Using neutron inelastic scattering, we investigate the low-energy spin fluctuations in ${\mathrm{Fe}}_{1+x}\mathrm{Te}$ as a function of both temperature and interstitial iron concentration. For ${\mathrm{Fe}}_{1.057\left(7\right)}\mathrm{Te}$, the magnetic structure is defined by a commensurate wave vector of ($\frac{1}{2},0,\frac{1}{2}$). The spin fluctuations are gapped with a sharp onset at 7 meV and are three dimensional in momentum transfer, becoming two dimensional at higher-energy transfers. On doping with interstitial iron, we find, in ${\mathrm{Fe}}_{1.141\left(5\right)}\mathrm{Te}$, the ordering wave vector is located at the ($0.38,0,\frac{1}{2}$) position and the fluctuations are gapless with the intensity peaked at an energy transfer of 4 meV. These results show that the spin fluctuations in the ${\mathrm{Fe}}_{1+x}\mathrm{Te}$ system can be tuned not only through selenium doping, but also with interstitial iron. We also compare these results with superconducting concentrations and, in particular, the resonance mode in the ${\mathrm{Fe}}_{1+x}{\mathrm{Te}}_{1-y}{\mathrm{Se}}_y$ system.

• Received 31 March 2011

DOI:https://doi.org/10.1103/PhysRevB.84.045124