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

Manganates of the compositions Nd0.5−xLaxCa0.5MnO3 and Pr0.5−xLaxCa0.5MnO3, with the average A-site cation radius, <rA>, in the range 1.17-1.20 Å, are charge ordered at ordinary temperatures (TCO~240 K), and undergo a re-entrant transition to a ferromagnetic state on cooling (TC<TCO). The ferromagnetic Curie temperature, TC, of the re-entrant transition increases markedly with x or <rA> in the two series of compounds. The plots of TC and TCO against <rA> show that the two curves intersect around a <rA> value of 1.195 Å, below which TCO>TC. Site disorder due to the size mismatch of the A-site cations, σ2, has a marked effect on the re-entrant transition temperature, TC. Thus, in a series of manganates of the type Ln0.5A0.5MnO3 with a fixed <rA> value of 1.185 Å, the TC 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 TCO and TC, the CO and FM states are likely to coexist, the coexistence temperature regime decreasing with increasing <rA>, and increasing with σ2 at a fixed <rA>