We have measured the bulk properties as well as the elastic and inelastic neutron scattering of ${\mathrm{YMnO}}_3$ 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.

• Received 28 June 2002

DOI:https://doi.org/10.1103/PhysRevB.68.104426