Castaing instabilities in spin-echo experiments

The effects of spin-wave instabilities on theta - Delta t - pi spilt-echo e xperiments in spin-polarized normal Fermi fluids are investigated. Using a linear stability analysis, the helical configuration of the magnetization d ensity which occurs in such experiments is shown to be unstable against lon g-wavelength perturbations-an example of the Castaing instability of the Le ggett equation. For large values of the spin rotation parameter, mu M much greater than 1, and large tip angles, perturbations develop into large ampl itude oscillations which destroy the helix before the transverse magnetizat ion decays by diffusion. This, of course, affects the interpretation of spi n echo data, The nonlinear dynamics of the instabilities in spin echo exper iments and their effects on spin-echo data are studied rising computer simu lations. The results are compared with the linear analysis and summarized i n a mu M -theta diagram which indicates when the standard Leggett-Rice spin -echo analysis breaks down.