  In this paper we study the effect of cosmic neutrino decoupling on the
spectrum of cosmological gravitational waves (GWs). At temperatures T>>1 MeV,
neutrinos constitute a perfect fluid and do not hinder GW propagation, while
for T<<1 MeV they free-stream and have an effective viscosity that damps
cosmological GWs by a constant amount. In the intermediate regime,
corresponding to neutrino decoupling, the damping is frequency-dependent. GWs
entering the horizon during neutrino decoupling have a frequency f ~ 1 nHz,
corresponding to a frequency region that will be probed by Pulsar Timing Arrays
(PTAs). In particular, we show how neutrino decoupling induces a spectral
feature in the spectrum of cosmological GWs just below 1 nHz. We briefly
discuss the conditions for a detection of this feature and conclude that it is
unlikely to be observed by PTAs.
