Magnetic excitations in neodymium antimonide

The magnetic excitation spectrum of the antiferromagnet NdSb (TN=13.6 K) has been studied by neutron inelastic scattering. At 4.9 K nearly all the spin-wave strength resides in the transition to the sixth excited state which originates from the exchange split Γ₈⁽²⁾ quartet ground state and Γ₈⁽¹⁾ quartet excited state of the Nd³⁺ ion. A weaker and lower-frequency transition between the ground state and the fourth excited state, the lower member of the exchange split Γ₆ doublet, has also been observed. These transitions give rise to spin-wave branches of transverse and longitudinal symmetry, respectively, whose frequencies, measured in this experiment in the 00ζ, ζζ0, and ζζζ directions, are surprisingly independent of wave vector. Measurements at higher temperatures show that there is surprisingly little downward renormalization ({~} 10%) of the frequency of the main branch of magnetic excitations on passing through the N\'eel point. The crystal-field parameters have been determined from the excitation spectra in the paramagnetic state. From the results at 4.9 K two bilinear exchange parameters and a large quadrupolar parameter have been obtained with the aid of a pseudoboson spin-wave theory that includes all nine excited states of the Nd³⁺ ion. A satisfactory description of the magnetic properties of NdSb is obtained from the model parameters.