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 $Z_{\mathrm{anion}}$ from the Fe plane; $Z_{\mathrm{Sb}}$ for CuFeSb is $\sim$1.84 Å, much larger than $Z_{\mathrm{As}}$ (1.31–1.51 Å) in FeAs compounds and $Z_{\mathrm{Te}}$ ($\sim$1.77 Å) in Fe${}_{1+y}$Te. 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 $T_{\mathrm{c}}=375$ 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.

Figure 1

Neutron powder diffraction spectra ($+$) of CuFeSb and the results of FullProf. refinements (solid line) at 400 K (a) and 20 K (b). The difference between the experimental data and the fitting line is shown at the bottom in both (a) and (b). Tick marks indicate nuclear Bragg diffraction peak positions of CuFeSb/minor impurity phases (FeSb and Fe${}_3$O${}_4$) and magnetic diffraction peaks of CuFeSb/minor Fe${}_3$O${}_4$ phase.

Figure 2

Crystal structure of CuFeSb.

Figure 3

(a) Magnetization as a function of temperature $M$($T$) of CuFeSb measured with ZFC and FC histories. (b) Magnetization as a function of magnetic field $M$($H$) of CuFeSb at 2 and 100 K. Left inset: the temperature dependence of resistivity. Right inset: the magnetic hysteresis loop at 2 K.

Figure 4

The ordered magnetic moment as a function of temperature extracted from the magnetic structure refinements of neutron powder diffraction spectra for CuFeSb. Inset: (001) diffraction peak at various temperatures.