Continuous magnetic and structural phase transitions in Fe1+yTe

We report a sequence of continuous phase transformations in iron telluride, Fe1+yTe (y~0.1), which is observed by combining neutron diffraction, magnetic susceptibility, and specific heat measurements on single crystal samples. While a gradual increase of magnetic scattering near the wave vector (0.5, 0, 0.5) is seen below T = 70 K, a temperature where the discontinuous first order magneto-structural phase transition is found in systems with small y (< 0.06), the reduction of the lattice symmetry in Fe1.1Te only occurs at Ts = 63 K. Below TN = 57.5 K the long-range magnetic order develops, whose incommensurate wave vector Qm varies with temperature. Finally, at Tm ~ 45 K the system enters the low-T phase, where Qm is locked at (0.48, 0, 0.5). We conclude that these instabilities are weak compared to the strength of the underlying interactions, and we suggest that the impact of the Fe interstitials on the transitions can be treated with random-field models.