The observed transition of the magnetic structure from a noncollinear state at x=0 to a ferromagnetic phase at x=1.75, through a series of spin spirals in the substitutional TlCo2Se2-xSx alloys, has been investigated by first principles theory. Our calculations successfully reproduced the magnetic structure of TlCo2S2 and the obtained magnetic moment is somewhat higher (0.82 mu(beta) per Co atom) than the experimental value. However, TlCo2Se2 was found to possess an antiferromagnetic state which is inconsistent with the observed spin spiral. We show that the correct magnetic structure of TlCo2Se2 can be obtained if the distance between the Co layers is reduced. Moreover, we demonstrate that the modification of the Co interlayer distance is a crucial parameter that governs the nature of the magnetism in these alloys. We discuss a mechanism that could favor noncollinear states in TlCo2Se2 over the ferromagnetic configuration and explain the change of the magnetic structure in the substitutional TlCo2Se2-xSx alloys.
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