Magnetic orders, excitations, and phase transitions in Fe1+yTe

Abstract

We study the magnetic properties of Fe1+yTe, a parent compound of the iron-based high-temperature superconductors. Motivated by recent neutron scattering experiments, we show that a spin S=1 exchange model, supplemented by a single-ion spin anisotropy, accounts well for the experimentally observed low-temperature magnetic phase diagram, that exhibits a commensurate bicollinear order at low Fe dopings (yâ‰0.12) and an incommensurate spin-spiral order at high Fe dopings (yâ‰0.12). We suggest that the commensurate-incommensurate transition at yâ‰0.12 is due to the competition between the exchange interaction and the local spin anisotropy. At low Fe dopings, the single-ion spin anisotropy is strong and pins the spins along the easy axis, which, together with the spatially anisotropic exchanges, induces a unusual bicollinear commensurate magnetic order. The low-energy spin-wave excitation is gapped due to the explicit breaking of spin-rotational symmetry by the local spin anisotropy. At high Fe dopings, the single-ion anisotropy is weak, and the exchange favors an incommensurate coplanar state. The incommensurate magnetic wave vector averages out the spin anisotropy so that a gapless low-energy spin-wave excitation is obtained. We also analyze the low-energy hydrodynamic model and use it to describe the magnetostructural transition and the static and dynamical spin structure factors across the magnetic ordering transitions. © 2013 American Physical Society.