Bandwidth-controlled magnetic and electronic transitions in La_0.5Ca_0.5-xSr_xMnO₃ (0<~x<~0.5) distorted perovskite

Abstract

We have investigated the effect of the increase of the one-electron e_g bandwidth (W) on the ferromagnetic and antiferromagnetic transitions in charge ordered La_0.5Ca_0.5MnO₃ by substituting bigger Sr²⁺ ions for Ca²⁺ ions through electrical resistivity and low-field ac-susceptibility measurements as a function of temperature. The system La_0.5Ca_0.5-xSr_xMnO₃ is shown to undergo structural transitions with increasing x. For x=0.0 and 0.1, the structure is orthorhombic with space group Pnma. For x=0.2, 0.3, and 0.4, the structure is consistent with a monoclinic space group I2/a. The end member (x=0.5) has a tetragonal structure (I4/mcm). 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. The nonmonotonous variation of T_N may indicate a possible change in the type of magnetic structure in the antiferromagnetic state.