Magnetic excitations in uranium nitride

The magnetic inelastic scattering from uranium nitride in the antiferromagnetic and paramagnetic phases has been measured by neutron spectroscopy. The magnetic response at low temperatures is found to be spread over a considerable frequency range and to have longitudinal polarization. No sharp transverse spin-wave modes are observed even though uranium nitride orders antiferromagnetically. With the raising of the temperature towards the Neel temperature, a second longitudinal component of inelastic scattering centered on zero frequency grows rapidly and dominates the low-frequency response. However, at frequencies beyond 5--6 THz there is virtually no change in the observed magnetic scattering