ORDER INSULATOR-METAL TRANSITIONS IN PEROVSKITE MANGANITES WITH CHARGE-ORDERING INSTABILITY

The metal-insulator (M-I) phase transitions relevant to charge/orbital-ordering have been investigated for single crystals of perovskite-type manganites with controlled doping level (x) and one-electron bandwidth (W). The magnetic-field-induced I-M transition accompanied by several orders of magnitude change in resistivity is observed concomitantly with metamagnetic transitions and lattice structural change for relatively small-W system, Nd_1/2Sr_1/2MnO₃ crystal. This is due to the field-induced transition from antiferromagnetic charge-ordered insulator to ferromagnetic metal. In the case of further reduced-W system, e.g., Pr_1−xCa_xMnO₃, charge-ordered ground states are dominant for a wider range of x around x=0.50 and the robustness of the charge-ordered state can be controlled by degree of discommensuration (i.e., deviation of x from 0.50). With decrease of W, the antiferromagnetic spin fluctuation subsists even above the ferromagnetic transition temperature and enhances the electron-lattice coupling. The antiferromagnetic fluctuation that may be associated with the orbital correlations and collective Jahn-Teller distortions can be suppressed by application of a relatively low magnetic field, producing the low-field colossal magnetoresistance effect.