Pressure effects on charge-ordering transitions in Perovskite manganites

Effects of chemical and external pressures have been investigated on the two types of charge-ordering (GO) systems of perovskite manganites with the use of single-crystal specimens: One is Nd1-xSrxMnO3 with moderate CO instability occurring only near x=1/2 and the other is Pr1-xCaxMnO3 with stronger CO instability extending over a wide x region 0.3 less than or equal to x less than or equal to 0.7. We have partially substituted the Nd ions of Nd1/2Sr1/2MnO3 with larger La ions or applied external pressure on them with the aim of destabilizing the CO state via an increase of the 3d-electron hopping interaction. An electronic phase diagram relevant to the CO transition was derived for (Nd1-zLaz)(1/2)Sr1/2MnO3 by such a control of the one-electron bandwidth (W). With an increase of W, the enhanced ferromagnetic double-exchange interaction increases the Curie temperature (T-C) and suppresses the charge-ordered state with a concomitant antiferromagnetic charge-exchange-type spin ordering (AF-CE). In a narrow window of z (0.4 less than or equal to z less than or equal to 0.6) or in the pressurized state for z = 0.4, another type of antiferromagnetic (perhaps the A type) phase replaces the AF-CE state. Application of external pressure and resultant enhanced carrier itineracy suppresses the CO transitions also for Pr1-xCaxMnO3. For the x = 0.30 crystal application of pressure induces a metallic phase from the low-temperature side in the charge-ordered insulating phase. The pressure-temperature phase diagrams relating to the CO transition or the concurrent insulator-to-metal transition were shown to scale well with the magnetic-field-temperature phase diagrams.