Comparative Mössbauer and magnetization study of 1% Sn 119 -doped La 0.67 Ca 0.33 Mn O 3 and La 0.67 Sr 0.33 Mn O 3

A detailed comparative M\"ossbauer study of ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ and ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ samples doped with 1% Sn is presented. The spectra at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ can be analyzed by a single ferromagnetic component. However, at higher temperatures two separate ferromagnetic components and a paramagnetic one are required to fit the spectra. The ferromagnetic component with the lower hyperfine field is attributed to clusters of weakened double-exchange interactions due to competition by localization mechanisms. Clear differences between the two samples in regard to the critical behavior arise: (i) The hyperfine field of ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{Mn}}_{0.99}{\mathrm{Sn}}_{0.01}{\mathrm{O}}_{3}$ disappears abruptly at ${T}_{c}$ in contrast to that of ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}}_{0.99}{\mathrm{Sn}}_{0.01}{\mathrm{O}}_{3}$ which varies continuously and goes to zero smoothly at the critical temperature. (ii) The second ferromagnetic phase appears above $0.35{T}_{c}$ in the Ca sample but only above $0.83{T}_{c}$ in the Sr sample. The paramagnetic component appears for $Tg0.67{T}_{c}$ in the former compared to $Tg0.9{T}_{c}$ in the latter. The results support recent studies that show differences in the polaronic character, and critical behavior thereof, between the two materials.