Synthesis and magnetic properties of Ca-La M-type hexaferrite

Abstract

Ca-La M-type hexaferrites have been reported to exhibit high saturation magnetization (Ms) and coercivity (Hc) comparable to those of Sr or Ba M-type hexaferrites. In this study, we tried to synthesize the single phase of Ca0.5La0.5Fe12-yO19-δ and investigated the effects of iron deficiency on structural and magnetic properties in Ca0.5La0.5Fe12-yO19-δ (0.75 ≦ y ≦2.15) M-type hexaferrites. Ca-La M-type hexaferrite powder samples having the nominal compositions of Ca1-xLaxFe12O19 (x=0.4, 0.5 and 0.6) and Ca0.5La0.5Fe12-yO19-δ (0.75 ≦ y ≦ 2.15) were prepared by conventional solid state reaction. All the precursor powders used (La2O3, CaCO3, and Fe2O3) in this study were 99.9% purity．Precursor powders were weighed, ball-milled for 24 h, and then uniaxially pressed into pellets. As-pressed pellets were calcined at 1150, 1200, 1250, and 1300 ℃ for 12 h in air, to check whether the single phase of M-type hexaferrite can be formed or not. The calcination was repeated twice by intermediate ball milling and pelletizing. As-calcined powders were pressed into pellets uniaxially. To measure the magnetic properties of samples, the pellets were put into a muffle furnace and sintered at 1275, 1300 and 1325℃ for 4 h in air. Phases and lattice parameters of sintered samples were analyzed by powder X-ray diffraction (XRD). Microstructure was observed by Field Emission-scanning electron microscopy (FE-SEM) and magnetic properties were measured by Vibrating Sample Magnetometer (VSM). XRD patterns showed all the phases in the Ca0.5La0.5Fe12-yO19-δ (1.75 ≦ y ≦ 2.15) calcined at 1250 for 12 h and Ca0.5La0.5Fe12-yO19-δ (0.75 ≦ y ≦ 2.15) calcined at 1300℃ for 12 h were the single phases with hexagonal structure. For the single phases of Ca0.5La0.5Fe12-yO19-δ, at a given sintering temperature, the lattice parameter a, c and unite cell volume of the Ca0.5La0.5Fe12-yO19-δ single phases were all firstly decreased and increased later with increasing y, at a given sintering temperature. The average grain sizes are firstly increased and then decreased with increasing y. The saturation magnetization Ms decreased insignificantly with increasing y. The highest Ms value of 77.5 emu/g was obtained from the Ca0.5La0.5Fe12-yO19-δ (y = 0.75) sample sintered at 1300℃ for 4 h in air, which is higher than those of pure phase SrM and BaM by 11.2% and 14.5%, respectively. As y is increased, Hc first was decreased and then increased for both systems. In conclusion, iron deficiency could facilitate the formation of single phase Ca-La M-type hexaferrite. The compound Ca0.5La0.5Fe12-yO19-δ was calcined for 12 h in air at 1250 and 1300℃, and their Fe solubility limits were 1.75 ≦ y≦ 2.15 and 0.75 ≦ y≦ 2.15 respectively. Iron deficiency could Iron deficiency could improve the magnetic properties of Ca-La M-type hexaferrites.