Abstract :
Some of the compositions of the half-doped rare-earth manganates, La0.5−xLnxCa0.5MnO3 (Ln=Nd, Pr) and Nd0.5Ca0.5−xSrxMnO3 with relatively small A-cation radii, 〈rA〉, show an unusual behavior wherein they become ferromagnetic (FM) on cooling the charge ordered (CO) state (TCO>TC). With increase in 〈rA〉, however, the TC becomes greater than TCO. Thus, plots of TC and TCO against 〈rA〉 for La0.5−xLnxCa0.5MnO3 (Ln=Nd, Pr) and Nd0.5Ca0.5−xSrxMnO3 show cross-over from the TCO>TC regime to the TC>TCO regime around 〈rA〉 values of 1.195±0.003 and 1.200±0.005 Å, respectively. Between TC and TCO, the CO and FM phases are likely to coexist. In Nd0.5Ca0.5Mn1−xMxO3 (M=Cr, Ru), TCO>TC when x≤0.10, suggesting the re-entrant nature of the FM transition.
