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

Abstract

Manganates of the compositions Nd0.5-xLaxCa0.5MnO3 and Pr0.5-xLaxCa0.5 MnO3, with the average A-site cationradius, ⟨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 aferromagnetic state on cooling (TC<TCO). Theferromagnetic Curie temperature, TC, of the re-entrant transition increases markedly with x or ⟨rA⟩in the two series of compounds. The plots of TC andTCO against ⟨rA⟩ show that the twocurves intersect around a ⟨rA⟩ value of1.195 Å, below which TCO>TC. Site disorder due tothe size mismatch of the A-site cations, σ2, has amarked 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 withincrease in site disorder, suggesting that the re-entranttransition can be entirely suppressed at a sufficiently highvalue of σ2. Between TCO and TC, the CO andFM states are likely to coexist, the coexistence temperatureregime decreasing with increasing ⟨rA⟩, andincreasing with σ2 at a fixed ⟨rA⟩.