New aspects of the magnetic excitations in HoCo2

We have obtained new results about the magnetic excitation spectrum of Ho${\mathrm{Co}}_2$ using inelastic neutron scattering. It has been possible to observe at 6 K a well-defined cobalt excitation mode up to 14 THz. The application of a magnetic field along the easy magnetization axis has revealed the longitudinal character of the excitation at 2 THz observed at 6 K. New modes below and above the rotation temperature have been measured. These results have been analyzed in a Green's-function formalism using the random-phase approximation to determine the crystalline electric field (CEF) and the exchange parameters. Despite the use of an extra quadrupolar term for the crystalline field it has not been possible to get a satisfactory agreement for the dispersion curves obtained, together with the macroscopic measurements. However, the values obtained $W=0.2983\mathrm{}$ K and $x=-0.4839\mathrm{}$ for the CEF parameters are in agreement with those determined by other authors. The exchange parameters have been determined as $J_{\mathrm{H}\mathrm{o}-\mathrm{H}\mathrm{o}}=0\mathrm{}\pm 0.0005$ THz, $J_{\mathrm{H}\mathrm{o}-\mathrm{C}\mathrm{o}}=-0.07\mathrm{}$ THz, and $J_{\mathrm{C}\mathrm{o}-\mathrm{C}\mathrm{o}}=9\mathrm{}$ THz. The main discrepancy concerns $J_{\mathrm{H}\mathrm{o}-\mathrm{C}\mathrm{o}}$ which may be determined from the gap of the cobalt mode or from the molecular field when the CEF transition levels are fitted regardless of the cobalt mode. The discrepancy suggests that it might be necessary to take into account a mixture of $3d$ and $4f$ magnetism at the very level of the exchange mechanism, including the itinerant character of the $3d$ electrons.