  A magnetic particle with atomic spins ordered in an unstable direction is an
example of a false vacuum that decays via excitation of internal spin waves.
Coupled evolution of the particle's magnetization (or the vacuum state) and
spin waves, considered in the time-dependent vacuum frame, leads to a peculiar
relaxation that is very fast at the beginning but slows down to a
nonexponential long tail at the end. The two main scenarios are linear and
exponential spin-wave instabilities. For the former, the longitudinal and
transverse relaxation rates have been obtained analytically. Numerical
simulations show that the particle's magnetization strongly decreases in the
middle of reversal and then recovers.
