Magnetic Excitations in Thulium

We have observed, for the first time, magnetic excitations in a single crystal of thulium, using inelastic neutron spectroscopy. In the ferrimagnetic phase three modes with energies around 4 meV, 8 meV and 15 meV were clearly seen. We have considered our results in terms of crystal field level schemes for thulium. The rare earth metal thulium (hcp, electronic configuration 4f12) exhibits a particularly interesting sequence of magnetic phases. Below TN = 56 K, the moments are sinusoidally modulated along the hexagonal axis [I, 21. This structure "squares up" at lower temperatures (below 40 K) as the amplitude begins to exceed the free-ion moment of 7 p ~ . Measurements of the temperature dependence of the magnetic periodicity show that there is a first-order lock-in transition at 32 K to an exact 7-layer periodicity. Below this temperature the structure is ferrimagnetic and comprises 4 layers of moments parallel to the aaxis, followed by 3 antiparallel layers. The ferrimagnetic coupling persists to the lowest temperatures. We performed a first study of the magnetic excita tions in a single crystal of thulium using the tripleaxis spectrometer TAS 3 at Riso National Laboratory. The single crystal was prepared by J. S. Abell at the Department of Metallurgy, University of Birmingham. Measurements were made at 12 K (in the ferrimagnetic phase) and at higher temperatures. The instrument was operated in the constant-kp mode (kp= 2.662 A-' . Figure 1 shows an energy scan for ) Q = (1, 0, 1) as a function of temperature. There is clear evidence for a mode at about 3.5 meV. The energy of the peak changes little with increasing temperature, whereas the intensity steadily decreases. At 80 K, above the NBel temperature, the magnetic scattering has all been shifted towards the elastic line and manifests itself as a broad tail indistinguishable from the elastic scattering. Further studies were made at the Institute LaueLangevin using the IN8 triple-axis spectrometer. The 1 experimental configuration (k~=constant =2.662 A, collimation: 40' 60' 60') was comparable to that at Riso. We performed constant-Q scans along the [001] and [loll directions. For some scans at high energy transfer we also used the Cull1 monochromator with k~ = 4.189~-l. Figure 2 shows a scan at 5 K for Q = (1, 0, 2.5) with energy transfers up to 20 meV. Three modes at 4.3 meV, 8 meV and 15.3 meV were Thulium 0 = (1.0.1)