What does it look like?
Thulium (Tm, Z=69) is a seemingly unremarkable- albeit uncommon- element lurking towards the far end of the rare earth group. At room temperature it has a common hexagonal close packed structure. The chemical unit cell is shown in the bottom of the image.
However, at 4.2K the magnetic moments are aligned in an alternating 3-4-3-4 arrangement along the c- axis (up the screen). This means that the magnetic unit cell in the c- axis is seven times the chemical unit cell. The magnetic structure was determined by single crystal neutron scattering.
This is one of a rich panoply of magnetic structures discovered in the rare earth elements.
Where does it come from?
The chemical structure of Thulium is 9010996 in the Crystallography Open Database. The figure was produced with VESTA.
Reference: Koehler et. al., Phys. Rev. 126 (1962) 1672