Abstract:
Magnetic and electron transport properties of four series of manganates of the composition La0.5−xLnxSr0.5MnO3 (Ln=Pr, Nd, Gd and Y) have been investigated to examine how the ferromagnetic metallic nature of the parent La compound changes over to antiferromagnetic insulating behavior, with change in Ln and x due to the associated changes in the A-site cation radius as well as the size disorder. When Ln=Pr and Nd, there is a transition from the tetragonal I4/mcm structure to the orthorhombic Immm and Imma structures at x=0.2 and 0.35, respectively. There is a gradual evolution of the properties from those of La0.5Sr0.5MnO3 to those of Pr0.5Sr0.5MnO3 or Nd0.5Sr0.5MnO3 with increase in x. Thus, when x>0.2 and >0.35, respectively, the Pr- and Nd-substituted manganates show ferromagnetic transitions followed by antiferromagnetic transitions at low temperatures, with the ferromagnetic TC decreasing with increasing x. The Gd and Y series of compounds are all orthorhombic and show a decrease in TC with the increase in x, the ferromagnetism disappearing at high x. At a value of x corresponding to the A-site cation radius of Pr0.5Sr0.5MnO3, the Gd and Y series of compounds exhibit ferromagnetism in the 250–300 K region and undergo an antiferromagnetic transition on cooling. The TC−TN gap is sensitive to the disorder arising from the size mismatch.
