Magnetic excitations in neodymium,a sinusoidally modulated antiferromagnet

We report the first unambiguous observations of the magnetic excitations in neodymium metal, which exhibits complex antiferromagnetic structures below T_N = 20 K. The wavevector and temperature dependence of the excitations have been determined from neutron inelastic scattering measurements made at the Institut Laue-Langevin. Two district, low-energy, magnetic modes are observed for the [100] basal plane direction (TM) whilst there is only a single mode in the c-direction (TA) These modes are only observed clearly at low temperatures (T?8 K): they broaden and soften as the temperature is raised through T_N. Measurements were also made of the longitudinal and transverse acoustic phonon spectra in neodymium. The predictions of current theories of the excitations in sinusoidally modulated magnetic structures are discussed and compared with the results of our observations.