Magnetic excitations in thulium metal

We have performed inelastic neutron scattering measurements on a single-crystal specimen of Tm at wave vectors κ=(1, 1,ζ) and (0,0,2+ζ) (ζ=0,.,1). Most of the measurements have been made at T=5 K, where Tm exhibits a seven-layer ferrimagnetic-antiphase-domain structure (four moments up, parallel to the c axis, followed by three moments down). At this temperature the excitation spectra consist of three peaks. The two lower energy excitations have been identified as originating from magnetovibrational scattering from the TA phonon, while the higher energy excitation is magnetic and exhibits only a weak dispersion (between 8.3 and 9.6 meV). At T=50 K, a temperature at which the system exhibits a c-axis sinusoidally modulated structure, the magnetic mode shows significant softening and broadening. The magnetovibrational scattering vanishes above the Néel temperature (T _N=58.5 K) while the magnetic mode persists at least up to T=70 K. These results suggest that the Hamiltonian in this system is dominated by the crystal-field-anisotropy energy and that the exchange interaction is relatively weak.