Inelastic neutron scattering investigation of the crystal field excitations of NdCo${}_5$

We present an inelastic neutron scattering study of the crystal electric field levels in the intermetallic ferrimagnets RECo${}_5$ (RE = Nd and Y). In NdCo${}_5$, measurements at $5$K reveal two levels at approximately 28.9 and 52.9 meV. Crystal field calculations including the exchange field $B_{\text{exc}}$ from the Co sites account for both of these, as well as the spectrum at temperatures above the spin-reorientation transition at $\sim 280$~K. In particular, it is found that both a large hexagonal crystal field parameter $A_6^6⟨r^6⟩$ and $B_{\text{exc}}$ 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 RECo${}_5$ systems, and is therefore expected to stimulate further experimental and computational work on the broader family of rare-earth permanent magnets.