Charge, orbital and magnetic ordering in the bandwidth-control manganites $Pr_0.55(Ca_{1-y}Sr_{y})_0.45MnO_3$ ($y=0.15,0.20$)

$\rm Pr_{0.55}(Ca_{1-y}Sr_{y})_{0.45}MnO_3$ has been reported to display bicritical features near y=0.25. We have utilized neutron and synchrotron x-ray scattering techniques to study the interactions between FM, AFM and charge/orbital ordering (CO-OO) for samples with y=0.15 and y=0.20 at H=0 and in a magnetic field up to 7 Tesla. In the absence of magnetic field, both samples display insulating AFM ordered ground states. The CO-OO shows an incommensurate-commensurate transition as the temperature is lowered and the correlation lengths of the CO-OO remain much shorter than those for the AFM ordering at all temperatures. With the application of a magnetic field there is a sharp first-order like transition from the CO-OO insulating phase to a metallic ferromagnetic state. This work was supported by the US DOE under Contract No. DE-AC05- 00OR22725 with UT-Batelle, LLC and by U.S. NSF DMR-0139882.