Magnetic anisotropy in hole-doped superconducting Ba 0.67K 0.33Fe 2As2 probed by polarized inelastic neutron scattering

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We use polarized inelastic neutron scattering (INS) to study spin excitations of optimally hole-doped superconductor Ba₀.₆₇K₀.₃₃Fe₂As₂ (Tc=38 K). In the normal state, the imaginary part of the dynamic susceptibility, χ′′(Q,ω), shows magnetic anisotropy for energies below 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 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 Eᵣ=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 resonance with different energy widths for the c-axis and in-plane spin excitations. The observation of spin anisotropy in both opitmally electron and hole-doped BaFe₂As₂ is due to their proximity to the AF ordered BaFe₂As₂ where spin anisotropy exists below Tₙ.