On the La2-xSrxCoRuO6 double perovskites:: Crystal structure, magnetic properties and transport

As double perovskites have attracted attention due to the unusual magnetic properties and half-metallic nature of the charge carrier transport, the study of the La(2-x)Sr(x)CoRuO(6) perovskite offers a unique opportunity to examine the interplay of the cationic order, charge balance, transport and magnetic interactions between Co and Ru species. We confirm the existence of the B-site ordered double perovskite phase for 0.0 < x < 1.6, whereas for higher x a single-phase perovskites with a statistical distribution of all cations is formed. Consistent picture of transport properties of B-site ordered perovskites with x similar to 1 is based on a supposition of the ideal valency of Co and Ru close to 2+ and 5+, respectively, when the real chemical composition leads to excess electrons (x < 1) or holes (x > 1). The magnetic data reveal that effective magnetic moment amounts similar to 5.5 mu(B) and does not significantly vary with x. Weiss constant Theta(c), however, shows a parabolic-like dependence on x: from the antiferromagnetic Theta(C) similar to -70 K for x = 0, via a broad maximum for x similar to 1.0 with Theta(c) similar to 15 K and a decrease to Theta(c) similar to -220 K for x = 2. To explain this evolution we suggest that antiferromagnetic superexchange between Co and Ru ions is for x similar to 1 compensated by the reinforcement of ferromagnetic interactions, most likely associated with the formation of quasi-itinerant pi* bands (Ru-O). (C) 2007 Elsevier Masson SAS. All rights reserved.