Magnetic behavior of Ca2BWO6 (B=Co,Ni) double perovskites

C.A. LOPEZ; M. DEL C. VIOLA; J.C. PEDREGOSA; R.D. SÁNCHEZ; J. CURIALE

Buenos Aires, Argentina.

Congreso; At the Frontiers of Condensed Matter III; 2006

Centro Atómico Constituyentes, Comisión Nacional de Energía Atómica.

The discovery of colossal magnetoresistance (CMR) properties in hole-doped manganese perovskites has stimulated the enthusiasm of solid-state chemists and physicists. As basic features, a CMR material must exhibit a semi-metallic, spin-polarized conductivity and experience a ferromagnetic (FM) ordering at Curie temperatures, TC, as high as possible, ideally well above room temperature (RT). It is now well established that Curie temperatures of mixed-valence manganites (based upon LaMnO3 perovskite) cannot be increased above 400 K by any chemical substitution. However this temperature is overcome in the some members of the family of double perovskites of composition A2B?B??O6 (A = alkali earths, B?, B?? = transition metals) proposed as half-ferromagnets (best described as ferrimagnets). These are an alternative to manganites. The revival of interest in this family was triggered by a report on Sr2FeMoO6, demonstrating that in the electronic structure only minority spins are present at the Fermi level; this material was shown to exhibit intrinsic tunneling-type magnetoresistance (MR) at RT. The ferrimagnetic structure can be described as an ordered arrangement of parallel Fe3+ (S = 5/2) magnetic moments antiferromagnetically coupled with Mo5+ (S = 1/2) spins. The occurrence of MR properties is a common feature in other members of the A2B?B??O6 family. The A = Ca and Ba analogues of the Sr2FeMoO6 were also found to exhibit semimetallic and ferrimagnetic properties. The CMR has also been induced in phases with B? = Co, as the recently reported in Sr2CoMoO6 upon chemical reduction, via topotactical removal of oxygen atoms. Because satisfactory results have been obtained with A2B?B??O6 double perovskites, have given our attention to phases Ca2CoWO6 and Ca2NiWO6. They were studied in the sixties and forgotten for more than 30 years. Recently these phases have been studied by Martínez-Lope and col. with data of neutron powder diffraction. These crystalline structures were reported as monoclinic perovskite and antiferromagnetic TN = 36 K and TN = 56 K for the Co and Ni, respectively. We have chosen to study the magnetic properties of Ca2CoWO6 and Ca2NiWO6 for several reasons. In this work these compounds are characterized by XRD on a well-crystallized samples and their monoclinic distorted structure is confirmed by Rietveld methods. A new study is presented starting from macroscopic magnetic in order to corroborating the magnetic structure obtained previously by ND data and to confront with previous magnetic data.