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, Ha, of Zn–Z rapidly from ∼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 TC 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 Ha in this range. Complications caused by rapid magnetic relaxation and magnetostriction associated with divalent Co are also avoided.