Influence of alkaline earth metal atoms on magnetic properties of Bi2MNb2-2xFe2xO9-δ (M - Ba, Sr, Ca)
Complex bismuth-containing oxides with a layered perovskite-like structure, called the Aurivillius phases, attracted the interest of researchers for their ferroelectric and oxygen-conducting properties. Bismuth niobates, described by the general formula Bi2MNb2O9 (M - Ca, Sr, Ba), are among the most studied ferroelectrics in the numerous family of Aurivillius phases, where the number of perovskite layers in the [MNb2O7]2- blocks between the bismuth-oxygen layers [Bi2O2]2+ equals two. Earlier studies of the magnetic dilution of iron-doped solid solutions of bismuth niobates revealed a number of features of the magnetic behavior of iron atoms that are atypical for other paramagnetic atoms. In order to continue the study of factors affecting the magnetic behavior of paramagnet atoms, this work is devoted to the influence of diamagnetic substituents on the character of interatomic interactions of iron atoms in Bi2MNb2-2xFe2xO9-δ solid solutions (M - Ba, Sr, Ca). The study of the magnetic susceptibility of iron-doped solid solutions with a layered perovskite-like structure has established the influence of atoms of the second coordination sphere on the degree of aggregation and the nature of exchange interactions in the exchange-bound clusters containing Fe (III) atoms. We have shown that an increase in the radius and a weakening of the polarization properties of atoms of alkaline earth metals lead to an increase in the degree of aggregation of paramagnetic iron atoms and the intensification of the antiferromagnetic type of exchange in iron-doped clusters. In the Bi2CaNb2-2xFe2xO9-δ solid solutions, the exchange parameters and the distribution of clusters have been calculated depending on the iron content. It has been established that the values of the magnetic moment of iron atoms in solid solutions exceed of the pure spin values of Fe (III) due to the presence of exchange-bound aggregates of Fe (III) atoms with antiferro- and ferromagnetic exchange types.