Unconventional spin fluctuations in the hexagonal antiferromagnet YMnO ₃

We used inelastic neutron scattering to show that well below its Néel temperature, T_N, the two-dimensional (2D) XY nearly triangular antiferromagnet YMnO₃ has a prominent central peak associated with 2D antiferromagnetic fluctuations with a characteristic lifetime of 0.55(5) ps, coexisting with the conventional long-lived spin waves. Existence of the two time scales suggests competition between the Néel phase favored by weak interplane interactions, and the Kosterlitz-Thouless phase intrinsic to the 2D XY spin system.