Extension of colossal magnetoresistance properties to small A site cations by chromium doping in Ln0.5Ca0.5MnO3 manganites

The Mn site substitutions of charge ordered (CO) manganites ${\mathrm{Ln}}_{\mathrm{0.5}}{\mathrm{Ca}}_{\mathrm{0.5}}{\mathrm{MnO}}_{\mathrm{3}}$ $(\mathrm{Ln=La–Tb})$ have been investigated. Among the different studied doping elements, chromium is shown to be the most efficient one to induce a metal insulator (MI) transition in the CO undoped insulators. A new $T_m{\mathrm{(〈r}}_A\mathrm{〉)}$ magnetic diagram, where ${\mathrm{〈r}}_A〉$ is the $A$ site cationic average radius, is established for the oxides ${\mathrm{Ln}}_{\mathrm{0.5}}{\mathrm{Ca}}_{\mathrm{0.5}}{\mathrm{Mn}}_{\text{1−x}}{\mathrm{Cr}}_x{\mathrm{O}}_{\mathrm{3}},$ which demonstrates that ${\mathrm{〈r}}_A〉$ is not a rhedibitory parameter for the occurrence of colossal magnetoresistance (CMR) properties. The average size limit ${\mathrm{〈r}}_A{〉}_L\text{≈1.19\hspace{0.17em}Å}$ below which the MI transition is suppressed in the undoped CO compounds is indeed lowered to ${\mathrm{〈r}}_A{〉}_L\text{≈1.14\hspace{0.17em}Å}$ in the Cr-doped materials. The electron microscopy study of these Cr doped oxides shows that this spectacular effect corresponds with the disappearance of the structural transition associated with CO.