  Pulsar spin frequencies and their time evolution are an important source of
information on compact stars and their internal composition. Oscillations of
the star can reduce the rotational energy via the emission of gravitational
waves. In particular unstable oscillation modes, like r-modes, are relevant
since their amplitude becomes large and can lead to a fast spin-down of young
stars if they are saturated by a non-linear saturation mechanism. We present a
novel mechanism based on the pronounced large-amplitude enhancement of the bulk
viscosity of dense matter. We show that the enhanced damping due to non-linear
bulk viscosity can saturate r-modes of neutron stars at amplitudes appropriate
for an efficient spin-down.
