Multiferroic properties and structural features of $M$-type Al-substituted barium hexaferrites

Precise studies of the crystal and magnetic structures of $M$-type substituted barium hexaferrites BaFe$_{12-x}$Al$_{x}$O$_{19}$ (0.1 $\le x\le$ 1.2) have been performed by powder neutron diffraction in the temperature range 300–730 K. The electric polarization and the magnetization, and also the magnetoelectric effect of the compositions under study have been studied in electric (to 110 kV/m) and magnetic (to 14 T) fields at room temperature. The spontaneous polarization and significant correlation between the dielectric and magnetic subsystems have been observed at room temperature. The magnetoelectric effect value is, on average, about 5%, and it increases slightly with the aluminum cation concentration. The precise structural studies made it possible to reveal the cause and the mechanism of formation of the spontaneous polarization in $M$-type substituted barium hexaferrites BaFe$_{12-x}$Al$_{x}$O$_{19}$ ($x\le$ 1.2) with a collinear ferromagnetic structure.