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 gammaH(a) 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 gammaH(a) 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 gammaH(a), which determines the frequency of maximum absorption, is found to decrease rapidly with increasing temperature near 300 K. Magnetometry indicates that this temperature dependence gammaH(a) is caused mostly by variation of H-a with temperature. The temperature dependence of gammaH(a) for Ni-Z suggests a strategy to compensate the strong and opposing temperature dependence of gammaH(a) induced by Co ions, which are used to reduce H-a. By independently varying the mix of Zn, Co, and Ni in these ferrites, a useful range of microwave resonance frequencies, gammaH(a), 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.