Magnetic anisotropy in hole-doped superconducting Ba0.67K0.33Fe2As2 probed by polarized inelastic neutron scattering

We use polarized inelastic neutron scattering ( INS) to study spin excitations of optimally hole-doped superconductor Ba0.67K0.33Fe2As2 (T-c = 38 K). In the normal state, the imaginary part of the dynamic susceptibility, chi ''(Q,omega), shows magnetic anisotropy for energies below similar to 7 meV with c-axis polarized spin excitations larger than that of the in-plane component. Upon entering into the superconducting state, previous unpolarized INS experiments have shown that spin gaps at similar to 5 and 0.75 meV open at wave vectors Q = (0.5,0.5,0) and (0.5,0.5,1), respectively, with a broad neutron spin resonance at E-r = 15 meV. Our neutron polarization analysis reveals that the large difference in spin gaps is purely due to different spin gaps in the c axis and in-plane polarized spin excitations, resulting in a resonance with different energy widths for the c-axis and in-plane spin excitations. The observation of spin anisotropy in both optimally electron- and hole-doped BaFe2As2 is due to their proximity to the AF ordered BaFe2As2 where spin anisotropy exists below T-N. DOI: 10.1103/PhysRevB.87.081101