Successive magnetic phase transitions in α-Tb₂S ₃ studied by neutron diffraction technique

Neutron diffraction measurements on a powder sample of α-Tb ₂S₃ were performed. This material consists of a stacking of square lattice Tb1 and triangular lattice Tb2 planes and exhibits two magnetic phase transitions at ∼3.5 and ∼12 K. It was found that the Tb1 and Tb2 moments behave quite independently. The Tb1 moments show a collinear antiferromagnetic ordering at ∼12 K with the magnetic unit cell same as the chemical one. On the other hand, the Tb2 moments start to order at ∼3.5 K without affecting the magnetic order of the Tb1 moments. The magnetic structure is noncollinear with a magnetic unit cell double of the chemical one along the a- and c-axes. The ordered moments of the Tb1 and Tb2 moments are ∼8μ_B, similar to the full moment for the free Tb³⁺ ion. These results indicate that the Tb1 and Tb2 moments are completely decoupled due to an orbital contribution, which probably gives rise to a noncollinear structure in the triangular lattice plane. The results are discussed in comparison with those of the related system α-Gd ₂S₃, which has no orbital degree of freedom.