The second-order spin-wave instability threshold in single-crystal yttrium-iron-garnet films under perpendicular pumping

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Abstract
The second-order spin-wave instability threshold in yttrium-iron-garnet single-crystal thin films has been investigated under conditions of ferromagnetic resonance (FMR). The samples were (111) etched circular films of 500 μm diam and 5.1 μm thickness, grown on gadolinium gallium garnet substrates by liquid-phase epitaxy. The pulsed microwave power FMR absorption curves were measured by standard signal averaging techniques at 9.18 GHz and room temperature for three different perpendicular pumping configurations: (1) in-plane static field and out-of-plane microwave field, (2) mutually perpendicular in-plane static and microwave fields, and (3) out-of-plane static field and in-plane microwave field. The experimental results show that the upsweep and downsweep FMR curves for lower pulsed microwave powers at low duty cycle of 1% are identical, but there are foldover effects in the FMR profiles at higher powers. Theoretical expressions for the instability threshold for these three configurations were obtained by extending previous theories. The calculated thresholds are in good agreement with the experimental results for configurations (1) and (2). The results for configuration (3) indicate the presence of a true foldover effect, also consistent with instability theory.