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 (CO) systems of perovskite manganites with the use of single-crystal specimens: One is Nd_1-x{Sr}ₓMnO₃ with moderate CO instability occurring only near x=1/2 and the other is Pr_1-x{Ca}ₓMnO₃ with stronger CO instability extending over a wide x region 0.3{}x{}0.7. We have partially substituted the Nd ions of Nd1/2{Sr}1/2MnO₃ 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 (Nd_1-z{La}z)1/2{Sr}1/2MnO₃ 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 (TC) 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{}z{}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 Pr_1-x{Ca}ₓMnO₃. 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.