  The spectrum of spin fluctuations in the stacked-triangular antiferromagnet
YMnO3 was studied above the Neel temperature using both unpolarized and
polarized inelastic neutron scattering. We find an in-plane and an out-of-plane
excitation. The in-plane mode has two components just above TN, a
resolution-limited central peak and a Debye-like contribution. The
quasi-elastic fluctuations have a line-width that increases with q like Dq^z
and the dynamical exponent z=2.3. The out-of-plane fluctuations have a gap at
the magnetic zone center and do not show any appreciable q-dependence at small
wave-vectors.
