Magnetic excitations in neodymium antimonide

Abstract

The magnetic excitation spectrum of the antiferromagnet NdSb ($T_N=13.6\mathrm{}$ 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 ${\Gamma }_8^{\mathrm{}\left(2\right)\mathrm{}}$ quartet ground state and ${\Gamma }_8^{\mathrm{}\left(1\right)\mathrm{}}$ quartet excited state of the ${\mathrm{Nd}}^{3+}$ ion. A weaker and lower-frequency transition between the ground state and the fourth excited state, the lower member of the exchange split ${\Gamma }_6$ 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 $00\zeta$, $\zeta \zeta 0$, and $\zeta \zeta \zeta$ 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 Néel 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 ${\mathrm{Nd}}^{3+}$ ion. A satisfactory description of the magnetic properties of NdSb is obtained from the model parameters.