Given the huge and increasing global demand for permanent magnet supply, even slight improvements of the magnetic properties and cost effectiveness of permanent magnet materials could incur huge energy and money savings. This article is concerned with the optimization of the experimental conditions for the production of pure M-type BaxSr1−xFe12O19 hexaferrites with improved magnetic properties. X-ray diffraction analysis revealed the formation of a single M-type hexaferrite in the ball milling route at a sintering temperature of 1100 °C for the samples with x = 0.0 and 0.5, and at 1200 °C for x = 1.0. In the sol–gel route, however, a single M-type phase was successfully synthesized at a significantly lower temperature of 890 °C. The magnetic parameters of the samples prepared by ball milling exhibited an improvement at lower Ba contents. On the other hand, the samples prepared by sol–gel method exhibited a significant improvement of the intrinsic coercivity (HcM) compared with those prepared by ball milling, with the highest value of 5.9 kOe observed at x= 0.0 and sintering temperature of 1000 °C. The saturation and remnant magnetization, however, were not influenced significantly by the synthesis route, and remained relatively high, comparable with the best parameters for ferrite isotropic magnets. The sample with x = 1.0 prepared by sol–gel method and sintering at 890 °C exhibited the highest residual induction Br = 2509 G, practical coercivity HcB = 1919 Oe, and maximum energy product (BH)max = 9.9 kJ m−3.
Barium hexaferrite (BaFe12O19; M-type; BaM) is an important, cost effective magnetic material for permanent magnet applications. The magnetic properties of the prepared samples, and the purity of the BaM phase depend critically on the synthesis route and experimental conditions. In this study, BaM hexaferrites were prepared by co-precipitation method using two different values of pH for the precursor solutions (11.0 and 12.5), and sintering pellets of the co-precipitates at 860, 920 and 990°C.The prepared samples were characterized using X-ray diffraction and magnetic measurements. X-ray diffraction patterns indicated that the samples prepared with pH = 12.5 consisted of a single BaM phase at all sintering temperatures. However, the patterns of the samples with
pH = 11.0 did not reveal the existence of BaM at 860°C, whereasa major BaM phase
(86 – 87 wt.%) was observed at 920 and 990°C with a minor α-Fe2O3 phase. The thermo magnetic curves confirmed the BaM magnetic phase in the samples. The hysteresis loops of the BaM samples showed characteristics of hard magnetic materials with relatively high saturation magnetization. Analysis of the magnetic data indicated an intrinsic coercivity Hci~ 5 kOe for all samples, and a saturation specific magnetization in the range
σs = 56.0 – 66.3 emu/g, which are suitable for permanent magnet applications. The practical coercivity (HcB), residual induction (Br) and maximum energy product (BH)max of the samples with pH = 12.5 are higher than those of the samples with pH = 11.0, and the highest magnetic parameters of HcB = 1871 Oe, Br = 2384 G, and (BH)max = 8.92 kJ/m3 were observed for the sample with pH = 12.5 and sintered at 860°C.
Rare earth substituted W-type hexaferrites were prepared by mixing and ball milling starting powders with molar ratios consistent with the stoichiometry of Ba1-xRexCo2ZnxFe16-xO27 (Re is a rare-earth element; x = 0.1 and 0.2), pelletizing, and sintering 1300° C. X-ray diffraction patterns showed a pure W-type phase in all samples, except in the Nd-Zn (x = 0.1) substituted sample which revealed the presence of an impurity α-Fe2O3 nonmagnetic phase. The crystallization of the W-type phase in all samples was further confirmed by the characteristic Curie temperature ranging between 476° C and 484° C as revealed by the thermomagnetic measurements. Scanning electron microscopy imaging revealed the variations of the particle size and morphology, and porosity of the prepared samples. The magnetic measurements indicated that the RE–Zn substitution improved the saturation magnetization slightly relative to the un-substituted Co2W hexaferrite, and resulted in a decrease of the coercivity and magnetocrystalline anisotropy field. In addition, the peaks below 300° C in the thermomagnetic curves is an indication of the occurrence of spin reorientation transitions in the prepared hexaferrites.
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