First Order Phase Transition to Strongly Anisotropic Ferromagnetism in Tl₂Mo₆Te₆

The magnetic susceptibility, magnetization, specific heat and electrical resistivity were measured on the new material Tl2Fe6Te6. All the data show a very sharp anomaly revealing a phase transition to ferromagnetism at Tc ≈ 220 K. The presence in the structure of one-dimensional metallic cluster ¦Fe3¦1∞ is evidenced by a very strong magnetic anisotropy in the ferromagnetic state, showing an interesting intermediate situation between a pure linear chain and 3-dimensional iron. Furthermore, the unusual sharpness of the transition leads us to anticipate a first-order phase transformation, but the lack of detectable thermal hysteresis was surprising. The molecular field model proposed by Bean and Rodbell in order to account for the similar behaviour of MnAs gives a close representation of our magnetic data. Thus, a pronounced dependence of the exchange interaction upon interatomic spacing may well be the dominant mechanism leading to the observed phenomena