Strong enhancement of the ferromagnetic phase and local order in Ru substituted manganites studied by EXAFS and dc magnetization measurements

We report extended X-ray absorption fine structure (EXAFS) and dc magnetization measurements performed on (La1-xCax) (Mn1-yRuy)O3 samples (x = 0.37 and 0.75 and y = 0.03 or 0.08). In the nanoscale phase inhomogeneity scenario characteristic of these complex oxides, we compare the effects induced by the partial substitution of Mn with mixed valence non-Jahn-Teller Ru5+ and at most very weak Jahn-Teller Ru4+ ions with those induced by the already studied analogous substitutions of Mn with non-Jahn-Teller Cr3+ or Ni2+ or Jahn-Teller Cu2+. We find evidence both from dc magnetization and EXAFS that the ruthenium doping induces a huge effect, clearly enhancing (more efficiently than is the case for the other dopants) the double exchange effect and consequently the metallic/ferromagnetic phase and the local structural order at the expenses of the charge-ordered phase component. In the x = 0.75 long-range charge-ordered system these effects are much more evident, as expected, while for x = 0.37 the magnetic ion partial substitution does not significantly modify the metal-insulator and ferromagnetic transitions, already present in the parent compound.