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 Nd 0.5- x La x Ca 0.5 MnO 3 and Pr 0.5- x La x Ca 0.5 MnO 3 , 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, σ 2 , has a marked effect on the re-entrant transition temperature, T C . Thus, in a series of manganates of the type Ln 0.5 A 0.5 MnO 3 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 σ 2 . 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 σ 2 at a fixed ⟨ r A ⟩.