Bandwidth-controlled magnetic and electronic transitions in ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ $(0\mathrm{}<~x<~0.5)$ distorted perovskite

We have investigated the effect of the increase of the one-electron $e_g$ bandwidth $\mathrm{}\left(W\right)\mathrm{}$ on the ferromagnetic and antiferromagnetic transitions in charge ordered ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ by substituting bigger ${\mathrm{Sr}}^{2+}$ ions for ${\mathrm{Ca}}^{2+}$ ions through electrical resistivity and low-field ac-susceptibility measurements as a function of temperature. The system ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ is shown to undergo structural transitions with increasing $x$. For $x=0.0\mathrm{}$ and 0.1, the structure is orthorhombic with space group $\mathrm{Pnma}$. For $x=0.2\mathrm{}$, 0.3, and 0.4, the structure is consistent with a monoclinic space group $I2/a$. The end member $\mathrm{}(x=0.5\mathrm{})\mathrm{}$ has a tetragonal structure $\mathrm{}(I4/mcm)\mathrm{}$. $T_C$ increases with $x$ monotonically, consistent with the fact that the increase of $W$ enhances double-exchange ferromagnetic interaction. On the other hand, in contrast to the expected monotonic suppression, $T_N$ increases initially with Sr concentration, followed by a decrease, finally disappearing for $x=0.4\mathrm{}$. The nonmonotonous variation of $T_N$ may indicate a possible change in the type of magnetic structure in the antiferromagnetic state.