Studies On Synthesis, Microstructures, Magnetic Properties Of Ions Substituted M-type Strontium Ferrites

M-type hexaferrites (SrFe12O19and BaFe12O19) have been widely used as permanent magnets, microwave devices, magneto-optics and magnetic recording media due to relatively low price, high coercivity, large magnetocrystalline anisotropy and outstanding chemical stability. In order to fulfill various applications, many attempts have been made to improve magnetic properties of hexagonal ferrites concerning cationic substitution. In this paper, the solid state reaction method was used to prepare the ions substituted M-type hexaferrite magnetic powders and magnets. The phase identification of the magnetic powders was performed by X-ray diffraction. The micro structures of the magnets were investigated by a field emission scanning electron microscopy. The magnetic properties of the magnets and magnetic powders were measured by a vibrating sample magnetometer and a permanent magnetic measure equipment, respectively.The effects of La-Co substitution, Co/La ratios, heat treatment temperatures, iron content and calcium content on the structural and magnetic properties of La-Co substituted Sr-Ca hexaferrites. The main results are as follows:1) For the M-type ferrite Sro.7-xCa0.3LaxFe12-yCoyO19, there is a single magnetoplumbite phase in the magnetic powders with the substitutiom of La (0.05≤x≤0.15) and Co (0.04≤y≤0.12) contents. For the magnetic powders containing La (x≥0.20) and Co (y≥0.16), magnetic impurities begin to appear in the structure. The magnets have formed hexagonal structures. Br continuously increases with increasing dopant contents. Whereas, HCj, HCb and (BH)max for the magnets first increases and then decreases with an increase in the La-Co contents.2) For the M-type ferrite Sr0.75-xCa0.25LaxFe12-yCoyO19, there is a single magnetoplumbite phase in the samples with y/x from0.3to0.6, and when y/x>0.6, the CoFe2O4phase is observed. The magnets have formed the hexagonal structures and the particles are distributed homogeneously. Br, Hcband (BH)max. of the magnets first increase with y/x from0.3to0.6, and then decrease when y/x>0.6. However, HCj of the magnets first increases with y/x from0.3to0.9, and decreases when y/x>0.9. The rectangularity of the demagnetizing curves for the magnets decreases with increasing y/x.3) For the M-type ferrite Sr0.50Ca0.20La0.30Fe11.15Co0.25O19, at calcination temperatures ranging from1170to1270℃, the phase compositions of the magnetic powders consist of M-type hexaferrites together with small amount of impurity phases such as α-Fe2O3, LaFeO3and CoFe2O4. At calcination temperatures above1270℃,single-phase M-type hexaferrites can be obtained. The particles appear in hexagonal plate-like shape and the particles are distributed homogeneously. The radial shrinkage of the magnets increases with the increase of calcination or sintering temperature. For high Br,Hcj,Hcb and (BH)max, the optimized calcination and sintering temperatures are1250℃and1190℃, respectively.4For the M-type ferrite Sr0.45Ca0.25La0.30FexCo0.25O19, there is only the magnetoplumbite-type phase in magnetic powders with iron content (x) from10.45to11.65. When iron content (x)≥11.85, hematite (α-FeaO3) phase begins to occur. The magnets have formed the hexagonal structure and the particles are distributed evenly. Br, Hcj, Hcb and (BH)max first increase with iron content (x) from10.45to11.05, and then, decrease when iron content (x) continues to increase. The magnetic properties at x=11.05reach the maximum values.5) For the M-type ferrite Sr0.65-xCaxLa0.35Fe11.32Co0.28O18.435, there is a single magnetoplumbite phase in the calcined magnetic powder with the increase of x from0to0.45. The sintered magnets have formed the perfect hexagonal structure and the particles are distributed evenly. The magnetic properties (including Br, HCj,Hcb and (BH)max) of the magnets first increase with x from0to0.20. and then, begin to decrease when x continues to increase. The magnetic properties of the magnet at x=0.20reach the maximum values.The effects of La-Zn substitution, La/Zn ratios, iron content, Co content and barium content on the structural and magnetic properties of La-Zn substituted strontium hexaferrites. The main results are as follows:1) For the M-type ferrite Sri-xLaxFe11.8-xZnxO19, the single magnetoplumbite phase is obtained in the magnetic powder with x from0to0.42. The perfect hexagonal structure is formed in the magnets and the particles are distributed homogenously. The radial