Paramagnons in the PrNi system with an induced magnetic moment

The recent studies of the PrNi system have shown that, at a temperature slightly higher than the ferromagnetic ordering temperature (T{sub C} {approx} 21 K), softening of some part of magnetic excitations occurs near the {gamma} point of the Brillouin zone. The magnetic modes observed have been qualitatively described by a model taking into account the crystal field and the exchange interaction within the mean-field random-phase approximation. To refine the model parameters and obtain a complete set of excitation modes, inelastic neutron scattering measurements on PrNi single crystals and polycrystals have been performed at T = 23 K. An acoustic branch related to the level with high excitation energy has been observed for the first time. The character of its dispersion suggests that the low-energy acoustic mode is responsible for the magnetic phase transition.