We have studied the magnetic ordering in Na doped BaFe2âAs2â by
unpolarized and polarized neutron diffraction using single crystals. Unlike
previously studied FeAs-based compounds that magnetically order,
Ba1âxâNaxâFe2âAs2â exhibits two successive magnetic transitions: For
x=0.35 upon cooling magnetic order occurs at âź70\ K with in-plane magnetic
moments being arranged as in pure or Ni, Co and K-doped BaFe2âAs2â samples.
At a temperature of âź46\ 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
Ba0.65âNa0.35âFe2âAs2â point along the c direction. Magnetic
correlations in these materials cannot be considered as Ising like, and
spin-orbit coupling must be included in a quantitative theory.Comment: 5 pages, 4 figure