Multiferroic materials, in which long-range magnetic and ferroelectric orders coexist, have recently been of great interest in the fields of both basic and applied sciences. The Y-type hexagonal ferrite Ba₂Mg₂Fe₁₂O₂₂ is an example of a multiferroic material. Its single crystals have a relatively high spiral-magnetic transition temperature (~200 K), show multiferroic properties at zero magnetic field, and the direction of the ferroelectric polarization can be controlled by a weak magnetic field (< 0.02 T) [1]. We present a study of the influence of substituting the Mg²⁺ cations in the Y-type Ba₂Mg₂Fe₁₂O₂₂ hexaferrites with a magnetic cation, such as Co²⁺, on the structural and magnetic properties. The Ba₂Mg_0.4Co_1.6Fe₁₂O₂₂ powder was synthesized by sonochemical co-precipitation. High-power ultrasound was applied to assist the co-precipitation process. The precursors produced were synthesized at 1170°С. The XRD spectra of the powders showed the characteristic peaks corresponding to the Y-type hexaferrite structure as a main phase and some CoFe₂O₄ impurity (< 2%) as second phase. This was also confirmed by Mössbauer spectroscopy measurements. The magnetization values at 50 kOe were 30 emu/g and 26.6 emu/g at 4.2 and 300 K, respectively. The ZFC and FC magnetization curves were obtained at a magnetic field of 500 Oe. The magnetic measurements revealed a magnetic phase transition at 200 K from ferrimagnetic-to-helical spin order. Such a transition is considered as a precondition for the material to exhibit multiferroic properties.

[1] K. Taniguchi, N. Abe, S. Ohtani, H. Umetsu, T. Arima, "Ferroelectric polarization reversal by a magnetic field in multiferroic Y-type hexaferrite Ba₂Mg₂Fe₁₂O₂₂", Appl. Phys. Express, vol. 1, art. num. 031301, 2008.