Riccardo Roveda

Grain‐Size Average and Distribution Effects on the Magnetic Losses and Threshold Fields in Nickel Ferrites at Microwave Frequencies

Nickel ferrites with scaled average grain sizes and lognormal size distributions were prepared by hot‐pressing and subsequent annealing and by conventional firing. Average grain sizes ranged from less than 1 to 10 μ. The dielectric loss, saturation magnetization, bulk density, and linewidth measurements confirmed that almost all samples were good nickel ferrites. Lognormal distributions allow with easy calculations to evaluate for each sample the fraction of grains having diameters below certain values that are critical for the loss mechanism. Orthogonal and parallel pump measurements of threshold fields at X band showed a very regular behavior with average grain sizes: ∥ hcrit varied from 80 Oe for 0.9‐μ sample to 8 Oe for a 10‐μ sample with a dependence of the type hcrit≈x−2 for x 2 μ. An analysis of the distribution of grains shows that ∥ hcrit increases steeply for samples having about 80% or more of the number of grains with sizes less than 3 μ. Regular behaviors of the ima...

Slowly dispersive, short time delay line based on very thick liquid phase epitaxially grown yttrium iron garnet

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.

Independent Grain Behavior of Parametrically Excited π/2 Magnons in Polycrystalline YIG

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.

Magnetostatic mode suppression in layered yttrium iron garnet films

A novel approach to the suppression of magnetostatic modes in layered Ca-doped/pure yttrium iron garnet (YIG) films is presented. By properly selecting the position and width of the broad background signal excited in the Ca:YIG layer, the magnetostatic spectrum of the coupled pure YIG can be drastically modified. In particular, selecting a proper calcium concentration of the damping layer, an entire surface or volume mode band can be suppressed. In this paper, experimental results for double-layered films are reported. An extension to triple-layered films, allowing the total suppression of all modes but one, is also proposed.