shrinkage (%) of the magnets at x=0.30reaches the maximum value of12.27. Br and (BH)max for the magnets first increase with x and reach the maximum values of421.8mT and33.43kJ m-3at x=0.30, respectively. Then, Br and (BH)max start to decrease when x continues to increase. Both Hcj and Hcb decrease linearly with the increase of x from0to0.42.2) For the M-type ferrite Sr1-xLaxFe12-yZnyO19, there is a single magnetoplumbite phase in the calcined powders with of1.2and1.4, and for other calcined powders, the secondary phase is observed. The samples have formed the hexagonal structures and the particles are distributed homogeneously. Br and (BH)max of the magnets first increase with x/y from0.8to1.2, and then decrease when x/y continues to increase. However, HCi and Hcb of the magnets decrease with x/y increased from0.8to2.2.3) For the M-type ferrite Sr0.80La0.20FexZn0.15O19, there is only the magnetoplumbite phase in the magnetic powder with the iron content (x) from10.65to11.65. However, as x increases to11.85, another phase of hematite (α-Fe2O3) begins to occur. The hexagonal ferrite magnet has formed the perfect hexagonal structure and the particles are distributed evenly. Br and (BH)max for the magnets at x=11.25reach the maximum value of414.5mT and32.24kJ/m3, respectively. HCj and Hcb for the magnets at x=11.65reach the maximum value of265.8kA/m and257.8kA/m, respectively.4) For the M-type ferrite Sr0.68La0.32Fe11.7Zn0.30-xCoxO19, all the X-ray diffraction peaks belong to the single hexagonal ferrite. The hexagonal ferrite magnets have formed the plate-like hexagonal structure.Brand (BH)max of the magnets first increase till x=0.10, and then decrease with increasing x from0.10to0.30. However, Hcj and Hcb of the magnets increase with the increase of x from0to0.30. The ratio of Hk/Hcj decreases gradually with the increase of x from0to0.30.5) For the M-type ferrite Sr0.7-xBaxLa0.3Fe11.8Zn0.2O19(0≤x≤0.70), X-ray diffraction patterns show that the hexagonal single phase is obtained in all samples. All magnets have formed hexagonal structures and the particles are distributed homoqeneousiy Br,Hcj, Hcb and (BH)max of the magnets continuously decrease with increasing barium content (x).The effects of La-Cu substitution, La/Cu ratios, Co content, calcium content and barium content on the structural and magnetic properties of La-Cu substituted strontium hexaferrites. The main results are as follows:1) For the M-type ferrite Sr1-xLaxFe12-xCuxO19(0≤x≤0.35), the single magnetoplumbite phase is obtained in magnetic powders with x from0to0.35. The hexagonal structure is formed and the particles are distributed evenly. Br and (BH)max for the magnets first increase with x from0to0.20and then, start to decrease when x continues to increase. Both HCj and Hcb decrease linearly with the increase of x from0to0.35.2) For the M-type ferrite Sr1-xLaxFe12-yCuyO19, there is a single magnetoplumbite phase in the magnetic powders with x/y from0.6to1.6, and for the magnetic powders with x/y from1.8to2.0, the LaFeO3phase is observed. The magnets are formed of hexagonal-shaped crystals and the particles are distributed homogeneously. Br of the magnets increases with x/y from0.6to2.0. However, HCj, Hcb and (BH/)max of the magnets first increase with x/y from0.6to1.2, and then begin to decrease when x/y continues to increase.3) For the M-type ferrite Sr0.65La0.35Fe11.7Cu0.30-xCoxO19, there is a single magnetoplumbite phase in the magnetic powders with Co content (x) from0to0.30. The magnets are formed of hexagonal-shaped crystals. Br,(BH)max. and Hk/Hcj ratio of the magnets first increase till x=0.20, and then decrease with the increase of x from0.20to0.30. However, Hcj and Hcb of the magnets increase with the increase of x from0to0.30.4) For the M-type ferrite Sr0.70-xCaxLa0.30Fe11.72Cu0.28O19, X-ray diffraction patterns show that the hexagonal single phase is obtained in all samples. The magnets are formed of hexagonal-shaped crystals and the particles are distributed evenly. Br, Hcj, Hcb and (BH)max. of the magnets first increase with x from0to0.20, and then decrease when x continues to increase. The magnetic properties of the magnet at x=0.20reach the maximum values.5) For the M-type ferrite Sr0.70-xBaxLa0.30Fe11.78Cu0.22O19, there is a single magnetoplumbite phase in the magnetic powders with barium content (x) from0to0.70. All magnets have formed hexagonal structures and the particles are distributed evenly. Br, HCj, Hcb, and (BH)max. of the magnets continuously decrease with the increase of barium content (x).