Magnetic excitations in Ho₂Co₁₇ and Ho₂Fe₁₇. An inelastic neutron scattering study

The low energy part (<20 meV) of the magnetic excitation spectrum of the uniaxial easy basal plane ferrimagnets Ho₂Co₁₇ and Ho₂Fe₁₇ have been measured along the three high symmetry directions at a temperature of 4.2 K, using the inelastic neutron scattering technique. The resulting magnon dispersion relations have been interpreted using linear spin wave theory with a Hamiltonian including single ion crystal field anisotropy and isotropic exchange between spatially well localized spins, i.e. we have used a localized pseudo spin description of the magnetism of the itinerant 3d-ions. The R₂T₁₇ structure contains two different Ho sites, with the same point symmetry, and from the spin wave results it was concluded that the crystal field anisotropy of the two Ho sites in both Ho₂Co₁₇ and Ho₂Fei₁₇ were identical. The deduced crystal field parameters for Ho₂Fe₁₇ were slightly larger than for Ho₂Co₁₇, and the parameters were of the same order of magnitude as for pure Ho. For Ho₂Fe₁₇ the Fe-Fe exchange was found to be anisotropic, and for both compounds the magnetic ordering temperatures T_c of 1178 K for Ho₂Co₁₇ and 335 K for Ho₂Fe₁₇ were determined by the strong positive 3d-3d exchange. The rare earth - 3d exchange constant, which is responsible for the ferrimaqnetic coupling scheme was observed to be small and neqative, and within experimental uncertainty identical for the two compounds. The rare earth - rare earth exchange in both cases found to be negligible, and consequently, a non dispersive crystal field like excitation was observed. Using the parameters deduced from the linear spin-wave fit to the observed magnon dispersive relations, the temperature dependence of the non dispersive modes and the magnetization curve (the latter only for Ho₂Co₁₇ because data was not available for Ho₂Fe₁₇) could be predicted and was found to agree with experiments. The
low temperature (4.2 K) macroscopic anisotropy constants predicted from the deduced crystal field parameters were an order of magnitude larger than the experimentally observed values.