Parametric Excitation and Amplification of Magnetoelastic Waves

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Abstract
A series of experiments is described showing parametric amplification of traveling magnetoelastic waves in single-crystal YIG in the frequency range 1–10 GHz. The process involves a pump field at right angles to the dc field, with almost exactly twice the signal frequency. Pulse echo techniques were used to observe amplified echoes in both axially and transversely magnetized rods and in normally magnetized disks. The time delay of the echo is a function of applied magnetic field strength and amplification is observed only if the pump is provided at a particular time. The amplification in a rod is largest when the field is directed ≈15° off the rod axis and in this case the pump must be applied approximately when the signal is in the magnetoelastic crossover region. A net gain exceeding 55 dB was obtained at 1.5°K for X-band signals with 10-μsec delay. At room temperature, 6-dB net gain was obtained in X-band at 3-μsec delay with 250 W of pump power, and 17-dB net gain was obtained in L band with only 0.6 W of pump power. The observed echo behavior indicates that the transverse pumping process involves a pair of shear magnetoelastic waves on the same branch of the dispersion curve and occurs near the magnetoelastic crossover point. The theoretical instability thresholds are derived for several processes involving transversely pumped, magnetoelastic shear waves, but the results do not agree with the most direct explanation of the experimental observations.