Phasons, amplitude modes, and spin waves in the amplitude-modulated magnetic phase of PrNi2Si2

Abstract

The magnetic excitations in the low-temperature amplitude-modulated magnetic structure of PrNi${}_{2}$Si${}_{2}$ have been investigated by inelastic neutron scattering. The dispersions and intensities of both longitudinal and transverse excitations are measured along the high-symmetry directions. The modulated magnitude of the ordered moments implies that the longitudinally polarized magnetic excitations are more intense and dispersive than the usual transverse spin waves. Several well-defined longitudinal amplitude modes are observed to coexist with the longitudinal phason mode. The experimental results are in good overall agreement with predictions from the random-phase approximation, using parameters already established from the macroscopic properties and the paramagnetic excitations. At low energies in the neighborhood of the magnetic zone center, the magnetic phason appears to hybridize with an unidentified dispersionless mode.