Improved coercivity of W- and M-type composite hexaferrites using two different synthesis routes

Two series of W-and M-type composite hexaferrites were synthetized using the conventional ceramic method in two different synthesis routes. In the first synthesis route, W-and M-type composite hexaferrites were formed by reaction in the pre-sintering stage. In the second synthesis route, W-and M-type composite hexaferrites were obtained by physically mixing W-and M-type hexaferrites in the second ball milling stage. The influence of M type phase on the structure and magnetic properties of W-type hexaferrites synthesized in both synthesis routes are investigated in detail. For both synthesis routes, X-ray diffraction (XRD) patterns of all samples indicated crystallization of W-and M-type hexaferrites phase and scanning electron microscopy (SEM) measurements revealed hexagonal platelet-like grains. In the pre-sintering stage, the saturation magnetization (4 pi Ms) of all samples ranged between 3.30 kG and 4.25 kG. The remanence ratio (Mr/Ms) changed from 0.77 to 0.88 with the increase of pre-sintering temperature. The coercivity (Hc) of all samples ranged between 456 Oe and 1846 Oe. The ferromagnetic resonance (FMR) linewidth (& UDelta;H) ranged between 466 Oe and 748 Oe. In the second ball milling stage, 4 pi Ms and Mr/Ms maintained a relatively stable value of-4.5 kG and-0.89, respectively. Hc increased from 519 Oe to 620 Oe with the increase of M-type hexaferrites. delta H ranged between 317 Oe and 573 Oe. We note that in both synthesis routes only the composite hexaferrites synthesized in the pre-sintering stage have a significantly improved coercivity from 456 Oe to 1846 Oe. This enhancement in coercivity is attributed mainly to the effect of exchange coupling among ferrite grains. In addition, the delta H decreases with the addition of M-type hexaferrites.