We performed elastic neutron-scattering and magnetization measurements on ${\text{Fe}}_{1.07}{\text{Te}}_{0.75}{\text{Se}}_{0.25}$ and ${\text{FeTe}}_{0.7}{\text{Se}}_{0.3}$. 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 ${\text{FeTe}}_{0.7}{\text{Se}}_{0.3}$, 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.

• Received 19 June 2009

DOI:https://doi.org/10.1103/PhysRevB.80.104506