Macroscopic and microscopic investigation of the antiferromagnetic phase of TbB6

The antiferromagnetic state of TbB$_6$ has been investigated by means of magnetic susceptibility, magnetostriction and X-ray diffraction measurements on a single crystal. The anisotropy of the magnetic susceptibility and the magnetostriction data point to a tetragonal symmetry in the antiferromagnetic state. Below T$_N$, the X-ray diffraction investigation shows charge satellites, related to the $\langle\frac{1}{2} 0 0\rangle$ and $\langle\frac{1}{2} \frac{1}{2} 0\rangle$ wave vectors stars, similar to those observed in GdB$_6$. The $\langle\frac{1}{2} 0 0\rangle$ satellites clearly result from the formation of static atomic displacements waves.\\ The displacive phenomenon is described as a compromise between the exchange coupling and a single-ion elastic energy. This mechanism is shown to be responsible for the recurrence of the $\langle\frac{1}{4} \frac{1}{4} \frac{1}{2}\rangle$ magnetic wave vectors in rare-earth hexaborides. A combined analysis of experimental and theoretical data points to only two stable tetragonal models for TbB$_6$ antiferromagnetic structure.\\ This study confirms the influence of exchange displacements in rare-earth hexaborides and highlights the need for a systematic investigation of the magnetic anisotropy of antiferromagnets.