Ru doping of perovskite manganites: An effective route to ferromagnetism, metallicity, CMR

The doping of Mn sites by Ru in manganites has been studied. It is shown th at. whatever the nature of the antiferromagnetic data of the undoped perovs kite, CE or pseudo-CE type, like for Ln(0.5)Ca(0.5)MnO(3) and Ln(0.4)Ca(0.6 )MnO(3), or A type as for Pr0.5Sr0.5MnO3, or C type as for Sm0.2Ca0.8MnO3, or G type as for CaMnO3, ferromagnetism and metallicity can be induced by R u doping in a very effective manner, leading to colossal magnetoresistance (CMR) properties. Thus a dramatic modification of the magnetic phase diagra ms can be obtained by Ru doping, as shown for the Sm1-yCayMn0.88Ru0.12O3 sy stem where the T-C of the ferromagnetic insulating (FMI) region (0.12 less than or equal to y less than or equal to 0.40) has been increased significa ntly by Ru doping. But more important, the charge-ordered state of the elec tron rich region (0.40 less than or equal to y less than or equal to 0.80) has been spectacularly replaced by a ferromagnetic metallic (FMM) state wit h a very high T-C, up to 235 K by Ru substitution. The peculiar nature of t he so obtained FMM state is shown to be related to the structural phase sep aration. This particular efficiency of ruthenium, compared to chromium, is discussed in terms of not only valency effect due to the presence of RU5+ t hat increases the Mn3+ content, but also valency fluctuation between RU5+ a nd RU4+, and in terms of strong ferromagnetic superexchange interactions be tween ruthenium and manganese, due to orbital hybridization, so that Ru4+ a nd Ru5+ participate in the band formation.