The first-order spin-wave instability threshold in saturated and partially magnetized spheres, rods, and disks of polycrystalline yttrium iron garnet at 9.1 GHz

The first-order spin-wave instability threshold has been measured at 9.1 GHz for polycrystalline yttrium iron garnet as a function of sample shape, sample orientation, pump configuration, and magnetization stake. Spheres, rods, and thin disks were used. The results for saturated samples show that sample shape has a strong influence on the threshold. The data are in good agreement with theory, except for the thin disk samples. The disk data are affected strongly by annealing and the threshold field dependence on static field is modified by inhomogeneous demagnetizing fields. Results for partially magnetized samples indicate that the theory for saturated samples is directly applicable to partially magnetized systems if demagnetizing factors appropriate to the domain shape and not sample shape are used. In the limit of zero external field, the threshold for thin disks with the microwave field in the disk plane is equal to the perpendicular pump threshold for axially magnetized rods. This result indicates that the thin disks demagnetize by the formation of rod-shaped domains oriented normal to the disk. The disk results for pumping with the microwave field normal to the disk are also consistent with this qualitative domain model. Domain shape appears to play essentially the same role in determining thresholds for partially magnetized systems that sample shape does for saturated systems.