Occurrence of re-entrant ferromagnetic transitions in rare-earth manganates on cooling the charge-ordered states

Some of the compositions of the half-doped rare-earth manganates, La0.5−x Ln x Ca0.5MnO3 (Ln=Nd, Pr) and Nd0.5Ca0.5−x Sr x MnO3 with relatively small A-cation radii, 〈r A〉, show an unusual behavior wherein they become ferromagnetic (FM) on cooling the charge ordered (CO) state (T CO>T C). With increase in 〈r A〉, however, the T C becomes greater than T CO. Thus, plots of T C and T CO against 〈r A〉 for La0.5−x Ln x Ca0.5MnO3 (Ln=Nd, Pr) and Nd0.5Ca0.5−x Sr x MnO3 show cross-over from the T CO>T C regime to the T C>T CO regime around 〈r A〉 values of 1.195±0.003 and 1.200±0.005Å, respectively. Between T C and T CO, the CO and FM phases are likely to coexist. In Nd0.5Ca0.5Mn1−x M x O3 (M=Cr, Ru), T CO>T C when x≤0.10, suggesting the re-entrant nature of the FM transition.