SPIN-WAVES AND TEMPERATURE-DEPENDENT BEHAVIOR OF THE QUASI-2-DIMENSIONAL ANTIFERROMAGNET KFEF4

A study of the quasi-two-dimensional antiferromagnet KFeF4 below the transition temperature, T(N) = 13 K, has been undertaken using neutron scattering techniques. The dispersion relation of the spin excitations is well described by a model Hamiltonian incorporating Heisenberg exchange interactions and uniaxial anisotropy. The anisotropy controls the energy of the zone centre mode whose frequency varies in the same way with temperature as the square of the staggered magnetization which is described by the two-dimensional Ising model. Dipole-dipole contributions are too small to account fully for the anisotropy. Different nearest-neighbour exchange energies are found along the a and b directions due to different superexchange paths, with J1 = -2.18 meV along a, and J2 = -2.73 meV along b.