We have studied the magnetic ordering in Na doped BaFe2As2 by unpolarized and polarized neutron diffraction using single crystals. Unlike previously studied FeAs-based compounds that magnetically order, Ba1−xNaxFe2As2 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 and 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. Magnetic correlations in these materials cannot be considered as Ising like, and spin-orbit coupling must be included in a quantitative theory. PACS numbers:There are two promising explanations for the appearance of high-temperature superconductivity in FeAsbased materials [1]. Orbital fluctuations may result in a s ++ superconducting state [2,3] and can reflect the fact that highest superconducting transition temperatures arise in materials with almost ideal FeAs 4 tetrahedrons [4] and, thus, with highest orbital degeneracy. On the other hand, there are strong magnetic fluctuations associated with the antiferromagnetic (AFM) order in the parent compounds which can explain a s ± superconducting state [5].Magnetism and orbital degrees of freedom are closely tied in FeAs-based compounds. Although the structural distortion accompanying AFM order in the parent materials remains small [6][7][8] its electronic signatures are rather strong as seen in the anisotropic resistance [9,10], in angle-resolved photoemission studies (ARPES) [11,12] or optical spectroscopy [13]. In addition the magnon dispersion in the AFM state is fully anisotropic [14] inspiring theoretical models of orbital order driving magnetic interaction similar to those applied to manganates [15][16][17]. Studying the interplay between orbital and magnetic degrees of freedom seems crucial for the understanding of FeAs-based materials.Here we focus on spin-space anisotropies arising from the spin-orbit coupling between spin and orbital moments and which thus allow for a direct view on orbital contributions. In AFM BaFe 2 As 2 , polarized neutron scattering shows that it costs more energy to rotate the magnetic moment within the planes than perpendicular to them [18]. Magnetic anisotropy clearly persists into the superconducting range of the phase diagrams [19][20][21][22][23].So far all AFM ordered FeAs-based compounds exhibit a single magnetic transition to a magnetic structure where moments are aligned parallel to the in-plane

Two strong arguments in favor of magnetically driven unconventional superconductivity arise from the coexistence and closeness of superconducting and magnetically ordered phases on the one hand, and from the emergence of magnetic spin-resonance modes at the superconducting transition on the other hand. Combining these two arguments one may ask about the nature of superconducting spin-resonance modes occurring in an antiferromagnetic state. This problem can be studied in underdoped BaFe2 As2, for which the local coexistence of large moment antiferromagnetism and superconductivity is well established by local probes. However, polarized neutron scattering experiments are required to identify the nature of the resonance modes. In the normal state of Co underdoped BaFe2 As2 the antiferromagnetic order results in broad magnetic gaps opening in all three spin directions that are reminiscent of the magnetic response in the parent compound. In the superconducting state two distinct anisotropic resonance excitations emerge, but in contrast to numerous studies on optimum and over-doped BaFe2 As2 there is no isotropic resonance excitation. The two anisotropic resonance modes appearing within the antiferromagnetic phase are attributed to a band selective superconducting state, in which longitudinal magnetic excitations are gapped by antiferromagnetic order with sizable moment.

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.

The mechanism of Cooper pair formation in iron-based superconductors remains a controversial topic. The main question is whether spin or orbital fluctuations are responsible for the pairing mechanism. To solve this problem, a crucial clue can be obtained by examining the remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase. The enhancement is called spin resonance for a spin fluctuation model, in which their energy is restricted below twice the superconducting gap value (2Δs), whereas larger energies are possible in other models such as an orbital fluctuation model. Here we report the doping dependence of low-energy magnetic excitation spectra in Ba1−xKxFe2As2 for 0.5 < x < 0.84 studied by inelastic neutron scattering. We find that the behavior of the spin resonance dramatically changes from optimum to overdoped regions. Strong resonance peaks are observed clearly below 2Δs in the optimum doping region, while they are absent in the overdoped region. Instead, there is a transfer of spectral weight from energies below 2Δs to higher energies, peaking at values of 3Δs for x = 0.84. These results suggest a reduced impact of magnetism on Cooper pair formation in the overdoped region.

Spin-resonance modes (SRM) are taken as evidence for magnetically driven pairing in Fe-based superconductors, but their character remains poorly understood. The broadness, the splitting and the spin-space anisotropies of SRMs contrast with the mostly accepted interpretation as spin excitons. We study hole-doped Ba 1−x Na x Fe 2 As 2 that displays a spin reorientation transition. This reorientation has little impact on the overall appearance of the resonance excitations with a high-energy isotropic and a low-energy anisotropic mode. However, the strength of the anisotropic low-energy mode sharply peaks at the highest doping that still exhibits magnetic ordering resulting in the strongest SRM observed in any Fe-based superconductor so far. This remarkably strong SRM is accompanied by a loss of about half of the magnetic Bragg intensity upon entering the SC phase. Anisotropic SRMs thus can allow the system to compensate for the loss of exchange energy arising from the reduced antiferromagnetic correlations within the SC state.