Oxygen content variation and cation doping dependence of (La)1.4(Sr1-yCay)1.6Mn2O7±δ (y=0,0.25,0.5) bilayered manganites

The results of the synthesis and characterization of the optimally doped (La)1.4(Sr1-yCay)1.6Mn2O7±δ solid solution with y=0, 0.25, and 0.5 are reported. By progressively replacing the Sr with the smaller Ca, while keeping fixed the hole concentration due to the divalent dopant, the 'size effect' of the cation itself on the structural, transport, and magnetic properties of the bilayered manganite has been analyzed. Two different annealing treatments of the solid solution, in pure oxygen and in pure argon, allowed us to also study the effect of the oxygen content variation. Structure and electronic properties of the samples have been investigated by means of x-ray powder diffraction and x-ray absorption spectroscopy measurements. Magnetoresistivity and static magnetization measurements have been carried out to complete the samples characterization. Oxygen annealing of the solid solution, which showed a limit for y∼0.5, induces an increase of the Mn average valence state and a transition of the crystal structure from tetragonal to orthorhombic while the argon annealing induces an oxygen understoichiometry and, in turn, a reduction of the Mn average valence state. Along with the Ca substitution, the Jahn-Teller distortion of the MnO6 octahedra is reduced. This has been directly connected to a general enhancement of the transport properties induced by the Ca doping. For the same cation composition, oxygen overstoichiometry leads to higher metal-insulator transition temperatures and lower resistivity values. Curie temperatures (TC) are reduced by increasing the Ca doping. The lower TC for all the annealed samples with respect to the as-prepared ones are connected to the strong influence on the magnetic interactions of the point defects due to the δ-variation