Glassy Spin Freezing and Gapless Spin Dynamics in a Spatially Anisotropic Triangular Antiferromagnet Ag2MnO2

Using elastic and inelastic neutron scattering techniques, we show that upon cooling a spatially anisotropic triangular antiferromagnet Ag${}_2$MnO${}_2$ freezes below $T_f\sim 50$ K into short range collinear state. The static spin correlations are extremely two-dimensional, and the spin fluctuations are gapless with two characteristic relaxation rates that behave linearly with temperature.