Ground‐ and excited‐state spin waves in PrAl2

The magnetic excitations in ferromagnetic PrAl2 have been studied by means of neutron inelastic scattering. Branches of excitations corresponding to four of the six allowed magnetic‐dipole transitions have been observed. At the magnetic zone boundary, where the excitation spectrum most closely approximates a single‐ion spectrum, the observed frequencies are 1.54±0.04, 2.05±0.05 and 3.76±0.06 THz. The results are analyzed in terms of exchange and crystal‐field parameters using the pseudoboson theory for all states of the J=4 manifold. The crystal‐field‐only ground state is found to be a non‐magnetic doublet but the exchange interaction is strong enough to induce almost the full moment at low temperatures. At elevated temperatures in the ordered phase, excited‐state spin waves have been observed at 0.55±0.10 THz. These correspond to a transition between the upper member of the exchange‐split ground doublet, and the second member of the exchange‐split triplet state.