Inelastic Neutron Scattering Investigation of the Crystal Field Excitations of NdCo5

We present an inelastic neutron scattering study of the crystal electric field (CEF) levels in the intermetallic ferrimagnets RECo5 (RE=Nd and Y). In NdCo5, measurements at 5K reveal two levels at approximately 28.9 and 52.9 meV. Crystal-field calculations including the exchange field B-exc from the Co sites account for both of these, as well as the spectrum at temperatures above the spin-reorientation transition at similar to 280 K. In particular, it is found that both a large hexagonal crystal field parameter A(6)(6)< r(6)> and B-exc are required to reproduce the data, with the latter having a much larger value than that deduced from previous computational and experimental studies. Our study sheds light on the delicate interplay of terms in the rare-earth Hamiltonian of RECo5 systems, and is therefore expected to stimulate further experimental and computational work on the broader family of rare-earth permanent magnets.