Anomalous field-dependent specific heat in charge-ordered Pr1-xCaxMnO3 and La0.5Ca0.5MnO3

We report low temperature specific heat measurements of Pr1-xCaxMnO3 (0.3 less than or equal to x less than or equal to 0.5) and Ln(0.5)Ca(0.5)MnO(3) with and without applied magnetic field. An excess specific heat, C'(T), of nonmagnetic origin associated with charge ordering is found for all the samples. A magnetic field sufficient to induce the transition from the charge-ordered state to the ferromagnetic metallic state does not completely remove the C' contribution. This suggests that the charge ordering is not completely destroyed by a ''melting'' magnetic field. In addition, the specific heat of the Pr1-xCaxMnO3 compounds exhibit a large contribution linear in temperature (gamma T) originating from magnetic and charge disorder.