Nonlinear magnetic properties of Mn-modified [Ba.sub.3][Co.sub.2][Fe.sub.23][O.sub.41] hexaferrite

We have investigated the structural and magnetic properties of a novel type of hexaferrite with composition of [Ba.sub.3][M.sub.2][Fe.sub.23[O.sub.41] (where M = Co and Mn) (Z hexaferrite) with tunable permeability. The material is promising for very high frequency (VHF) control when used in a voltage-controlled oscillator (VCO) as a tunable inductor. We substituted Mn for Co in the composition [Ba.sub.3][Co.sub.2-x][Mn.sub.x][Fe.sub.23][O.sub.41] from x = 0.0 to 1.2 to improve the inductance tunability and quality factor of the inductor material. Mn substitution had significant influence on the grain size of the sintered hexaferrites, and the coercivity was greatly reduced. The initial permeability and quality factor increased with increasing substitution of Mn for Co until x reached 0.5, an effect attributed to the decrease of the magnetocrystalline anisotropy constant of the hexaferrite and increase of average grain size. The tunability of the samples also increased with increasing Mn substitution, reaching a maximum at x = 0.5, and then decreased with further substitution. This behavior is similar to that of the initial permeability and coercivity, and can be explained by the same mechanism. Index Terms--Magnetic properties, nonlinear magnetic properties, tunability, Z-type hexaferrite.