The role of impurities and oxygen vacancies in the magnetic response of FexCoySn1-x-yO2. Experimental and ab initio study

In this work we present an experimental and theoretical study of the magnetic and hyperfine properties of (Fe, Co) co-doped rutile SnO2 (FexCoySn1-x-yO2). Ab initio calculations were performed in the framework of the Density Functional Theory (DFT) using the full-potential linearized augmented plane wave (FP-LAPW) method. The effect of the oxygen vacancies on the magnetic and hyperfine properties and on the magnetic alignment of Fe and Co impurities was studied considering different vacancy concentrations and distributions in the host. Our calculations predicted that the Fe and Co impurities tend to be located as close as possible and favors the generation of oxygen vacancies, forming a pair of magnetic impurities sharing oxygen vacancies with an antiparallel spin alignment, giving rise to a ferrimagnetic entity. Ab initio predictions were compared with experimental results: magnetization curves obtained by vibrating sample magnetometry (VSM) at room temperature and Mössbauer spectroscopy (MS) studies obtained for SnO2 samples doped with 1.0% of Fe and co-doped with Co concentrations ranging from 0.0 to 0.5%, grown by sol-gel and thermal decomposition method. The comparison enabled us to identify the observed hyperfine interactions in MS experiments and characterize the local structure around the Fe atoms unambiguously. Finally, based on our theoretical results for the lowest energy Fe–Fe, Fe–Co and Co–Co magnetic configurations we can understand and reproduce the experimental behavior obtained by VSM measurements of the saturation magnetization (MS) (similar to the magnetic moment per magnetic atom) as a function of the Co concentration.