The effect of Co and Ni on magnetic properties and microstructure of BaFe$_{12-x}$Ni$_x$O$_{19}$ and BaFe$_{12-x}$Co$_x$O$_{19}$ powders synthesized by the hydrothermal method

In this work, nanoscale powders of Co- and Ni-substituted barium hexaferrites were obtained by hydrothermal synthesis (BaFe$_{12-x}$Ni$_x$O$_{19}$ and BaFe$_{12-x}$Co$_x$O$_{19}$ with $x$ = 0.1, 0.3, 0.5). The obtained samples were analyzed by several methods, including XRD, EDX, VSM, FTIR and TEM. It is shown that despite the close chemical nature of Co$^{2+}$ and Ni$^{2+}$, the effect of substitution of Fe$^{3+}$ by these elements is completely different. Thus, nickel has virtually no effect on the shape and size of the resulting BaFe$_{12-x}$Ni$_x$O$_{19}$ particles (plate-shaped crystallites with a diameter of about 200 nm and a thickness of 60 nm). With an increase in the nickel concentration, the magnetic parameters of the resulting ferrites decrease almost linearly. Cobalt, on the contrary, in a certain concentration leads to a significant change in the morphology of the particles (thinning of the crystallites to 30 nm or less), resulting in a sharp decrease in the coercive force of the resulting powders. This effect is due to the fact that cobalt promotes the formation of the BaFe$_{12-x}$Co$_x$O$_{19}$ phase directly during the hydrothermal treatment of the precursors, whereas additional high-temperature treatment is required to form BaFe$_{12-x}$Ni$_x$O$_{19}$ and BaFe$_{12}$O$_{19}$.