Spin-wave dispersion and sublattice magnetization in NiCl2

Abstract

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 T_N=52.3K. The sublattice magnetization has been measured from 1.5K to T_N. 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 J_NN=21.70K and J_NNN=-4.85K (H=-2 Sigma _i>jJ_ijS_i.S_j). 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 T_N are properly predicted.