Preparation, crystal structure and magnetic properties of Ruddlesden-Popper phases La1.2Sr1.8Mn2−xRuxO7 (x=0, 0.05, 0.1, 0.2, 0.5)

Single phase products of La1.2Sr1.8Mn2−xRuxO7 (0⩽x⩽0.5) were prepared by solid state reaction of La2O3, SrCO3, Mn2O3 and RuO2 in several heating cycles. The samples have been studied by X-ray powder diffraction techniques and the crystal structures were refined using the Rietveld method. The compounds La1.2Sr1.8Mn2−xRuxO7 belong to the Ruddlesden-Popper phases An+1BnO3n+1 with n=2, which crystallize in the tetragonal space group I4/mmm (No. 139). In the crystal structure two crystallographic sites are occupied by La and Sr: the perovskite site shows a higher occupation with La, while the rocksalt site refines to a higher occupation with Sr. The lattice parameters a and c as well as the unit cell volume V increase monotonically with increasing ruthenium content. Probably because of the larger cation radius of ruthenium ions in comparison to manganese ions the average distance between transition metal and oxygen atoms was found to increase. Magnetization and AC susceptibility measurements show an enhancement of the ferromagnetic Curie temperature with increasing Ru substitution from 121 to 160 K, which gives evidence for the unique role of Ru when substituted in manganates. A sequence of magnetic transitions observed above TC is probably related to effects of quasi-two-dimensional magnetic ordering (in stacking faults or of intrinsic nature). Metallic conduction is maintained up to x=0.1 in La1.2Sr1.8Mn2−xRuxO7. Possible magnetic exchange interactions of Ru and Mn in the compounds are discussed.