Longitudinal spin fluctuations in thermal and magnetic properties of TlCo2Se2−xSx

At isovalent substitution of Se for S, the layered chalcogenides TlCo2Se2−xSx (0⩽x⩽2) exhibit a change of magnetic order type, from an antiferromagnetic helix at low S content to a ferromagnet at high S content. The effective magnetic moments peff∼2μB per Co, as estimated from the magnetic susceptibility at high temperatures, significantly exceed what would be the result of the Co magnetic moments μsat∼0.5μB in both the ferromagnetic and antiferromagnetic phases. This signifies the crucial role of longitudinal spin fluctuations in the magnetic behaviour of these conducting compounds. The spin fluctuations reveal themselves also in specific heat measurements, leading to a diminution of the expected anomalies at the Curie/Néel temperatures and to an excess of heat capacity at high temperatures as compared to phonon contributions.