The theory of three-magnon relaxation processes is summarized, and it is shown that such processes are very likely to be important at low microwave frequencies. The instability threshold in an rf magnetic field applied parallel to the dc field has been measured in yttrium iron garnet at room temperature as a function of the dc field at five frequencies between S and X band. The results of these measurements are in qualitative agreement with the theoretical predictions.

On the garnets Y3−2xCa2xFe5−x−yVxInyO12, with x ≤ 1.5 and y ≤ 0.5, we have measured the polycrystalline linewidth ΔHpoly at X-band and room temperature and have determined from single crystals the first-order anisotropy constant, K1. Since the single-crystal linewidth of these materials is but a few oersteds and the polycrystalline samples had less than 1% porosity and second phase, the observed ΔHpoly is attributed to anisotropy broadening, according to Schlömann. The introduction of indium reduces both | K1 | and ΔHpoly, for example: x = 0.63y = 0TC = 280°C4πMs = 650 GΔHpoly = 98 Oex = 0.80y = 0.50TC = 155°C4πMs = 750 GΔHpoly = 8 Oe. That last value is the lowest yet reported in the range of T ≈ 23 TC.

The imaginary part of the parallel pump susceptibility has been measured over a wide range of power levels and applied dc fields. Some of the structure present in the susceptibility curves is explained by a confluence process between two parametrically excited spin waves. The square of the frequency of the relaxation oscillations, which are frequently present in parallel pump excitation, has been found to increase linearly with the square of the magnetic field.

Fine-grain ferrites were fabricated by hot-pressing ball-milled and flame-sprayed powders of Ni1−xCoxFe2O4 (where 0≤x≤0.04) and Ni0.973Mn0.027Fe2O4. Threshold fields for subsidiary absorption at Ku-band were significantly increased by decreasing the grain size to 5 μ or less. Between 5 and 10 μ, the critical field hcrit decreased gradually and above 10 μ was independent of grain size. The highest value observed was 192 Oe on a fine-grained nickel-cobalt ferrite vs ∼24 Oe for conventionally fired nickel ferrites. Dielectric losses were reduced to values below 5×10−4 at X-band, and low-field magnetic losses were approximately two to four times those of conventionally fired ferrites of the same composition. No systematic variation in threshold field or magnetic loss was observed with cobalt substitution in fine-grain samples, although threshold fields in general were found to be higher when cobalt was present. The threshold field increased linearly with cobalt substitution on large-grain samples. A Ku-band 90° ferrite phase shifter using a fine-grained, hot-pressed ferrite was capable of operating at power levels in excess of 750 kW/750 W, whereas the same device with a conventional ferrite exhibited nonlinearity below 20-kW peak power.

The threshold microwave-field amplitude h.rl t required for the unstable growth of spin waves has been investigated for oblique pumping with a linearly polarized microwave field applied at an arbitrary angle with respect to the static magnetic field. The experiment was performed on YIG (1% Dy) spheres at 9.2 GHz, using a rectangular TE102 cavity and a pulsed magnetron source. Data were obtained at static external fields of 400, 600, 1100, and 1500 Oe. In general, herit increases with the pump angle. Theoretical expressions for the oblique pumping h.rl t have been derived for ferromagnetic insulators of ellipsoidal , shape and with axial symmetry about the direction of the static magnetic field sufficient to saturate the sample. The threshold field is determined by maximizing l/h=cosif;/hll±sinif;/hJ. with respect to the angle between the internal field and the spin-wave propagation direction. Here, hI! and hJ. are the parallel and perpendicular pump threshold fields of Schlemann and Suhl. For external fields sufficient to magnetize the spherical samples to saturation, the theoretical predictions are in good agreement with the experimental data. For small fields, the agreement is poor. However, fair agreement is obtained by assuming a transverse demagnetizing factor Nc greater than the value of }, which is appropriate for saturated spherical samples. The value of Nc for the best theoretical fit depends on the way in which the sample is demagnetized. This result indicates that for samples not magnetized to saturation, the magnetic structure consists of elongated domains, and that the oblique-pumping data can be explained from the theory for saturated systems, assuming demagnetizing factors appropriate to the domain structure.• The portion of this research performed by J. J. Green and C. E.