We study the influence of the Joule effect on the nonlinear behavior of the transport I - V curves in polycrystalline samples of the manganite Pr 0.8 Ca 0.2 MnO 3 by using the crystalline unit-cell parameters as an internal thermometer in x-ray and neutron diffractions. We develop a simple analytical model to estimate the temperature profile in the samples. Under the actual experimental conditions we show that the internal temperature gradient or the difference between the temperature of the sample and that of the thermal bath is at the origin of the nonlinearity observed in the I - V curves. Consequences on other compounds with colossal magnetoresistance are also discussed.
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