This presentation will cover two topics in nonlinear spin wave dynamics, (1) Fermi-Pasta-Ulam recurrence and (2) random solitary waves.
The discovery of Fermi-Pasta-Ulam (FPU) recurrence in 1955 marked the birth of nonlinear science and ushered in the age of computational science. A magnetic film strip-based active feedback ring system has now been used to demonstrate bona fide FPU recurrence. At some threshold ring gain level, a wide spin wave pulse is self-generated in the ring. As this wide pulse circulates, it separates into two envelope solitons with different speeds. When the fast soliton catches up to and collides with the slow soliton, the initial wide pulse is perfectly reconstructed. The repetition of this soliton process leads to periodic recurrences to the initial wide pulse.
The random generation of coherent solitary waves from incoherent waves in a medium with an instantaneous nonlinearity has been observed. If one excites a temporal packet of incoherent spin waves in a magnetic film strip, a fundamentally new type of solitary wave, a random solitary wave (RSW), can be formed from the propagating wave packet. The RSW pulses materialize from the propagating wave packet randomly in time, randomly in position relative to the overall wave packet, and with a random amplitude. In spite of the incoherent nature of the propagating wave packet and the random nature of the generation process, the RSW pulses show coherent properties of the sort found for traditional solitary waves.
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