The isovalent substitution of Gd for La in ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ lowers the paramagnetic-ferromagnetic transition temperature $T_c$ and metal-insulator transition temperature $T_{\mathrm{MI}}$ with a substantial increase in the electrical resistivity ρ. Reduction of the ferromagnetic saturation moment M (5 K) below that expected for complete ferromagnetic alignment of the Mn magnetic moments occurs and $T_{\mathrm{MI}}$ is significantly lower than $T_c$ for $x>\mathrm{}\mathrm{0.2.}\mathrm{}$ The observations are consistent with the existence of a granular mixture of ferromagnetic/conducting La-Ca-Mn-O and ferrimagnetic/insulating Gd-Ca-Mn-O. The ionic radius of the substituted rare-earth ion is found to be an important factor in determining whether or not granular behavior occurs. The results provide insight into why $T_c\ne T_{\mathrm{MI}}$ in some colossal magnetoresistance oxides.

• Received 27 October 2000

DOI:https://doi.org/10.1103/PhysRevB.63.174436