Short-range incommensurate magnetic order near the superconducting phase boundary in Fe1+δ Te1-x Sex

We performed elastic neutron-scattering and magnetization measurements on Fe1.07 Te0.75 Se0.25 and FeTe0.7 Se0.3. Short-range incommensurate magnetic order is observed in both samples. In the former sample with higher Fe content, a broad magnetic peak appears around (0.46,0,0.5) at low temperature, while in FeTe0.7 Se0.3, the broad magnetic peak is found to be closer to the antiferromagnetic (AFM) wave vector (0.5,0,0.5). The incommensurate peaks are only observed on one side of the AFM wave vector for both samples, which can be modeled in terms of an imbalance of ferromagnetic/antiferromagnetic correlations between nearest-neighbor spins. We also find that with higher Se (and lower Fe) concentration, the magnetic order becomes weaker while the superconducting temperature and volume increase.