Interstitial Iron Effects on Magnetic Excitations in Parent Phases Fe$_{1+x}$Te from Polarized and Inelastic Neutron Scattering

One of the simplest systems of the iron-based superconducting family, Fe$_{1+x}Ch$ (where $Ch$ = S, Se, or Te) presents ample opportunity to study the relationship between antiferromagnetism and superconductivity. Several studies have demonstrated how the makeup of the $Ch$ anions changes the electronic properties drastically, but the effect of excess interstitial iron, the $x$ in Fe$_{1+x}Ch$, is not as well understood. Our previous diffraction experiments on samples varying $x$ from 4 \% to 16 \% demonstrated how the magnetic ordering changes from collinear antiferromagnetic to helical incommensurate via a spin-density wave state at the special composition of $x \approx 12\%$. We present inelastic neutron scattering measurements of the phases Fe$_{1+x}$Te for two amounts of interstitial iron in the lattice, 5\% and 14 \%. We have combined data from cold neutron triple-axis, thermal neutron triple-axis, and spallation source time-of-flight to provide a full picture of the magnetic excitations in Fe$_{1+x}$Te for $x=14 \%$ from 0.5 meV to 150 meV. In addition, we present polarized inelastic studies on this particular composition to investigate the nature of the spin waves, $i.e.$ longitudinal $vs.$ transverse. The results are compared with those found in the phase with low amounts of interstitial iron ($\approx$ 5 \%), in order to understand the nature of the exchange interactions in this important parent compound.