Nonlinear spin waves in magnetic systems are of both fundamental and technological interest. This presentation will cover two topics on the chaotic excitation of nonlinear spin waves. One will be on the excitation of chaotic surface spin waves through four-wave modulational instability, and the other will be on the excitation of chaotic backward volume spin waves through three-wave nonlinear processes.
The high order nonlinear Schrödinger equation predicts that the modulational instability of nonlinear waves could lead to the chaotic behaviors of the waves. Such chaotic excitations have been observed experimentally for the waves that have attractive nonlinearity. For the waves with repulsive nonlinearity, however, there have been no experimental observations so far. The first half of this talk will present our recent work on the excitation of chaotic surface spin waves through the four-wave modulational instability processes. The work was conducted in a magnetic film strip-based active feedback ring. The magnetic film/field configuration was set to allow for the propagation of surface spin waves that have repulsive nonlinearity. The self-generation of chaotic spin waves in the ring was observed. The chaotic nature of the signals was confirmed by irregular waveforms, broad spectra, and correlation dimension saturation.
Previous work has demonstrated the three-wave interactions between the surface and backward volume spin waves as well as the excitation of chaotic spin waves through those interactions. In principle, the three-wave interactions of pure backward volume spin waves are also possible at low magnetic fields. In practice, these interactions have never been observed experimentally. The second half of this talk will present our recent experimental observation on the three-wave interactions of backward volume spin waves and the excitation of chaotic spin waves through the three-wave processes. As in the four-wave process experiments, the three-wave interaction experiments were also carried out with a feedback ring structure, and the chaotic natures of the signals were confirmed by the broad power-frequency spectra and the saturation of correlation dimensions.