Microwave properties, such as the threshold field h c for the parallel pump instability, the effective linewidth W , and the dielectric loss tangent have been measured for a number of fine-grain Ni and Mg ferrites. Ferrite powders, prepared by spray drying of sulfate solutions and subsequent roasting, were sintered by continuous hot pressing. Grain sizes as small as 0.5 μm in dense materials were so obtained. The Mg ferrites showed excellent microwave properties ( h_{c} = 110 Oe, W(H_{i} \approx 0) = 55 Oe, \tan \delta_{\epsilon) = 2 \times 10^{-4} ). The experimental data suggest that the mean free path of parametrically excited spin waves is of the order of the grain diameter. The Ni ferrites showed comparable values of h c but they had much larger dielectric losses ( \tan \delta_{\epsilon} \simeq 1 \times 10^{-3} ). Also W(H_{i} \approx 0) is much more strongly affected by the fine graining (up to 410 Oe). The bad magnetic properties of the fine-grain Ni ferrites are tentatively attributed to stresses near the grain boundaries.

J. De Lau

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