Structural and magnetic ordering in Pr{sub 0.65}(Ca{sub y}Sr{sub 1-y}){sub 0.35}MnO{sub 3} : quantum critical point versus phase segregation scenarios.

The phase diagram of Pr{sub 0.65}(Ca{sub y}Sr{sub 1-y}){sub 0.35}MnO{sub 3}, 0.6{approx}<y{approx}<0.8, has been determined by neutron diffraction, magnetization, and electrical conductivity measurements in order to investigate the nature of the transition between ferromagnetic metallic and charge-ordered insulating states near y=0.75. Two possible scenarios for this transition have been proposed: a 'quantum-critical-point'-like feature, near which an associated charge-disordered paramagnetic phase is present, or a phase coexistence region. We demonstrate that the latter case is true, phase segregation occurring on a mesoscopic macroscopic length scale (several hundred angstroms to several microns). Our results show that no significant amount of the charge-disordered paramagnetic phase is present at low temperatures. Our data also indicate that the charge-ordered insulator to ferromagnetic metal phase boundary is temperature as well as composition dependent.