Fine-Grained Ferrites. III. Ni1−xCoxFe2O4. High-Power Microwave and rf Properties

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Abstract
Dense Ni1−xCoxFe2O4, with x=0, 0.028, 0.030, and 0.067, was prepared with grain sizes down to 0.1 μ by using a previously described technique. This consists of a combination of flame‐spraying to obtain the initial fine ferrite particles (down to 0.03 μ), and hot‐pressing to sinter these into single‐phase bodies of near theoretical density (to 98%). The high‐power microwave properties were measured in a cavity at X band with perpendicular pumping for a series of grain sizes ranging from 0.10 to 0.35 μ. Co=0.027 was used, since the magnetocrystalline anisotropy constant K1 is theoretically zero and thus would be expected to provide minimum linewidth ΔH in this compositional system. Measured values of the microwave magnetic loss susceptibility under perpendicular pumping χ⊥″, the threshold field for the onset of the subsidiary absorption hc, ΔH, 4πMs, and density are given as a function of grain size. hc was found to increase with decreasing grain (or pore) size. This was explained on the basis that spin waves which are important at the subsidiary resonance are longer than the grain size, and therefore interact effectively with such inhomogeneities. This explanation is supported by the finding that [(hc)2000− (hc)1500]/(hc)1500, for Hdc=1500 and 2000 Oe, decreases with decreasing grain size. χ⊥″ passes through a minimum near 0.18 μ in unannealed samples, but in annealed samples it decreases rather linearly with grain size. Of practical concern are the following combinations of hc and χ⊥″ which are obtainable by this method: >105 Oe and 0.6, >75 Oe and 0.15, compared to 16 Oe and 0.03 for a large‐grained ferrite. The power ratio between the first and last is over 40. Radio‐frequency properties such as μ′ and μ″ were measured from 0.1 Mc/sec to 7 Gc/sec. In the dispersion curves obtained, the rf resonance frequency compared well with that calculated on the basis of the domain rotation mechanism. The static μ′ had similar values to those in large‐grained samples, contrary to the case of NiZn ferrites, where wall motion plays a role. The resonance frequency of the microwave peak also agreed well with that calculated from a previously derived equation.

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