Grain-size effects on microwave ferrite magnetic properties

This paper presents a review of polycrystalline microwave ferrites with specific reference made to the grain size. Fine-grained ferrites and large-grained ferrites as referred to in this article stand for the ferrites with an average grain size of 2 μm or less and those of 100 μm or larger, respectively. Fine-graining of ferrites is one of the state-of-the-arts techniques for obtaining high-power ferrites. Large-graining is a newly developed technique for further improving the family of narrow resonance linewidth garnet materials. Starting with some ceramics-oriented topics including the definition of the average grain-size and so forth, the present review covers the grain-size dependence of initial permeability and the grain-size dependence of microwave properties (ΔH, ΔHeff, and ΔHk) both in the theoretical and in the experimental phases. Successful device applications of newly developed "single-crystal-like" large-grained ferrites are also reviewed.