Ground- and excited-state spin waves in PrAl2

The magnetic excitations in ferromagnetic PrAl2 were 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