Improved CMR properties of RE-doped (La,Sr)MnO$_{3}$ single crystals

The relationships among crystal structure, $T_{C}$ and CMR effect have been eagerly studied for (La,Sr)MnO$_{3}$ system mainly as a functions of the Sr composition, $x$ thus far. In the present study, we have attempted to improve the CMR properties near room temperature of the present system by optimizations of $T_{C}$ and phase transition temperature between orthorhombic and rhombohedral through RE mixing and elimination of excess oxygen, $i.e.$ cation vacancies, for (La$_{1-x}$Sr$_{x})$MnO$_{3}$ single crystals with $x$ = 0.2 and 0.25, which have higher$ T_{C}$ than room temperature and essentially high electronic conductivity. Crystal boules with nominal compositions of La$_{0.8-z}$RE$_{z}$Sr$_{0.2}$MnO$_{y}$ and La$_{0.75-z}$RE$_{z}$Sr$_{0.25}$MnO$_{y}$ (RE = Pr, Nd, Sm : $z$ = 0 $\sim $ 0.3) were grown by the floating zone method. Crystal structure of La$_{0.75-z}$Pr$_{z}$Sr$_{0.25}$MnO$_{3}$ at $\sim $300 K changed from rhombohedral ($z$ = 0, 0.15, 0.25) to orthorhombic ($z$ = 0.3) due to a decrease in mean ionic radius of A site. In addition, Pr-doping systematically decreased $T_{C}$. Similar tendencies were confirmed for Nd- or Sm-doped samples. The RE-doped samples exhibited large CMR ratio at $\sim $300 K comparable to that of La$_{0.825}$Sr$_{0.175}$MnO$_{3}$ and much higher conductivity reflecting high Sr concentration when phase transition temperature and $T_{C}$ were optimized.