Spin-Wave Dispersion and Sublattice Magnetization in NiCl_2

NiCl2 is a Heisenberg planar antiferromagnet composed of hexagonal ferromagnetic Ni2+ sheets with effective XY symmetry weakly coupled antiferromagnetically to adjacent Ni2+ sheets. The near two-dimensionality dimples a directionally-dependent spin-wave renormalization together with an unusual temperature dependence on the sublattice magnetization, gap energy and specific heat. The authors report an inelastic neutron scattering study of the spin waves both at low temperatures and, for selected q-vectors, for temperatures up to TN=52.3K. The sublattice magnetization has been measured from 1.5K to TN. A renormalized spin-wave theory, which does not contain any assumptions about the temperature dependence of the energy gap, has been developed. The authors deduce JNN=21.70K and JNNN=-4.85K (H=-2 Sigma i>jJijSi.Sj). Using these interaction constants and the renormalized spin-wave theory, the temperature-dependent dispersion relations (together with the sublattice magnetization) and the gap energy up to approximately 0.4 TN are properly predicted.