Chemical tuning of the interlayer magnetic coupling in TlCo2Se2-xSxTlCo2Se2-xSx

The system TlCo2Se2-xSxTlCo2Se2-xSx has been thoroughly investigated by neutron powder diffraction and SQUID magnetometry. TlCo2Se2-xSxTlCo2Se2-xSx is a layered tetragonal structure containing atomic cobalt layers separated by a distance of 6.4 Å in the sulphide and 6.8 Å in the selenide. The solid solubility of isovalent selenium and sulphur atoms in the structure makes it possible to continuously vary the interlayer distance and thereby tune the magnetic coupling between the Co-layers. At low temperatures, the Co-atoms are ferromagnetically ordered within the layers and magnetic moments lie in the abab-plane. However, these Co-moments form a helical magnetic structure that prevails for 0⩽x⩽1.50⩽x⩽1.5 with a gradual decrease of the angle between adjacent Co-layers from 122° to 39°. For x⩾1.75x⩾1.75, a collinear ferromagnetic structure is stable. The relationship between the coupling angle and the Co-interlayer separation shows an almost linear behaviour. The helical phase contains no net spontaneous magnetic moment up to TlCo2SeS, where a small net magnetic moment appears that increases until the ferromagnetic structure is found for 1.75⩽x⩽2.01.75⩽x⩽2.0.