Identifying further inelastic neutron crystal field transitions in ErNiAl₄

The orthorhombic, intermetallic series RNiAl₄ (R = rare earth) exhibits interesting magnetic behaviour, including the potential for low temperature, inverse, magnetic cooling. Given that the RNiAl₄ magnetism is associated solely with the R sub-lattice and is influenced strongly by the local crystal field (CF) interaction at the R-site, it is important that the CF interaction be characterised. However, the R site's local, orthorhombic (C_2v) symmetry requires nine CF parameters to be determined. Early CF characterization attempts reported in the literature were based on the observed bulk, single-crystal, magnetic anisotropy but ignored the important rank 6 terms. Our first inelastic neutron scattering (INS) measurements were performed on Er³⁺ (J = 15/2) in ErNiAl₄ at the Helmholtz Zentrum Berlin's NEAT time-of-flight instrument. Despite the limited incident neutron energy range, well-defined transitions were observed for the first three (3, 7.4 and 11.4 meV) of the 7 excited Kramers doublets. A semi-empirical approach was applied in a first effort to interpret these data in terms of a full CF Hamiltonian. We report here on our more recent, on-going, INS measurements that exploit the thermal energies and polarisation neutron capability of the Triple Axis Spectrometer (C5) facility at the Canadian Neutron Beam Centre in Chalk River. In addition to the low energy transitions observed at HZB there already appear to be further peaks at 15.5, 21 and 42 meV. (author)