Charge disordering induced by electron irradiation in colossal magnetoresistant manganites

The electron microscopy investigation of ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ shows that a complete charge disordering is induced in this phase by electron irradiation at 92 K, the disordered state remaining stable at this temperature, even in the absence of irradiation. A model is proposed for this transition, according to which an electron jumping into the manganite matrix creates a local charge disordered state and extends to the whole matrix through a chain mechanism, leading to a ferromagnetic metallic state. This model is supported by the fact that the charge ordered state (evidenced by the doubling of the a parameter) is restored by heating the material above 120 K $\left(\sim T_C\right)$ and then disappears above 150 K $\left(\sim T_{\mathrm{CO}}\right).\mathrm{}$ Similar results are obtained for ${\mathrm{Nd}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3,$ whereas ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ is not sensitive to electron exposure, in agreement with the larger stability of its 1:1 ${\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{4+}$ charge ordering.