Z type Ba hexagonal ferrites with tailored microwave properties

The microwave properties of Z type Ba hexagonal ferrites with uniaxial anisotropy can be controlled over a wide range by choice of an appropriate mixture of Zn, Ni, and Co as divalent species. Using unmagnetized powders of these ferrites, the microwave permeability spectra were taken in a swept frequency mode in the temperature range 200–550 K. From these data, the product γHa was derived. SQUID magnetometry was used to measure the magnetization and anisotropy separately. Pure Zn or Co Z type ferrites were found to have temperature dependences of γHa similar to those of other hexagonal ferrites containing Zn or Co, i.e., nearly flat for Zn and strongly increasing with T for Co. Ni–Z ferrite is notably different: the combination γHa, which determines the frequency of maximum absorption, is found to decrease rapidly with increasing temperature near 300 K. Magnetometry indicates that this temperature dependence γHa is caused mostly by variation of Ha with temperature. The temperature dependence of γHa for Ni–Z suggests a strategy to compensate the strong and opposing temperature dependence of γHa induced by Co ions, which are used to reduce Ha. By independently varying the mix of Zn, Co, and Ni in these ferrites, a useful range of microwave resonance frequencies, γHa, were obtained without undesirable temperature dependence near 300 K. This strategy may allow use of these materials as self-biased microwave components or for absorbing microwave energy in the 5–20 GHz range.