Longitudinal imaginary susceptibilities ∥χ″ in the spin-wave instability region were measured in YIG samples having bimodally distributed grain sizes. The samples were prepared by hot pressing mixtures of powders having grain sizes x of 1 and 10 μ. Two threshold fields exist for such samples: the low and high one, respectively, due to the coarse and fine grains. Two saturation levels are found for ∥χ″, approximately proportional to the respective volume fractions of fine and coarse grains. An expression for continuously x-distributed polycrystals is worked out on the basis of the single grain scattering mechanism, which compares favorably to experimental results.

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Three series of samples are examined : 1. coarse grained, rare earth-doped YIG ; 2. fine grained YIG and 3. fine grained, rare earth-doped YIG. ⊥ DHk increases very rapidly with low-field DHeff in the fine grained YIG, as compared with the linear, 1/2-slope in coarse grained, rare earth-doped YIG. ||herit behaves as x-1 in YIG, as x-0.4 in 0.03 Sm3+-doped YIG. A simple tentative explanation is given. The high power figure of merit, F*, raises from 2.7 to 12.5 on passing from a 14-micron to a 1-micron YIG sample

Preliminary results on a magnetostatic volume wave delay line based on very thick yttrium iron garnet films (=110 μm) and operating in the 2–12 GHz frequency range, are presented. As expected from the theoretical dependence on the film-thickness, dispersion characteristics appear to be particularly interesting whenever short and slowly varying (slope of 0.05 ns/MHz) delay times are needed. Typically, for forward modes in the 9.5-10.5 interval, the average insertion loss is 12 dB with a −3 dB bandwidth of 550 MHz (reaching almost 900 MHz in the −10 dB limit). The precautions taken to eliminate (within a ±1.5 dB tolerance) band ripples, and relative consequences on the bandwidth and delay time linearity, are also discussed.