  By numerically solving the equations of motion for atomic spins we show that
internal spin-wave processes in large enough magnetic particles, initially in
unstable states, lead to complete magnetization reversal and thermalization.
The particle's magnetization strongly decreases in the middle of reversal and
then recovers. We identify two main scenarios, exponential and linear spin-wave
instabilities. For the latter, the longitudinal and transverse relaxation rates
have been obtained analytically. Orientation dependence of these rates leads to
a nonexponential relaxation of the particle's magnetization at long times.
