Magnetic-crystallographic phase diagram of superconducting parent compound Fe$_{1+x}$Te

TL;DR

This study maps the magnetic and crystallographic phase diagram of Fe$_{1+x}$Te, revealing how interstitial iron influences magnetic orderings and phase separation in the superconducting parent compound.

Contribution

It provides detailed neutron diffraction data showing the evolution of magnetic order and phase separation as a function of interstitial iron concentration in Fe$_{1+x}$Te.

Findings

Identification of a short-range incommensurate spin-density wave at x ≈ 12%

Observation of a transition to incommensurate helical order with increased interstitial iron

Evidence of electronic phase separation around the critical iron concentration

Abstract

hrough neutron diffraction experiments, including spin-polarized measurements, we find a collinear incommensurate spin-density wave with propagation vector $ \mathbf k = $ ($0.4481(4) \, \,0 \, \, \frac1 2$) at base temperature in the superconducting parent compound Fe$_{1+x}$Te. This critical concentration of interstitial iron corresponds to $x \approx 12%$ and leads crystallographic phase separation at base temperature. The spin-density wave is short-range ordered with a correlation length of 22(3) \AA, and as the ordering temperature is approached its propagation vector decreases linearly in the H-direction and becomes long-range ordered. Upon further populating the interstitial iron site, the spin-density wave gives way to an incommensurate helical ordering with propagation vector $ \mathbf k =$ ($0.3855(2) \, \,0 \, \, \frac1 2$) at base temperature. For a sample with $x \approx 9(1) %$, we also find an incommensurate spin-density wave that competes with the bicollinear commensurate ordering close to the N\'eel point. The shifting of spectral weight between competing magnetic orderings observed in several samples is supporting evidence for the phase separation being electronic in nature, and hence leads to crystallographic phase separation around the critical interstitial iron concentration of 12%. With results from both powder and single crystal samples, we construct a magnetic-crystallographic phase diagram of Fe$_{1+x}$Te for $ 5% < x <17%$