Brillouin light-scattering observation of the nonlinear spin-wave decay in yttrium iron garnet thin films

Abstract

The direct observation of nonlinear spin-wave decay in thin yttrium iron garnet (YIG) films is reported. The second-order parametric spin waves were excited by a series of nonlinear magnetostatic backward volume wave (MSBVW) pulses propagated in a microstrip structure at 5 GHz. Brillouin light-scattering techniques were used to detect spin waves in the YIG at a point along the propagation path. Data were obtained as a function of the input pulse duty cycle and peak power level. These data yielded the decay rate ${\ensuremath{\eta}}_{\mathrm{eff}}$ for the parametric spin waves vs input power ${P}_{\mathrm{in}}.$ This ${\ensuremath{\eta}}_{\mathrm{eff}}$ was equal to the usual spin-wave relaxation rate at low power, but decreased rapidly with increasing ${P}_{\mathrm{in}}.$ As ${P}_{\mathrm{in}}$ approached the threshold power for complete MSBVW soliton formation at the observation point, ${\ensuremath{\eta}}_{\mathrm{eff}}$ leveled off to a relatively constant value that was about a factor of 5--10 smaller than the low power value. The initial decrease in ${\ensuremath{\eta}}_{\mathrm{eff}}$ with power is due to the compensation of the usual spin-wave decay by the simultaneous parametric pumping of the spin waves by Suhl processes. The leveling off in ${\ensuremath{\eta}}_{\mathrm{eff}}$ at the soliton threshold is due to the spin-wave shedding that is needed to maintain order one eigenmode soliton propagation at high input power levels. These experiments demonstrate the reduction in the spin-wave decay rate with power below the threshold for spin-wave instability. This reduction is a key element in spin-wave instability theory that has never been observed directly. The data also demonstrate the enhanced production of parametric spin waves that accompanies microwave magnetic envelope soliton formation.