Spin reorientation in Ba $_{0.65}$ Na $_{0.35}$ Fe $_{2}$ As $_{2}$ studied by single-crystal neutron diffraction

摘要

We have studied the magnetic ordering in Ba1−xNaxFe2As2 with 0.25≤x≤0.4 by unpolarized and polarized neutron diffraction using single crystals. Unlike most FeAs-based compounds that magnetically order, Na-doped BaFe2As2 exhibits two successive magnetic transitions: For x=0.35, upon cooling, magnetic order occurs at ∼70 K with in-plane magnetic moments being arranged as in pure or Ni-, Co-, or K-doped BaFe2As2 samples. At a temperature of ∼46 K a second phase transition occurs, which the single-crystal neutron-diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in Ba0.65Na0.35Fe2As2 point along the c direction. The two nearly degenerate magnetic states document orbital degeneracy to persist in the superconducting phase.