Effects of bismuth and bismuth-copper substitutions on structure, morphology, and magnetic properties of sol-gel derived barium hexaferrites

The partial substitution by bismuth (Bi) is aimed to improve magnetic properties of barium hexaferrites from the sol–gel auto-combustion synthesis. The exceptionally high saturation magnetization of 99 emu/g is attributed to the substitution of Fe3+ in tetrahedral 4f1 sites by Bi3+and the highly compact structure without impurity phase. On the other hand, the coercivity is reduced with increasing Bi substitution because the hexagonal particles grow larger and single magnetic domains become multi-domains. Similarly, the co-substitution by bismuth-copper (Bi-Cu) ions substantially enhances the particle size and hence the reduction in coercivity is even more drastic. The reduction in coercivity of barium hexaferrites with high magnetizations are desirable for microwave absorbing and data storage materials. The magnetizations of barium hexaferrites with either Bi or Bi-Cu substitutions follow the law of approach saturation. However, the saturation magnetization is significantly lower by the Bi-Cu substitutions into octahedral 4f2 sites and the presence of the secondary hematite phase.