Changes of the magnetic interactions in TlCo2Se2 upon metal substitution

Various solid solutions TlCo2−xMexSe2 (Me=Fe, Ni and Cu) have been investigated by neutron powder diffraction, supplemented by magnetometry. The incommensurate spin-helix running along the c-axis in tetragonal TlCo2Se2 prevails for low concentrations of copper and iron but changes pitch. In the copper case, only cobalt carries a magnetic moment. On nickel substitution, however, collinear antiferromagnetic coupling between the ferromagnetic layers occurs. The magnetic moment distribution between the two transition metals in the solid solution TlCo2−xNixSe2 was tentatively probed with first principle calculations on fictive ordered TlCoNiSe2, modelled by two types of superstructures. Also the ternary mother compounds, Pauli paramagnetic TlNi2Se2 and antiferromagnetic TlCo2Se2, were investigated with the same LMTO method.