SPIN-WAVE INSTABILITIES IN HE-3-HE-4 SOLUTIONS AT HIGH MAGNETIC-FIELD

During our recent high field pulsed NMR experiments on a spin-polarize d saturated He-3-He-4 solution, we observed a long-lived oscillating s ignal in addition to the usual spin-echo. The oscillations were excite d by the application of a single RF tipping pulse of angle greater tha n some critical value. They were obseravble only for temperatures belo w 20mK corresponding to values of the spin-rotation parameter mu M(0) greater than or equal to 3. The signals were not present during experi ments on lower concentration solutions. The frequencies of the oscilla tions were found to scale as the cosine of the tipping angle. The natu re and shape of the oscillations suggest that they may be driven by in stabilities through a mechanism proposed by Castaing. These instabilit ies arise due to a modification of the non-linear term in the Leggett equation by the large magnetization gradients present at the edges of the pulsed region.