Effect of electron irradiation on the paramagnetic state of La$_{0.67}$Ca$_{0.33}$MnO$_3$

The effect of point defects on the magnetic properties of La$_{0.67}$Ca$_{0.33}$MnO$_3$ polycrystals and single crystals has been studied. The magnetic susceptibility $\chi_{dc}$ of the initial samples and samples irradiated by electrons to the maximum dose $F$ = 9 $\times$ 10$^{18}$ cm$^{-2}$ has been measured in the temperature region 80 K $< T <$ 650 K. Local variations of Mn–O–Mn bond angles and lengths result in a nonmonotonic dose dependence of the Curie temperature $T_C$. At high doses of electron irradiation, $F\ge$ 5 $\times$ 10$^{18}$ cm$^{-2}$, the temperature of the transition from the ferromagnetic to polaron state in a single crystal is found to increase. In the paramagnetic region close to $T_C$, ferromagnetically ordered polarons are observed to exist, while at $T>$ 1.2 $T_C$, localization of $e_g$ electrons initiates formation of paramagnetic polarons with a higher magnetic moment. Electron irradiation stimulates persistence of magnetic polarons up to higher temperatures $T>$ 2$T_C$.