Structural defects in Sr2FeMoO6 double perovskite:: Experimental versus theoretical approach -: art. no. 033905

The lower than expected magnetization of imperfect Sr2FeMoO6 (SFMO) double perovskites is usually attributed to the presence of Fe at antisite positions that would be antiferromagnetically coupled to their regular neighbors. However, ab initio calculations suggest strongly that such defective Fe sites would be ferromagnetically coupled and, consequently, the magnetization reduction would originate from other kinds of defects. The magnetic, hyperfine, and structural properties of SFMO perovskites prepared by solid-state reaction under a variety of conditions are reported and correlated with ab initio calculations of the magnetic moments and hyperfine fields of Mo and Fe ions in different local environments (antisites, antisite neighbors, and neighbors of an oxygen vacancy). When plotted against the order parameter the experimental magnetization is found to decrease at a rate of about -7.6 mu(B) per Mo-Fe antisite pair as in other previous experiments, where the theoretical calculation predicts -6.56 mu(B) per antisite pair if the moments of Fe antisites are antiparallel to the regular Fe moments. Unfortunately, the energy of this configuration is found to be 0.7 eV higher than that of the parallel configuration for which the magnetization reduction is only -0.19 mu(B) per antisite pair. Sources for the supplementary reduction of magnetization have then been considered. The presence of spurious phases cannot account for the observation. Oxygen vacancies do reduce significantly the magnetization (-2.00 mu(B)/vacancy), but no significant sign of their effect is found in the Fe Mossbauer and Mo nuclear magnetic resonance spectra. Moreover, the position of the spectral lines of defects are compatible with the theoretical findings for Fe antisites in the antiparallel spin configuration. (c) 2005 American Institute of Physics.