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We report on an a $\mu$SR and $^{55}$Mn NMR investigation of the magneticorder parameter as a function of temperature in the optimally dopedLa$_{5/8}$(Ca$_y$Sr$_{1-y}$)$_{3/8}$MnO$_3$ and in the underdopedLa$_{1-x}$Sr$_{x}$MnO$_3$ and La$_{1-x}$Ca$_{x}$MnO$_3$ metallic manganitefamilies. The study is aimed at unraveling the effect of lattice distortions,implicitly controlled by the Ca-Sr isoelectronic substitution, from that ofhole doping $x$ on the Curie temperature $T_c$ and the order of the magnetictransition. At optimal doping, the transitions are second order at all $y$values, including the $y=1$ (La$_{5/8}$Ca$_{3/8}$MnO$_3$) end member. Incontrast, they are first order in the underdoped samples, which show a finite(truncated) order parameter at the Curie point, includingLa$_{0.75}$Sr$_{0.25}$MnO$_3$ whose $T_c$ is much higher than that ofLa$_{5/8}$Ca$_{3/8}$MnO$_3$. The order parameter curves, on the other hand,exhibit a very minor dependence on $x$, if truncation is excepted. Thissuggests that the effective exchange interaction between Mn ions is essentiallygoverned by local distortions, in agreement with the original double-exchangemodel, while truncation is primarily, if not entirely, an effect of under- oroverdoping. A phase diagram, separating in the $x-y$ plane polaron-driven firstorder transitions from regular second order transitions governed by criticalfluctuations, is proposed for the La$_{1-x}($Ca$_y$Sr$_{1-y}$)$_{x}$MnO$_3$system.

Band filling effect on polaron localization in La$_{1-x}($Ca$_y$Sr$_{1-y}$)$_{x}$MnO$_3$ manganites