Neutron Diffraction Study of the Magnetic Long-Range Order in Tb

Like other heavy rare-earth metals, Tb exhibits a magnetic phase with a spiral structure. This appears within the temperature region from 216 to 226^∘{}K between the ferromagnetic phase and the paramagnetic phase. The transition between ferromagnetic and spiral structure is of first order and implies no change in the long-range order. In the close vicinity of the N\'eel temperature TN=226^∘K the spiral magnetic long-range order varies as (TN-T)^0.25±0.01, whereas the total order within a wider temperature range roughly follows (TN-T)1/3. The turn angle per layer varies from 16.5^∘{} at 216^∘{}K to 20.7^∘{} at 226^∘{}K. The temperature variation of the transverse magnetostriction has also been measured and was found to vary approximately in proportion to the square of the magnetic long-range order.