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, La_0.5−xLn_xCa_0.5MnO₃ (Ln=Nd, Pr) and Nd_0.5Ca_0.5−xSr_xMnO₃ 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 La_0.5−xLn_xCa_0.5MnO₃ (Ln=Nd, Pr) and Nd_0.5Ca_0.5−xSr_xMnO₃ 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±0.005 Å, respectively. Between T_C and T_CO, the CO and FM phases are likely to coexist. In Nd_0.5Ca_0.5Mn_1−xM_xO₃ (M=Cr, Ru), T_CO>T_C when x≤0.10, suggesting the re-entrant nature of the FM transition.