Determination of frustrated and non-frustrated magnetic structures of hexagonal and orthorhombic TbPdAl

TbPdAl is of particular interest, because different heat treatment allows to transform it from a geometrically frustrated hexagonal modification to a non-frustrated orthorhombic modification. Using powder neutron diffraction, we have determined the complex magnetic structures of the two TbPdAl modifications, both with two successive phase transitions at TN1 ≈ 43 K and TN2 ≈ 22 K. In orthorhombic TbPdAl, the magnetic Tb3+ ions form a three-dimensional network of zigzag chains running along the a-axis and the b-axis. The magnetic structure below TN1 corresponds to a propagation vector k1 = [0.271(1), 0.523(1), 0] with incommensurate modulations along all chain directions. At TN2 a lock-in transition to a commensurate structure with k2 = [0, 1/2, 0] occurs. Large Tb moments of 8.6(1) μB at saturation are ordered along the chains with a “+−” sequence (a-axis) and a “++−−” sequence (b-axis). In hexagonal TbPdAl, the magnetic Tb3+ ions form a triangular lattice in the basal plane and geometrically frustrated magnetic structures are observed. Two third of non-frustrated Tb moments order at TN1 with a commensurate structure (k1 = [1/2, 0, 1/2]) and a large saturation value of 8.7(2) μB. One third of frustrated Tb moments change from a commensurate structure with k1 below TN1 to a purely incommensurate structure with k2i = [0.50(1), 0.12(1), 1/2] below TN2. A strong sample dependence is observed for the very unusual magnetic properties of the frustrated Tb moments.