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-xLaxCa0.5 MnO3, 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⟩.