Synthsis and Magnetic Properties of Sr-La M-type Hexaferrite

Abstract

Since the discovery of SrFe12O19 M-type hexaferrite (SrM), SrFe12O19 has drawn attention of both researchers and entrepreneurs because of its low production cost, good corrosion resistance, and good magnetic properties. In this study, samples having the compositions of Sr0.5La0.5Fe12-yO19-δ were synthesized, and the single phase hexaferrites with iron deficiency 0 ≦ y ≦ 0.5 were obtainable. The effect of iron deficiency on lattice parameters and magnetic properties of hexaferrites were investigated in this study. Sr-La M-type hexaferrites of Sr0.5La0.5Fe12-yO19-δ (-0.25 ≦ y ≦ 2) were synthesized by solid state reaction. All the precursor powders of La2O3, SrCO3, and Fe2O3 had 99.9% purity．Precursor powders were weighed, ball-milled for 24 h, and then pressed into pellets. As-pressed pellets were then calcined at 1150, 1200, 1225, 1250, and 1300 °C for 8 h in air, to check if the single phase M-type hexaferrites could be obtained. The calcination was repeated after grinding, ball milling, and pelletizing. As-pressed pellets were put into a muffle furnace and sintered at 1275, 1300 and 1325 °C for 2 h in air. Phases and lattice parameters of sintered samples were analyzed by powder X-ray diffraction (XRD). Field Emission-scanning electron microscopy (FE-SEM) was employed to observe the microstructure. Magnetic properties were measured by a Vibrating Sample Magnetometer (VSM). Powder XRD patterns indicated that the Sr-La M-type hexaferrites of Sr0.5La0.5Fe12-yO19-δ showed well-defined peaks of single phase with a limited range of iron deficiency y (0 ≦ y ≦ 0.5) after calcination at 1250 and 1300 °C for 8 h in air. When these samples were sintered at 1275, 1300, and 1325 °C for 2 h in air, those samples were not decomposed but still remained as single phase. For the Sr0.5La0.5Fe12-yO19-δ single phase hexaferrites sintered at 1300 °C, the lattice parameter c and unite cell volumes of the Sr0.5La0.5Fe12-yO19-δ single phases have the lowest value for the sample with y = 0.25, which can be explained by the ionic radius decrease due to the La3+ substitution for the Sr2+ site of SrM lattice and the ion radius increase from Fe3+ to Fe2+ ions. The average grain sizes are slightly decreased with increasing the iron deficiency y. The saturation magnetization (Ms) was slightly increased with increasing y compared with pure SrM. The highest Ms value of 72.8 emu/g and coercivity of 3300 Oe were obtained from the Sr0.5La0.5Fe11.5O19-δ (y = 0.5) samples sintered at 1325 °C for 2 h in air. In conclusion, single phase is obtainable from Sr0.5La0.5Fe12-yO19-δ hexaferrites for 0 ≦ y ≦ 0.5, which have never been reported yet, these may have a promising application in permanent magnet industry since their MS values are higher than 70 emu/g.