Atomic-scale coexistence of short-range magnetic order and superconductivity in Fe$_{1+y}$Se$_{0.1}$Te$_{0.9}$

The ground state of the parent compounds of many high temperature superconductors is an antiferromagnetically (AFM) ordered phase, where superconductivity emerges when the AFM phase transition is suppressed by doping or application of pressure. This behaviour implies a close relation between the two orders. Understanding the interplay between them promises a better understanding of how the superconducting condensate forms from the AFM ordered background. Here we explore this relation in real space at the atomic scale using low temperature spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy. We investigate the transition from antiferromagnetically ordered $\mathrm{Fe}_{1+y}\mathrm{Te}$ via the spin glass phase in $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.1}\mathrm{Te}_{0.9}$ to superconducting $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.15}\mathrm{Te}_{0.85}$. In $\mathrm{Fe}_{1+y}\mathrm{Se}_{0.1}\mathrm{Te}_{0.9}$ we observe an atomic-scale coexistence of superconductivity and short-ranged bicollinear antiferromagnetic order.