Magnetic excitations in terbium antimonide

The magnetic-excitation spectrum of the single-ground-state antiferromagnet TbSb has been studied by neutron inelastic scattering. At 4.4 K, the dispersion relations for the two lowest branches of the spectrum were determined, and evidence was also obtained for transitions between the ground state and the seventh and ninth excited states of the ${\mathrm{Tb}}^{3+}$ ion. Measurements were also carried out at several other temperatures both below and above $T_N$ (14.9 ± 0.2 K). At 4.4 K, the lowest branch has a large energy gap (frequency 0.54 ± 0.04 THz) caused by crystalline-field effects. The gap decreases as the temperature is raised and near $T_N$ the spin-wave scattering merges with a broad distribution of quasielastic critical scattering whose intensity reaches a maximum at $T_N$. Above $T_N$, the critical scattering gradually decreases with increasing temperature. The quasielastic scattering at the zone boundary has a somewhat larger frequency width than that at the zone center and also exhibits a weak shoulder in the temperature range 20-40 K. This behavior is inconsistent with current theories based on truncated-energy-level schemes, which predict that there should be a well-defined mode in the disordered phase whose frequency tends to the crystal-field splitting $\Delta$ at high temperature and to zero as $T\to T_N$. Exchange and crystal-field parameters for TbSb have been obtained by analyzing the results at 4.4 K in terms of a pseudoboson theory which takes into account transitions between the ground state and all 12 excited states of the lowest spin-orbit multiplet of the ${\mathrm{Tb}}^{3+}$ ion ($J=6\mathrm{}$). A very good description of the observed frequencies and intensities is obtained.