Abstract: The Ni, Mg, Al co-doped M-type barium ferrite powders (Ba(NiMg)0.2AlxFe11.4-xO19, x=0-0.4 ) were prepared via a sol-gel auto-combustion method using citric acid as complexing agent .The structure and room-temperature magnetic properties of the as-samples were studied by X-ray diffraction ( XRD) , transmission electron microscope ( TEM) and physical property measurement system ( PPMS) . The results show that the phase constitutent of the samples is closely related to the amount of Al doping . When the x value exceeds 0.2, the impurity alpha-Fe2O3 appears in the products, and the content of alpha-Fe2 O3 increases with the increase of the amount of Al substitution .The saturation magnetization (10 kOe) decreases with the increase of x value, while the coercivity (Hc) value increases first and then decreases with the increase of x value, and the coercivity reaches the maximum value of 5644 Oe at x=0.2, corresponding to the saturation magnetization of 46 emu/g.

Key words: The Ni, Mg, Al co-doped M-type barium ferrite powders (Ba(NiMg)0.2AlxFe11.4-xO19, x=0-0.4 ) were prepared via a sol-gel auto-combustion method using citric acid as complexing agent .The structure and room-temperature magnetic properties of the as-samples were studied by X-ray diffraction ( XRD) , transmission electron microscope ( TEM) and physical property measurement system ( PPMS) . The results show that the phase constitutent of the samples is closely related to the amount of Al doping . When the x value exceeds 0.2, the impurity alpha-Fe2O3 appears in the products, and the content of alpha-Fe2 O3 increases with the increase of the amount of Al substitution .The saturation magnetization (10 kOe) decreases with the increase of x value, while the coercivity (Hc) value increases first and then decreases with the increase of x value, and the coercivity reaches the maximum value of 5644 Oe at x=0.2, corresponding to the saturation magnetization of 46 emu/g.