Unconventional spin fluctuations in the hexagonal antiferromagnet YMnO${}_3$

Abstract

We used inelastic neutron scattering to show that well below its Néel temperature, $T_{\mathrm{N}}$, the two-dimensional (2D) XY nearly-triangular antiferromagnet YMnO${}_3$ has a prominent {\it central peak} associated with 2D antiferromagnetic fluctuations with a characteristic life time 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.

4pages, 5figures