Preparation And Magnetic Properties Of High Coercivity M-type Ferrite Powders

Due to the fact that M-type strontium ferrite（SrM） has high coercivity, high saturation magnetization, large magnetocrystalline anisotropy, high curie temperature and high chemical stability, such excellent functional properties have attracted many researchers attention and made it popular for industrial application such as permanent magnets, magnetic recording media and millimeter wave devices etc. The structure, morphology and magnetic properties of magnetic powders for M-type ferrite magnets have been studied by doping Al³⁺ ion and adjusting preparation technology. The major results of the thesis are as follows:1. It was systematically investigated that the influences of the Fe2O3 types, Fe content and calcining temperature on microstructure and magnetic properties of SrM with Al³⁺ doping. The results show that the purity SrM phases were obtained using two different particles size of iron oxide as raw materials, and higher coercivity can be obtained using iron oxide with smaller particles as raw materials. At the same calcining temperature, the coercivity and magnetization of the samples increased with the Fe content increasing. By contrast, at the same Fe content, the coercivity of the samples increased at first, and then decreased with the increasing the calcining temperature, while the magnetization was scarcely changed. The coercivity decreased when the calcining temperature is more than 1200℃, which attributed to the particle size of the samples being larger than the critical size of single-domain particle.2. The influences of the Al³⁺ content and calcining temperature on microstructure and magnetic properties were also investigated. We can found that the coercivity of samples increased with increasing Al³⁺content at the same temperature, while magnetization reduced. The lattice constant, lattice volume and lattice density decreased with the increase of the Al³⁺ doping because the radius of the Al³⁺ is smaller than that of Fe3+. With the same Al³⁺ doping amount, the coercivity of the samples were increased at first, and then decreased with the increasing the calcining temperature, while the magnetization was still scarcely changed.3. The atoms sites occupation were investigated when the amount of the Al³⁺ doping is 0.0, 0.8, 1.4 and 2.0 by analyzing the M?ssbauer Spectroscopy. We attempt to understand in detail the reasons and the mechanism of the coercivity enhancing as the amounts of Al³⁺ doping is in a range（0ï¿¡ x ï¿¡ 2.0）. At x <2.0, The Al³⁺ mainly substituted the Fe3+ of 2a, 4f1 and 12 k sites, which decreases the magnetic moment per unit cell, saturation magnetization and magnetic anisotropy constant of the samples. The enhancement of coercivity derived from the increasing magnetic anisotropy field, which attributed to that the saturation magnetization（Ms） decreases more rapidly as compared to the magnetic anisotropy constant（K1） with the increasing of Al³⁺ ions content.4. The influences of the different molten salts on the magnetic properties of the strontium ferrite powders with Al³⁺ doping were investigated. The results indicated that the pure M-phase could be obtained using cosolvent A at the same Al³⁺ doping amount and at the same calcining temperature. On the other hand, when cosolvent B was used, impurity phase Fe2O3 was observed. At the same Al³⁺ doping amount, the coercivity of the magnetic powders prepared with cosolvent A increased at first, and then decreased with the increasing the calcining temperature, while the coercivity of the magnetic powders prepared with cosolvent B decreased with increasing the calcining temperature. At the same calcining temperature, the coercivitiy of the both kinds of powders increased with the increase of the Al³⁺ doping amount, and their saturation magnetization decreased. At the same Al³⁺ doping amount and calcining temperature, the coercivity of the magnetic powders synthesized with cosolvent A is greater than that of with cosolvent B. 5. The magnetic powders of BaM partly replaced by Al³⁺ ion were prepared. The pure phase BaM could be achieved when the calcining temperature is more than 1180℃, and the magnetic particles showed a regular hexagonal plate shape. At the same Al³⁺ doping amount, the coercivity of the samples decreased with the increasing the calcining temperature, while their magnetization almost kept constant. At the same calcining temperature, the coercivity of the samples increased with the increasing of the Al³⁺ doping amount, while the magnetizaiton decreased.