J. J. Green

Linewidth Reduction through Indium Substitution in Calcium‐Vanadium Garnets

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 Schlomann. 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.

Threshold Microwave‐Field Amplitude for the Unstable Growth of Spin Waves under Oblique Pumping

The threshold microwave‐field amplitude hcrit 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, hcrit increases with the pump angle. Theoretical expressions for the oblique pumping hcrit 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 1/h=cosψ/h∥±sinψ/h⊥ with respect to the angle between the internal field and the spin‐wave propagation direction. Here, h∥ and h⊥ are the parallel and perpendicular pump threshold fields of Schlomann and Suhl. For external fields sufficie...

Fine‐Grain Nickel Ferrite for Microwave Applications at High Peak‐Power Levels

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 9...

Ferromagnetic Relaxation at Low Microwave Frequencies

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.

Subthreshold Steady‐State Absorption under Parallel Pumping

If the rf and dc magnetic fields are parallel, the absorption coefficient μ″ at low power levels is theoretically expected (for YIG at room temperature, a frequency of 10 Gc/sec, and a dc field of 1 kOe) to be of the order of 2×10−4. We have calculated how the steady‐state absorption varies with dc and rf field strength below the instability threshold. Experiments on a high‐purity YIG single crystal are in reasonably good agreement with the theoretical expectations.

Microwave Properties of Fine‐Grain Nickel Ferrite

Microwave measurements have been made on fine‐grain nickel ferrite prepared by a flame spray‐hot pressing technique. The fine‐grain samples have linewidths, below resonance magnetic losses, dielectric loss tangents, and subsidiary absorption thresholds which are considerably larger than those found in samples prepared by conventional techniques.

Parallel Pumping in Planar Zn2Y

Parallel pump absorption measurements have been made on a single‐crystal sphere and a single‐crystal thin flake of Zn2Y. The imaginary part of the susceptibility in each sample reaches a maximum value of about 2 for an rf magnetic field in the vicinity of 25 Oe. The dependence of the critical rf magnetic field upon dc magnetic field is compared to the theory of Bady and Schlomann and the agreement is found to be reasonably good. From the minimum rf critical field, spin‐wave linewidths of 8.0 Oe for the sphere and 9.24 Oe for the thin flake have been calculated.

Investigations of Spin‐Wave Interactions by the Parallel Pumping Technique

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.

Spin‐Wave Growth Under Parallel Pumping

The transient growth of the spin‐wave population immediately after the application of a strong microwave pulse has been calculated, assuming that the spin waves are in thermal equilibrium before the pulse is applied. The theory is compared with experiments performed at room temperature on a spherical single crystal of yttrium iron garnet at X band. Good agreement is found for a time interval of a few μsec starting several μsec after the pulse has been applied. Before this time interval, the absorption coefficient is too small to be measured. After this interval, the transient behavior is influenced by saturation effects which are not taken into account in the theory.

Magnetic Properties of Yttrium‐Gadolinium‐Aluminum‐Iron Garnets

The magnetic and microwave properties of polycrystalline garnets of the Gd3−xYxFe5−yAly O12 type were investigated. Compositions of low magnetization (∼600 gauss) and narrow linewidth (∼125 oe) were found which are suitable for operation in devices where temperature compensation is required.