High coercivity Gd-substituted Ba hexaferrites, prepared by chemical coprecipitation

A series of Gd-substituted Ba hexaferrites with nominal formula (Ba1-x Gdx) O5.25 Fe2 O3 (x=0-0.30) were prepared by the chemical coprecipitation method from nitrate precursors and heating at T=800-1200 °C for 2 h. The samples have been examined by x-ray diffraction, vibrating-sample magnetometer, and scanning electron microscopy methods. Gd substituted samples form single phase materials with the M -type hexaferrite structure at all heating temperatures, in the range of x≤0.10-0.20. The saturation magnetization (at 1.8 T) varies slightly with x in most cases and, for x=0.05-0.10, it increases up to 66.7 A m2 kg, exceeding the value of the unsubstituted hexaferrite. A strong enhancement of the coercivity is observed for all substituted samples, with maximum values Hc =457 kAm for the single-phase x=0.10 sample annealed at 1000 °C and Hc =477 kAm for the x=0.25 sample annealed at 1100 °C which contains Fe2 O3 and GdFe O3 impurities. As the variation of coercivity with either substitution rate (x) or annealing temperature is not monotonic, three different factors may account for the high coercivities that are obtained: (a) an inhibition of grain growth due to the presence of Gd, (b) a possible inherent effect on magnetocrystalline anisotropy, especially for single phase samples, and (c) a microstructural effect of secondary phases.