Magnetic excitations in the longitudinally modulated spin structure of erbium metal

The magnetic excitation spectrum for the longitudinally modulated incommensurate spin structure of Er metal has been studied by inelastic neutron scattering. Since the Er spins order parallel to the $c$ axis of the hcp crystal structure, it is possible to separate the longitudinal and transverse spin fluctuations. We find no sharp spin waves for the transverse fluctuations owing to the spatial variations of the exchange and anisotropy energies. The longitudinal fluctuations, however, produce neutron scattering, at small wave vectors, which possesses rather sharp peaks that have a linear dispersion relation.