Nonlinear magnetostatic spin waves: induced modulational instability and envelope solitons

Abstract

An experimental investigation of nonlinear magnetostatic spin wave (MSW) excitations in low loss yttrium iron garnet (YIG) films has been made. In particular, the induced modulational instability of MSW excitations and the formation of spin wave envelope (SWE) solitons were observed. For this investigation, a frequency filtered active resonant ring (ARR) and an inductive magnetodynamic probe (IMP) system were developed. The experiments utilized GHz MSW excitations with wave numbers of approximately 100 rad/cm.

A frequency filtered ARR structure consists of a YIG film delay line with a specially tailored frequency characteristic connected in a feedback loop with a microwave amplifier. At high ring gains a comb of frequencies is self generated. The proper choice of filtering, combined with the nonlinear dispersive properties of the ring, allowed for the self generation of stable trains of SWE solitons. The primary nonlinear mechanism responsible for such soliton formation was the induced modulational instability of the self generated MSW excitations. In the experiments, this technique allowed for the observation of SWE bright and dark soliton trains for volume and surface MSW excitations, respectively. The amplitude, phase and frequency characteristics were used to confirm the soliton nature of the output pulses. This observation agrees with the predictions of the nonlinear Schrodinger (NLS) equation model.

The newly developed IMP system consists of a high resolution scanning inductive probe used in conjunction with a YIG film delay line. The IMP system allowed for a time and space resolved investigation of linear and nonlinear MSW excitations. In the experiments, SWE solitons formed due to the induced modulational instability of co-propagating MSW excitations were investigated. Bright and dark SWE solitons were observed for volume and surface MSW excitations, respectively. In addition, bright nonlinear pulses were observed for surface MSW excitations and dark nonlinear pulses were observed for volume MSW excitations. The NLS equation model forbids such formation. The amplitude, phase, and frequency characteristics for these nonlinear pulses were observed. Furthermore, a spatial recurrence phenomenon was observed for bright soliton trains formed from volume MSW excitations. Simulations based on the NLS equation model agreed well with the spatial recurrence experiments.