PRESSURE EFFECTS ON CHARGE-ORDERING TRANSITIONS IN PEROVSKITE MANGANITES

Effects of chemical and external pressures have been investigated on t he two types of charge-ordering (GO) systems of perovskite manganites with the use of single-crystal specimens: One is Nd1-xSrxMnO3 with mod erate CO instability occurring only near x=1/2 and the other is Pr1-xC axMnO3 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 appli ed external pressure on them with the aim of destabilizing the CO stat e via an increase of the 3d-electron hopping interaction. An electroni c phase diagram relevant to the CO transition was derived for (Nd1-zLa z)(1/2)Sr1/2MnO3 by such a control of the one-electron bandwidth (W). With an increase of W, the enhanced ferromagnetic double-exchange inte raction increases the Curie temperature (T-C) and suppresses the charg e-ordered state with a concomitant antiferromagnetic charge-exchange-t ype spin ordering (AF-CE). In a narrow window of z (0.4 less than or e qual 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 r eplaces the AF-CE state. Application of external pressure and resultan t enhanced carrier itineracy suppresses the CO transitions also for Pr 1-xCaxMnO3. For the x = 0.30 crystal application of pressure induces a metallic phase from the low-temperature side in the charge-ordered in sulating phase. The pressure-temperature phase diagrams relating to th e CO transition or the concurrent insulator-to-metal transition were s hown to scale well with the magnetic-field-temperature phase diagrams.