Neutron scattering investigation of the temperature dependence of long-wavelength spin waves in ferromagnetic Rb₂CrCl₄

The long-wavelength spin waves in Rb₂CrCl₄, a nearly two-dimensional ferromagnet, have been investigated at several temperatures below T_c=52.4K using neutron inelastic scattering techniques. The data have been analysed in terms of a Hartree-Fock theory using matching-matrix elements to give correctly the effects of anisotropy. Values for the parameters in the spin Hamiltonian have been found, and the theory accounts well for the energy renormalisation of the spin waves and for the transition temperature and variation of magnetic moment with temperature. Due to weak uniaxial anisotropy terms the spin wave spectrum has a small energy gap of approximately=1K at zero wavevector, which precludes true XY-type behaviour, and a canting of the spins of +or-1.1 degrees from (110) directions is predicted.