Magnetic ordering and spin-liquid state of YMnO₃

We have measured the bulk properties as well as the elastic and inelastic neutron scattering of YMnO₃ in order to understand the static and dynamic properties of the Mn moments. Our measurements clearly show that above T_N there are short range correlations between spins at the nearest neighbor and next nearest neighbor Mn sites, which also fluctuate in time. This, together with other bulk properties, demonstrates the presence of a spin liquid phase above T_N arising from the geometrically frustrated Mn moments. Below T_N, a well-defined spin wave develops and we can understand the experimentally measured spin wave spectrum in terms of a Heisenberg Hamiltonian with a small easy plane anisotropy. However, even in the ordered phase we have found evidence of short range spin correlations.