Incommensurate magnetic order in TbTe3
In addition to hosting an incommensurate CDW, for magnetic rare earth ions, RTe3 also hosts long range antiferromagnetic order at low temperatures. The nature of the long-range magnetic order in this family of compounds has not yet been studied by diffraction techniques, and the effect of the incommensurate lattice modulation on the magnetic structure is unknown. To bridge this gap we have performed neutron scattering experiments in the representative compound TbTe3, which exhibits three closely located magnetic phase transitions at the following temperatures: Tmag1 = 5.78 K, Tmag2 = 5.56 K and Tmag3 = 5.38 K. We find that in the paramagnetic phase, slightly above Tmag1, there are pronounced 2D-like magnetic correlations. At Tmag1 long-range magnetic order emerges as a result of a continuous phase transition. In all three magnetically ordered phases, incommensurate modulations are present. This observation is at variance with the typical behavior of unmodulated rare-earth intermetallic compounds. Incommensurate magnetic structures often appear in these materials just below the phase transition into the magnetically ordered state. However, in most cases magnetoelastic coupling and crystal-field effects typically result in a lock-in transition to a commensurate magnetic structure. The behavior found in TbTe3 only partially resembles these general expectations. However, the magnetic Bragg peaks observed below Tmag1 near the position (0,0,0.24) (Fig. 3.16), stabilize at low temperature at the incommensurate position (0,0+δ,0.21). This effect is possibly due to the incommensurate lattice modulation present in TbTe3 in the CDW state. The propagation vectors of the magnetic structures, which we succeeded to identify in TbTe3, have components either along the <0,K,0> or <0,0,L> directions, or a linear combination of both. We could not assign any of the magnetic peaks, observed in powder diffraction pattern, as a Bragg reflection with non-zero component <H,0,0> of the propagation vector. Therefore, our results do not point towards a link of the propagation vector associated with the second CDW phase transition and of the magnetic order in TbTe3. Nonetheless, we discover that near the temperature Tmag1 the magnetic Bragg peaks appear around the positions (0,0,0.24) (or its rational multiples). This value is fairly close to the propagation vector (0,0,0.29) associated with the high-temperature CDW phase transition, raising the possibility that correlations leading to the long-range magnetic order in TbTe3 might be linked to the modulated chemical structure in the CDW state.