New substitutions in hexagonal ferrites to reduce anisotropy without using Co

Hexagonal ferrites that have a previously unexplored class of substitutions have been made and evaluated by observing their microwave absorption spectra as a function of temperature. The temperature dependences of the uniaxial anisotropy and the magnetic moment were determined. Substitutions were investigated in hexagonal ferrites normally having a small divalent ion in their composition, e.g., Zn-Z(Zn2Ba3Fe24O41), Ni-W, etc. Substitution of a further Zn and Ti for Fe reduced the uniaxial anisotropy field, H-a, of Zn-Z rapidly from similar to 5 kOe to a few kilo-oersteds, while replacing only about one of the 24 Fe atoms in the formula unit. This rapid reduction in anisotropy along with moderate reductions in T-C and moment at 300 K, imply that these substitutions preferentially fill the high anisotropy, up-spin, trigonal bipyramidal sites. The anisotropy field is only weakly temperature dependent near 300 K. This contrasts strongly with the case of Co substitutions often used to obtain H-a in this range. Complications caused by rapid magnetic relaxation and magnetostriction associated with divalent Co are also avoided. [S0021-8979(00)26008-8].