PARAMETRIC-EXCITATION OF SPIN-WAVES IN FERROMAGNETS BY LONGITUDINAL PUMPING LOCALIZED IN-SPACE

The equations of motion for the slowly varying complex amplitudes of s pin waves parametrically excited by a localized pumping magnetic field have been derived. A solution of these equations satisfying given bou ndary and initial conditions has been obtained. The energy dissipated by spin waves decreases with the pumping intensity beyond a certain pu mping power, which can be termed the regeneration threshold. The losse s vanish and change sign at the instability threshold. Both thresholds depend heavily on the linear dimension L of the pumping zone, increas ing with decreasing L. Owing to the regeneration process, the dissipat ion length of spin waves increases without bound as the pumping power approaches the instability threshold. Consequently, perturbations of a uniform state due to the boundary penetrate throughout the pumping zo ne, regardless of the dimension L. As a result, the full pattern of pa rametric instability is strongly affected by the zone boundary: 1) the spatial distribution of wave amplitudes becomes nonuniform everywhere inside the zone; 2) the amplitude growth rate in the unstable regime decreases at all points when perturbations due to the boundary reach t hese points; 3) the instability threshold is independent of the spin-w ave frequency offset from the parametric resonance frequency. The calc ulated minimum instability threshold as a function of the bias magneti c field (the ''butterfly'' curve) changes shape with L, in agreement w ith the available experimental data. (C) 1997 American Institute of Ph ysics.