Magnetostatic wave propagation at microwave frequencies through a periodic structure formed by selectively etching the surface of an epitaxial yttrium iron garnet (YIG) film is reported. Experimental insertion loss data obtained in the 2–4-GHz range from a structure comprising 20 grooves each 1 μm deep and 30 μm wide, and each separated by 120 μm, formed in a 9-μm-thick YIG film are in good agreement with theory based on repetitively mismatched transmission lines.
[Plate 15]
I n t r o d u c t io nIn a recent investigation (Sykes and Jones 1936) of the properties of the alloy Cu3Au we described the effect of the ordering process on the energy content of the alloy. The observed behaviour was found to be more com plicated than was anticipated from the theoretical work (Bragg and Williams 1934; Peierls 1936), and the existence of a variety of metastable states of an unexpected type was established. The face-centred lattice of Cu3Au may be considered in terms of the four interpenetrating simple cubic lattices to which it is equivalent. When completely ordered one lattice contains nothing but gold atoms and the other three, copper atoms. As the alloy cools through the critical temperature gold atoms may segregate on any of the four lattices, consequently any individual crystal in the alloy will contain a large number of small volumes or nuclei, each of which has its own consistent scheme of order, but different nuclei will be out of phase with one another. The various metastable states observed experimentally are associated with the presence of these nuclei, since their rate of growth, which determines the rate of disappearance of disordered material from the boundaries, is small compared with the time of relaxation of the material inside the nuclei. The size of the nuclei can be obtained from the width of the superlattice lines present in the X-ray powder diagram; this information, together with a number of specific-heat temperature curves, was used to determine the effect of the nuclei on the energy content of the alloy. It was shown that the experimental results were consistent with the assumption that the nuclei in the individual crystals were separated by narrow boun daries (from about one to two atomic distances in width), the atoms inside the nuclei having the equilibrium degree of order at the temperature of the experiment.The electrical resistance of an alloy in equilibrium depends on the degree of order; the presence of nuclei corresponding to a metastable state intro duces an extra resistance which is a function of the size of nuclei. The present
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