Hydrothermal Preparation And Magnetic Properties Of Composite SrFe₁₂O₁₉/（Ni,Zn）Fe₂O₄ Ferrite With Exchange Spring Behavior

M-type magnetoplumbite SrFe12O19 ferrite is widely used in the fields of microwave devices, magnet-optic systems and high density magnetic recording medium for its high coercivity and fine chemical stability. However, the saturation magnetization of SrFe12O19 ferrite is much lower than that in the spinel soft magnetic ferrite(Ni,Zn)Fe2O4. Nowadays,the exchange coupling between magnetically hard and soft phases is usually used to improve the magnetic performance of SrFe12O19 ferrite, which may result in the high coercivity and the high saturation magnetization simultaneously.In this thesis, composite SrFe12O19/Ni0.4Zn0.6Fe2O4 ferrite powders and bulks were prepared via hydrothermal method. The crystalline structures, magnetic properties,exchange coupling and magnetic microstructures were systematically studied, and some meaningful results were obtained.It was found that for pure SrFe12O19 ferrite, the optimal synthesis conditions were as follows: the hydrothermal temperature, 220oC; the reaction time, 5 h; the Fe/Sr atomic ratio in raw material, 1:4. However, those for pure Ni0.4Zn0.6Fe2O4 ferrite were as follows: the hydrothermal temperature, 200oC; the reaction time, 8 h; the excess of Zn ions in raw materials, 5%.For hydrothermal SrFe12O19/Ni0.4Zn0.6Fe2O4 composite ferrite powders, the average grain size of specimens was about 10 nm, and presented a core/shell-like microstructure.Their smooth magnetic hysteresis loops suggested a typical single behavior and fine exchange coupling in the specimens. When the mass ratio of SrFe12O19 and Ni0.4Zn0.6Fe2O4(Rm) was 1:1 or 1:2, a lot of Fe2O3 impurity was found in the corresponding specimen. For specimen with Rm= 2:1, the saturation magnetization obtained the maximum value(59.1emu/g).For sintered bulk SrFe12O19/Ni0.4Zn0.6Fe2O4 composite ferrites with different Rm prepared using hydrothermal powders, only SrFe12O19 and Ni0.4Zn0.6Fe2O4 phases without other impurity existed in the specimens sintered at 700oC and 900oC. It was found that all the specimens exhibited a typical smooth single-phase magnetic hysteresis loop, whichsuggested the fine exchange coupling between magnetically hard and soft phases that was further confirmed by the study of the parameter of exchange coupling, ?M. Also found were that the magnetic properties and the strength of exchange coupling were closely tied to the average grain size. In addition, the novel stripe domains were found in the composite ferrites, which were attributed to the soft magnetic Ni0.4Zn0.6Fe2O4 phase.