Spin reorientation transition in Na‐doped BaFe2As2 studied by single‐crystal neutron diffraction

Abstract

Several hole‐doped BaFe2As2 compounds were recently shown to exhibit a second magnetic phase transition in the concentration range close to the full suppression of antiferromagnetic (AFM) order. At this additional transition ordered magnetic moments reorient from in‐plane to out‐of‐plane alignment associated with a suppression of the orthorhombic distortion. We have studied the magnetic properties of such a representative hole‐doped system, Ba1–xNaxFe2As2 with 0.25 ≤ x ≤ 0.40, by neutron diffraction on large single crystals. With increasing Na substitution (0.25 ≤ x ≤ 0.39) the AFM transition temperature sharply decreases, while the spin‐reorientation transition temperature is rather constant, until both magnetic phases are completely suppressed at x = 0.40. For all studied Na concentrations the additional transition is related to the spin reorientation, which, however, is complete only in the middle of the concentration range of the out‐of‐plane phase. In the superconducting state, the intensities of magnetic Bragg reflections become heavily suppressed; this effect seems to increase for larger Tc and reaches ∼50% in Ba0.61Na0.39Fe2As2. In samples with coexisting in‐plane and out‐of‐plane ordering, this superconductivity induced suppression of ordered moments is significantly stronger for the out‐of‐plane components indicating that this phase more strongly competes with superconductivity.