Neutron spectroscopy in the cerium monopnictides

Neutron inelastic scattering experiments have been performed on polycrystalline samples of the cerium monopnictides CeX (X=P, As, Sb, Bi). The energy-level structure of the lowest multiplet 2 F 5/2 of the Ce 3+ ion has been determined at temperatures below 293K. The level splitting can be reproduced by a single-ion Hamiltonian that takes into account crystal-field, bilinear and quadrupolar pair interactions. The magnetic moments align parallel to the cubic (001) axis. The result gives an excellent description of the measured zero-field magnetisation of CeP and CeAs. In the paramagnetic state, the crystal-field splitting of CeP and CeAs increases by 15% with decreasing temperature.