Unconventional spin fluctuations in the hexagonal antiferromagnet YMnO 3

We used inelastic neutron scattering to show that well below its N\'eel temperature, ${T}_{\mathrm{N}},$ the two-dimensional (2D) XY nearly triangular antiferromagnet ${\mathrm{YMnO}}_{3}$ 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 N\'eel phase favored by weak interplane interactions, and the Kosterlitz-Thouless phase intrinsic to the 2D XY spin system.