Inuence of the B site ordering on the magnetic properties in the new La3Co2MO9 double perovskites with M=Nb or Ta.

V.C. FUERTES; M.C. BLANCO; D.G. FRANCO; J. M. DE PAOLI; E. V. PANNUNZIO-MINER; R. D. SANCHEZ; M.T.FERNÁNDEZ-DÍAZ; R. E. CARBONIO

Buenos Aires

Conferencia; At the Frontiers of Condensed Matter IV,; 2008

Tandar

Perovskite compounds are very interesting materials because of their potential technological applications. The ideal cubic perovskite structure can be seen like a three-dimensional corner-sharing network of BO6 octahedra, with a large A-cation occupying the cuboctahedral cavities. Double perovskites are formed with the general formula A2B'B"O6 were ions B' and B", in the ideal case, occupy dierent crystallographic sites. Usually, however, there is a certaindegree of "anti-site" disorder, where a fraction of B'occupies the B" sites and vice versa. Usually, A is an alkali-earth ion, like Ca2+, Sr2+, Ba2+. However, there are few reports of double perovskite materials in which the A site ion has been totally replaced by a small lanthanide Ln3+. Magnetism in these compounds can be achieved when de B site ion is a transition metal cation with unmatched electrons. Magnetic properties, like ferromagnetism, orferrimagnetism, in double perovskites are strongly dependent on their B cations order-disorder relationship. In the present work we describe the synthesis, structures and magnetic characterization of a new perovskite type family: La3Co2MO9 with M = Nb, Ta. These phases were synthesized as polycrystalline powders by both standard ceramic and sol gel method, this last one by a tartrate-precursor decomposition. Their crystallographic formula can be writtenas La3(Co)2d(CoM)2cO9 for samples synthesized by solid state and as La3(Co2 for those synthesized by the sol-gel method. Structural analysis was made through Rietveld renementfrom XRPD and NPD data. All are insulator monoclinic double perovskites and can be well characterized in P 21/n SG. Anti-site disorder is noticeable larger in the sol-gel compounds case. Magnetic measurements were made in a commercial SQUID in the 10-300 Ktemperature range, with the objective to understand the effect that causes this disorder on the magnetic properties of these materials. M vs T measurements indicated that samples synthesized by solid state are ferromagnetic with TC c.a. 60 K, while those synthesized by the sol-gel method are antiferromagnetic materials with TN c.a. 20 K. The solid state methodsynthesized compounds have shown to be more B site ordered than sol-gel synthesized compounds. We assign dfferent magnetic behaviours displayed by both compounds to these differences in antisite disorder.The refined magnetic cells are in agreement with magnetic  measurements in both, ferromagnetic and antiferromagnetic cases.