Phase transitions and magnetostructural couplings in Fe1+yTe

Fe1+yTe, the parent compound of the chalcogenide superconductors displays a complex phase diagram with several structural and magnetic phase transitions within the homogeneity range 0.06 ≤ y ≤ 0.15. Recent experiments suggest that, Fe1+yTe behaves differently than the pnictide counterparts: it exhibits an antiferromagnetic order that is not driven by Fermi surface nesting and a relatively large ordered moment on the Fe sublattices. Here, we show that the magnetic and structural phase transitions are extremely sensitive to such tuning parameters as Fe composition and application of external pressure. Further, for y 0.12, upon cooling, a continuous magnetic transition takes place at temperatures above the structural transition. In such cases we identify a magnetically inhomogeneous precursor state above the Néel temperature by using Mössbauer spectroscopy. The observed quasi-static moments with short-range correlations can be understood based on a phenomenological Landau theory as a soliton-liquid state that precedes the low-temperature incommensurate helimagnetic phase. This helimagnetic phase undergoes a further magneto-elastic transition at lower temperatures which can be interpreted as a lock-in transition, where the magnetic transformation from the helical into collinear structure drives a structural first-order transition. The succession of the different phases suggests a strong magnetostructural coupling in Fe1+yTe.
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