Effect of Y substitution in La-Ca-Mn-O perovskites showing giant magnetoresistance

Substitution of yttrium (${\mathrm{Y}}^{3+}$) in La-Ca-Mn-O perovskites of the ${\mathrm{ABO}}_{3}$ structure markedly affects the crystal structure and lattice constants as well as the resistivity (\ensuremath{\rho}), ferromagnetic transition temperature (${\mathit{T}}_{\mathit{c}}$), and the temperature corresponding to the insulator-metal-like transition (${\mathit{T}}_{\mathit{p}}$). The two important parameters which control the electronic transport and the magnetic behavior are the ${\mathrm{Mn}}^{4+}$ content and the effective radius of the A-site cation. The substitution by Y, which changes the A-site radius, is compared to pressure effect. For certain Y concentrations we observe substantial enhancement of the negative magnetoresistance at temperatures T${\mathit{T}}_{\mathit{c}}$ and a large positive magnetoresistance at T\ensuremath{\gg}${\mathit{T}}_{\mathit{c}}$. \textcopyright{} 1996 The American Physical Society.