Abstract
Inelastic neutron scattering has been employed to study the magnetic excitations of Ho3+ in a grain-aligned sample of HoBa2Cu3O7. The lowest-lying branch exhibits dispersion for spin waves propagating in the (a, b)-plane, whereas excitations along the c-axis remain constant in energy. The data are analysed in the random phase approximation in terms of the Heisenberg model, resulting in an enormous spatial anisotropy of the magnetic coupling parameters, which are weakly ferromagnetic, antiferromagnetic and almost zero for nearest-neighbor Ho3+ pairs along the a, b, and c axes, respectively.