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 Tc 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.
|Place of Publication||Roskilde|
|Publisher||Risø National Laboratory|
|Number of pages||85|
|Publication status||Published - 1981|
|Series||Denmark. Forskningscenter Risoe. Risoe-R|