Enhancement of ferromagnetism and ferroelectricity by oxygen vacancies in mullite Bi2Fe4O9 in the Bi2(Sn0.7Fe0.3)2O7-x matrix

•A new composite Bi2(Sn0.7Fe0.3)2O7-x/Bi2Fe4O9 was synthesized.•The nonlinear M(H) is related to the spin polaron mobility.•A mechanism for the formation of ferrons was proposed.•The impedance jump at the Néel temperature in mullite was found.•The magnetic hysteresis and remanent magnetization was found.•The remanent polarization hysteresis in the composite was established.•The dipole-type electric polarization below the Néel temperature was found.•Migration polarization is found at higher temperatures than the Néel temperature. A new bismuth pyrostannate-based composite Bi2(Sn0.7Fe0.3)2O7-x/Bi2Fe4O9 (BSFO/BFO) has been obtained by the solid-state synthesis. Temperature dependences of the magnetic hysteresis and remanent magnetization and the nonlinear field dependence of the magnetization for the Bi2Fe4O9 antiferromagnet have been established. A temperature of the formation of canted sublattices in the antiferromagnet with the occurrence of a spontaneous moment in mullite has been determined. The mechanism of the electric polarization hysteresis and the temperature dependence of the remanent polarization have been established. It has been shown, that, below the Néel temperature, the dipole polarization is induced by a lone electron pair of bismuth ions. At T > TN, the migration polarization is caused by the charge carrier density at the chemical potential. A significant increase in the remanent magnetization of mullite in the bismuth pyrostannate matrix with oxygen vacancies over a value typical of polycrystalline mullite has been established. Remanent magnetization is explained in terms of ferron model.