BANDWIDTH-CONTROLLED MAGNETIC AND ELECTRONIC-TRANSITIONS IN LA0.5CA0.5-XSRXMNO3 LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.5) DISTORTED PEROVSKITE

We have investigated the effect of the increase of the one-electron e( g) bandwidth (W) on the ferromagnetic and antiferromagnetic transition s in charge ordered La0.5Ca0.5MnO3 by substituting bigger Sr2+ ions fo r Ca2+ ions through electrical resistivity and low-field ac-susceptibi lity measurements as a function of temperature. The system La0.5Ca0.5- xSrxMnO3 is shown to undergo structural transitions with increasing x. For x=0.0 and 0.1, the structure is orthorhombic with space group Pnm a. For x=0.2, 0.3, and 0.4, the structure is consistent with a monocli nic space group I2/a. The end member (x=0.5) has a tetragonal structur e (I4/mcm). T-C increases with x monotonically, consistent with the fa ct that the increase of W enhances double-exchange ferromagnetic inter action. On the other hand, in contrast to the expected monotonic suppr ession, T-N increases initially with Sr concentration, followed by a d ecrease, finally disappearing for x=0.4. The nonmonotonous variation o f T-N may indicate a possible change in the type of magnetic structure in the antiferromagnetic state.