ChemInform Abstract: Magnetic Structure of TbMn6Ge6 from Neutron Diffraction Study

Investigations made by susceptibility measurements and neutron diffraction experiments have been performed on the ternary germanide TbMn 6 Ge 6 of HfFe 6 Ge 6 -type structure ( P6/mmm ). TbMn 6 Ge 6 orders ferrimagnetically at T c ≈ 427 K but an antiferromagnetic state occurs below about 410 K. Neutron diffraction study shows that both rare earth and manganese sublattices order simultaneously at room temperature. Over the whole 2–300 K temperature range the magnetic structure is a flat spiral consisting of ferromagnetic (001) sheets (with the moments in the layers) of Mn and Tb atoms respectively stacked along the c axis in the sequence Mn1TbMn2Mn1TbMn2. At 2 K, according to the modulation vector k = (0, 0, ∼0.125), the magnetic moments change their orientation by about 45° within the basal plane on going from one Tb sheet to another, while one observes a spiral turn angle θ 1 ≈ 31° between adjacent Mn1TbMn2 layers and θ 2 ≈ 14° between Mn2Mn1 layers, i.e. a repeat distance of about 65 A. The moment values are μ Tb = 8.43(27) μ B and μ Mn = 1.84(18) μ B . The magnetic behaviour of TbMn 6 Ge 6 is similar to that observed for DyMn 6 Ge 6 above 100 K. In the latter case our refinements lead us to propose a new arrangement for the helimagnetic structure previously published, with a Dy magnetic moment corrected to about 9 μ B at 2 K. The results confirm the interplay between the magnetic coupling and the unit cell volume observed in GdMn 6 Ge 6 . A discussion and some comparisons with the parent RMn 6 Sn 6 compounds are given in conclusion.