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

We report our elastic and inelastic neutron scattering measurements on a spatially-anisotropic triangular antiferromagnet metal compound of Ag$_2$MnO$_2$. Upon cooling, it undergoes a structural phase transition at 540 K from trigonal to monoclinic due to a ferro-orbital order of the Jahn-Teller active Mn$^3+$ ion resulting in spatially anisotropic magnetic interactions in the triangular plane. Despite the large T$_CW$ $\sim$ 400 K, it does not order down to T$_f$ = 48(6) K below which the Mn spins freeze into a collinear spin state with the frozen moment, M = 2.4(2) $\mu_B$/Mn << gs$\mu_B$/Mn, and the frozen spin order is short ranged indicating extreme two-dimensional magnetic interaction. Moreover, the two-dimensional spin fluctuations have a gapless spectrum with two characteristic relaxation rates, an overall relaxation rate that behave linearly above freezing temperature . We argue that Ag$_2$MnO$_2$ might be an excellent candidate for a gapless spin liquid phase.