Ferromagnetism in CuFeSb: Evidence of competing magnetic interactions in iron-based superconductors

We have synthesized a new layered iron-pnictide CuFeSb. This material shares a similar layered tetragonal structure with iron-based superconductors, with Fe square planar sheets forming from the edge-sharing iron antimony tetrahedral network. CuFeSb differs remarkably from Fe-based superconductors in the height of anion Z _anion from the Fe plane; Z _Sb for CuFeSb is ∼1.84 Å, much larger than Z _As (1.31-1.51 Å) in FeAs compounds and Z _Te (∼1.77 Å) in Fe _1+yTe. In contrast with the metallic antiferromagnetic (AFM) or superconducting state of iron pnictides and chalcogenides under current studies, CuFeSb exhibits a metallic, ferromagnetic (FM) state with T _c=375 K. This finding suggests that the competition between AFM and FM coupling may exist in Fe-based superconductors and that the nature of magnetic coupling within the Fe plane is indeed dependent on the height of anion as predicted in theories.