An investigation of the re-entrant ferromagnetic transition in rare earth manganates in the regime of competing charge-ordering and ferromagnetic interactions

Abstract

Manganates of the compositions Nd_0.5−xLa_xCa_0.5MnO₃ and Pr_0.5−xLa_xCa_0.5MnO₃, with the average A-site cation radius, <r_A>, in the range 1.17-1.20 Å, are charge ordered at ordinary temperatures (T_CO~240 K), and undergo a re-entrant transition to a ferromagnetic state on cooling (T_C<T_CO). The ferromagnetic Curie temperature, T_C, of the re-entrant transition increases markedly with x or <r_A> in the two series of compounds. The plots of T_C and T_CO against <r_A> show that the two curves intersect around a <r_A> value of 1.195 Å, below which T_CO>T_C. Site disorder due to the size mismatch of the A-site cations, σ², has a marked effect on the re-entrant transition temperature, T_C. Thus, in a series of manganates of the type Ln_0.5A_0.5MnO₃ with a fixed <r_A> value of 1.185 Å, the T_C decreases markedly with increase in site disorder, suggesting that the re-entrant transition can be entirely suppressed at a sufficiently high value of σ². Between T_CO and T_C, the CO and FM states are likely to coexist, the coexistence temperature regime decreasing with increasing <r_A>, and increasing with σ² at a fixed <r_A>.