Parallel pumping instabilities of spin wave modes in a nanodisk

The confined spin wave modes in a magnetic nanostructure are fundamental to the magnetization dynamics, and the majority of studies of these spin waves have used uniform transverse pumping fields to excite the modes. However, our recent ferromagnetic resonance force microscopy measurements have shown that parallel pumping reveals a richer set of resonances than the more conventional transverse pumping. This talk presents measurements and micromagnetic modeling of the parallel pumping process in a 500 nm diameter 25 nm thick Permalloy disk with fields applied in plane. In the experiments, the linear, transversely pumped spectrum at 5.2 GHz is simple, with a main resonance at 38 mT applied field and a weaker mode at 54 mT. At a doubled excitation frequency of 10.4 GHz and high pumping power, five resonances are excited by parallel pumping. Two of these resonances occur at the same fields as the modes observed under transverse pumping, but the most intense mode is one that does not appear in the transverse spectrum. The modeling results show similar behavior, and also provide images of the excited modes. The lowest thresholds for parallel pumping belong not to the nearly uniform ``main'' precession mode, but to standing waves that propagate along the field direction.