Interatomic versus intraatomic Ru interactions in perovskites

The competition between interatomic Ru-O-Ru interactions responsible for itinerant-electron ferromagnetism in SrRuO₃ and intraatomic spin-orbit coupling that suppresses some Ru-O-Ru magnetic interactions in CaRuO₃ so as to prevent long-range magnetic order has been further tested by investigating the double perovskites La₂RuZnO₆, La₂RuCoO₆, La₂TiCoO₆, and the solid solution Ca_2−2xLa_2xRu_2−xCo_xO₆ (0.0?x?1.0). The paramagnetic state of the low-spin, localized Ru⁴⁺:t⁴e⁰ configuration of LaRuZnO₆ undergoes a progressive transition on lowering the temperature to a non-magnetic state (J=0) as a result of spin-orbit coupling, λL·S. Comparison of the susceptibilities of La₂RuCoO₆ and La₂TiCoO₆ shows a similar behavior with long-range antiferromagnetic cobalt ordering via non-magnetic RuO_6/2 and TiO_6/2 bridges. Comparison is also made of the effect of substituting Ga for Ru in CaRuO₃ with the effect of other dopants M in CaRu_1−xM_xO₃. The evolution of transport properties in Ca_2−2xLa_2xRu_2−xCo_xO₆ is also presented.