Change from antiferromagnetic to ferromagnetic order in the pseudo-ternary series TbNi1−xCuxAl

Abstract In TbNiAl, which adopts the hexagonal ZrNiAl-structure, the Tb-site forms a triangular lattice in the a - b plane. The coupling between nearest neighbours was shown to be antiferromagnetic, giving rise to the formation of frustrated moments of Tb. TbNiAl has two antiferromagnetic phases which differ in the ordering of the frustrated spins. On the other hand, TbCuAl is a ferromagnet. It is shown by means of neutron diffraction that the stoichiometric substitution of Ni by a few per cent of Cu leads to an instant suppression of antiferromagnetism. The ordering temperature increases from T N = 47 K for TbNiAl to T c = 55 K for TbNi 0.90 Cu 0.10 Al since the number of frustrated bonds is strongly reduced. In the range 0.01 ⩽ x ⩽0.10, surprsingly, both ferro- and antiferromagnetic types of order coexist in different domains. The volume fraction with antiferromagnetic order decreases with both increasing x in different samples and with increasing temperature below T c in one sample. Compounds in the range 0.1 x ⩽1.0 are ferromagnets but at intermediate concentrations 0.60 ⩽ x ⩽0.80, a dominat short-range magnetic order is observed. The results are discussed with a local magnetic exchange coupling between Tb spins.