Superstructures and magnetic order in heavily Cu-substituted (Fe1-xCux)1+yTe

Most iron-based superconductors exhibit stripe-type magnetism, characterized by the ordering vector Q=(12,12). In contrast, Fe1+yTe, the parent compound of the Fe1+yTe1-xSex superconductors, exhibits double-stripe magnetic order associated with the ordering vector Q=(12,0). Here, we use elastic neutron scattering to investigate heavily Cu-substituted (Fe1-xCux)1+yTe compounds and reveal that (1) for x0.4, short-range magnetic order emerges around the stripe-type vector at Q=(12±δ,12±δ,12) with δ≈0.05; (2) the short-range magnetic order is associated with a superstructure modulation at Q=(13,13,12), with the magnetic correlation length shorter than that for the superstructure; and (3) for x0.55, we observe an additional intergrown phase with higher Cu content, characterized by a superstructure modulation vector Q=(13,13,0) and magnetic peaks at Q=(23,13,12)/(13,23,12). The positions of superstructure peaks suggest that relative to the tetragonal unit cell of Fe1+yTe, heavy Cu substitution leads to Fe-Cu orderings that expand the unit cell by 2×32 times in the ab plane, corroborated by first-principles calculations that suggest the formation of spin chains and spin ladders. Our findings show that stripe-type magnetism is common in magnetically diluted iron pnictides and chalcogenides, despite the varying associated atomic orderings.