Influence of the B-site ordering on the magnetic properties of the new La3Co2MO9 double perovskites with M = Nb or Ta

V.C. FUERTES; M.C. BLANCO; D.G. FRANCO; J.M. DE PAOLI; R.D. SÁNCHEZ ; R.E. CARBONIO

MATERIALS RESEARCH BULLETIN

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2011 vol. 46 p. 62 - 69

0025-5408

Double perovskites La3Co2NbO9 and La3Co2TaO9 have been prepared by both solid state and sol?gelsynthesis. The crystal structures have been studied from X-ray and neutron powder diffraction data.Rietveld refinements show that the crystal structure is monoclinic (P21/n), with different degrees ofordering of B0 and B'0 cations, with octahedra tilted according to the Glazer notation a-b-c+. Occupancyrefinements show that the solid state materials are more B-site ordered than the sol?gel ones.Magnetization measurements show that these perovskites show two magnetic contributions, one withspontaneous magnetization and other with linear behaviour with the magnetic field associated toantiferromagnetic correlations. In the samples synthesized by solid state the spontaneousmagnetizationis more important than those synthesized by the sol?gel and present TC of 62 K for Nb and 72 K for Ta. Onthe other hand,materials prepared by sol?gel have TC 20 K for Nb and 40 K for Ta, respectively and majorpresence of the antiferromagnetic contribution. The competition between these magnetic behaviours isinterpreted, by a microscopic point of view, as to be due to the different degrees of Co2+ ions disorder onthe B site of the double perovskite structure. This disorder affects the ratio between theantiferromagnetic Co2+?O?Co2+ and the ferromagnetic Co2+?O?M5+?O?Co2+ couplings proposed forthe system.