Magnetic inelastic scattering in uranium nitride

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Abstract
Of the cubic uranium pnictides, uranium nitride has the smallest U-U separation, the smallest magnetic moment (0.75 µB) and the lowest Néel temperature (TN = 49.6 ± 0.3 K). It is of interest to know the nature of the excitation spectrum of UN to gain an understanding of the electronic structure of the uranium ion and of actinide ions in general. Such questions as whether the 5f electrons are localized or itinerant for the metallic pnictides have not yet been answered. We report neutron inelastic scattering measurements at 4.2 K from a single crystal of UN which reveal magnetic scattering that is weak in intensity and differs in symmetry and nature from the sharp, low-velocity, largely transverse spin waves of the 4f cubic compounds. The magnetic response at the (110) magnetic reciprocal lattice point is a broad (FWHM = 5 ± 1 THz) distribution of intensity peaked at 4.2 ± 0.5 THz. Constant-frequency-transfer scans for ν between 2.5 and 8.0 THz show a single peak centred on the (110) point. The results indicate that UN possesses a steeply rising branch of excitations with an anisotropy gap. Measurements near (330) where the form factor is half that at (110) confirmed the magnetic character of the scattering. The symmetry of the magnetic response is primarily longitudinal. Intensity was observed where transverse scattering is anticipated, i.e. near (010), but the scattering was somewhat sharper in frequency and more than a factor two weaker than at (110). The transverse component at (110) is therefore less than 25 % of the observed response. The effect on the scattering near (110) of raising the temperature to 0.8 TN is to enhance the low-frequency response strongly (by a factor of 4 at ν = 1.5 THz) so as to give a broad distribution of inelastic scattering centred on ν = 0. At high frequencies this distribution is similar in form to the high-frequency part of the 4.2 K distribution. The results provide the first direct experimental evidence for high-velocity longitudinal spin excitations in ordered actinide compounds, and have important consequences for the theory of itinerant magnetism

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