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 Néel temperature $T_N=226\mathrm{}°$K the spiral magnetic long-range order varies as ${(T_N-T)}^{0.25\pm 0.01}$, whereas the total order within a wider temperature range roughly follows ${(T_N-T)}^{\frac{1}{3}}$. 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.