Measurement of the magnetic response function in UAs

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Abstract
Neutron inelastic scattering has been used to determine the imaginary part of the susceptibility chi "(Q,h(cross) omega ) in uranium arsenide. The measurements were performed on a time-of-flight spectrometer using a polycrystalline sample. In the paramagnetic state a broad (in energy) spectrum is found which can be characterised as a Lorentzian centred at zero energy with a half width 1/2 Gamma =17+or-3 meV. No evidence is found for any crystal-field levels below 40 meV. At the ordering temperature (TN=126K) the overall susceptibility decreases, as expected, but the dynamical part may still be characterised as a Lorentzian whose half width decreases gradually to 8+or-3 meV at 5K. Such a response in the ordered state, in which an apparently localised moment of 2.2 mu B resides on each U atom, is quite surprising, but is in agreement with experiments on single crystals in which no well defined excitations were found. At all temperatures the susceptibility peaks at the magnetic zone centre, showing the dominance of antiferromagnetic correlations. The results are discussed in terms of the systematics of the uranium rocksalt compounds and the need for realistic calculations of the ground state.