Colossal magnetoresistance in Ce doped manganese oxides

In this study we investigate the effects of Ce doping in ${\mathrm{R}}_{\text{1−x}}{\mathrm{A}}_x{\mathrm{MnO}}_{\mathrm{3}}$ $(\mathrm{R}=\mathrm{La},$ Ce, and $\mathrm{A}=\mathrm{Sr},$ Ce) on the magnetic and transport properties of this system. For ${\mathrm{La}}_{\text{1−x}}{\mathrm{Ce}}_x{\mathrm{MnO}}_{\mathrm{3}}$ (LCMO), an increase in Ce concentration is accompanied by an increase in $T_C$ from 225 to 236 K, as well as an increase in the electrical resistivity. An extremely high resistivity is observed in the new system ${\mathrm{Ce}}_{\text{1−x}}{\mathrm{Sr}}_x{\mathrm{MnO}}_{\mathrm{3}}$ (CSMO) which becomes insulating below its Curie temperature of 43 K. A maximum magnetoresistance (MR) ratio of 40% for CSMO and 53% for LCMO is observed. A larger change in resistivity is seen to correspond to an increase in the Ce concentration, however this is offset by an overall resistivity increase which keeps the MR ratio low. The high resistivity may be due to unreacted oxides in the samples. If true, the amount of impurity appears to be proportional to the Ce doping. If this impurity level can be reduced, a significant colossal magnetoresistance effect could be exhibited by these systems.