  Ag2MnO2 is studied as a possible candidate compound for an antiferromagnetic
XY spin model on a triangular lattice. In spite of the large Curie-Weiss
temperature of -430 K found in magnetic susceptibi-lity, Mn3+ spins with S = 2
do not undergo a conventional long-range order down to 2 K probably owing to
the geometrical frustration and two dimensionality in the system. Instead, a
unique phase transition is found at 80 K, where specific heat exhibits a clear
sign of a second-order phase transition, while magnetic susceptibility changes
smoothly without a distinct anomaly. We think that this transition is related
to the chirality degree of freedom associated with a short-range order, which
has been expected for the classical XY spin model on a triangular lattice. On
further cooling, spin-glass-like behavior is observed below 22 K, possibly
corresponding to a quasi-long-range order.
