Origin of the colossal dielectric response of Pr_0.6Ca_0.4MnO₃

We report the detailed study of dielectric response of Pr_0.6Ca_0.4MnO₃ (PCMO), a member of the manganite family showing colossal magnetoresistance. Measurements have been performed on four polycrystalline samples and four single crystals, allowing us to compare and extract the essence of dielectric response in the material. High-frequency dielectric function is found to be ε_HF=30, as expected for the perovskite material. Dielectric relaxation is found in the frequency window of 20 Hz to 1 MHz at temperatures of 50–200 K that yields to colossal low-frequency dielectric function, i.e., the static dielectric constant. The static dielectric constant is always colossal, but varies considerably in different samples from ε(0)=10³ to 10⁵. The measured data can be simulated very well by blocking (surface barrier) capacitance in series with sample resistance. This indicates that the large dielectric constant in PCMO arises from the Schottky barriers at electrical contacts. Measurements in magnetic field and with dc bias support this interpretation. Colossal magnetocapacitance observed in the title compound is thus attributed to extrinsic effects. Weak anomaly at the charge ordering temperature can also be attributed to interplay of sample and contact resistance. We comment on our results in the framework of related studies by other groups.