The effect of Mn-site doping on the magnetotransport properties of CMR manganites

Doping of the Mn-site in Ln(1-y)A(y)Mn(1-x)M(x)O(3) manganites has been extensively investigated. For the manganites exhibiting a Curie temperature (T-C) these substitutions depress T-C. However, the effect on T-C is doping-element dependent. It decreases from dT(C)/dx = -25 K %(-1) for M = Fe down to dT(C)/dx similar to 0 K %(-1) for M = Cr in the series Sm0.56Sr0.44Mn1-xMxO3. The peculiar role of chromium doping is also shown in the case of the charge-ordered Ln(0.5)Ca(0.5)MnO(3)-substituted manganites that are characterized by low average cationic size on the A-site and cationic size mismatch. For the latter, metal-insulator transitions together with CMR properties are induced by Cr and to a smaller extent by Co and Ni. The hindering of the charge-ordering establishment by these 'impurity effects' is also observed in the case of the two-dimensional manganites La0.5Sr1.5MnO4: and LaSr2Mn2O7. The highest efficiency of Cr among all doping elements may be ascribed to the similar t(2g)(3) tronic configurations of Cr3+ and Mn4+ which allow a double-exchange between the Cr3+ and Mn3+ species.