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The insulator-metal transition in single crystal La 5/8Ϫy Pr y Ca 3/8 MnO 3 with yϷ0.35 was studied using synchrotron x-ray diffraction, electric resistivity, magnetic susceptibility, and specific heat measurements. Despite the dramatic drop in the resistivity at the insulator-metal transition temperature T MI , the charge-ordering ͑CO͒ peaks exhibit no anomaly at this temperature and continue to grow below T MI . Our data suggest then, that in addition to the CO phase, another insulating phase is present below T CO . In this picture, the insulator-metal transition is due to the changes that occur within this latter phase. The CO phase does not appear to play a major role in this transition. We propose that a percolationlike insulator-metal transition occurs via the growth of ferromagnetic metallic domains within the parts of the sample that do not exhibit charge ordering. Finally, we find that the low-temperature phase-separated state is unstable against x-ray irradiation, which destroys the CO phase at low temperatures.

We have studied the magnetic relaxation properties of the phase-separated manganite compound La 0.250 Pr 0.375 Ca 0.375 MnO 3 . A series of polycrystalline samples was prepared with different sintering temperatures, resulting in a continuous variation of phase fraction between metallic (ferromagnetic) and charge-ordered phases at low temperatures. Measurements of the magnetic viscosity show a temperature and field dependence which can be correlated to the static properties. Common to all the samples, there appears to be two types of relaxation processes -at low fields associated with the reorientation of ferromagnetic domains and at higher fields associated with the transformation between ferromagnetic and non-ferromagnetic phases.