Continuous magnetic and structural phase transitions in Fe _1+yTe

We report a sequence of continuous phase transformations in iron telluride, Fe _1+yTe with y=0.10(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 T70 K, a temperature where the discontinuous first-order magnetostructural phase transition is found in systems with small y (0.06), the reduction of the lattice symmetry in Fe _1.1Te only occurs at T _s63 K. Below T _N 57.5 K, the long-range magnetic order develops, the incommensurate wave vector Q _m of which varies with temperature. Finally, at T _m45 K, the system enters the low-T phase, where Q _m 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.