High pressure study on maganites - The double exchange and polaron effects

We have examined the influence of oxygen-isotope on the pressure effects on the metal-insulator (M-I) transition in La0.65Ca0.35MnO3 (A) and on both t he M-I and charge ordering (CO) transitions in La0.57Ca0.43MnO3 (B) under h ydrostatic pressures. The pressure was found to enhance Tc at different rat es, depending on the O-isotopic mass of the sample. The greater dlnTc/dP ob served in the O-18 sample could be understood qualitatively in terms of the double exchange and the band reduction effect of polarons. In the B-sample s, the situation is much more complex. Pressure enhances Tc linearly at rat es independent of the O-isotopic mass but varies Tco nonlinearly in a compl ex way. The thermal hysteresis of the CO transition diminishes with increas e in pressure suggesting the suppression of the first order CO transition u nder pressure in both isotopes.