Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.

Neutron and x-ray diffraction studies show that the simultaneous first-order transition to an orthorhombic and antiferromagnetic (AFM) ordered state in BaFe2As2 splits into two transitions with Co doping. For Ba(Fe0.953Co0.047)2As2, a tetragonal-orthorhombic transition occurs at TS=60 K, followed by a second-order transition to AFM order at TN=47 K. Superconductivity occurs in the orthorhombic state below TC=17 K and coexists with AFM. Below TC, the static Fe moment is reduced along with a redistribution of low energy magnetic excitations indicating competition between coexisting superconductivity and AFM order.

We use magnetic long range order as a tool to probe the Cooper pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron diffraction experiments, in particular for the unusual behavior of the magnetic order below $T_{c}$, demonstrates the robustness of our conclusions. Our findings strongly suggest that superconductivity is unconventional in all members of the iron arsenide family.Comment: 3 figures and 4 pages; final version as published

Neutron and x-ray diffraction studies of Ba(Fe1−xMnx)2As2 for low doping concentrations (x 0.176) reveal that at a critical concentration, 0.102 < x < 0.118, the tetragonal-to-orthorhombic transition abruptly disappears whereas magnetic ordering with a propagation vector of ( 1 2 1 2 1) persists. Among all of the iron arsenides this observation is unique to Mn-doping, and unexpected because all models for "stripe-like" antiferromagnetic order anticipate an attendant orthorhombic distortion due to magnetoelastic effects. We discuss these observations and their consequences in terms of previous studies of Ba(Fe1−xT M x)2As2 compounds (T M = Transition Metal), and models for magnetic ordering in the iron arsenide compounds. PACS numbers: 74.70.Xa, 74.25.Dw Recent systematic neutron and x-ray diffraction studies of underdoped Ba(Fe 1−x Co x ) 2 As 2 superconductors have revealed fascinating results regarding the interactions among structure, magnetism and superconductivity. The undoped AEFe 2 As 2 parent compounds (AE = Ba, Sr, Ca) manifest simultaneous transitions from a high-temperature paramagnetic tetragonal phase to a low-temperature orthorhombic antiferromagnetic (AFM) structure.

Magnetic correlations in the paramagnetic phase of CaFe 2 As 2 ͑T N = 172 K͒ have been examined by means of inelastic neutron scattering from 180 K ͑ϳ1.05T N ͒ up to 300 K ͑1.8T N ͒. Despite the first-order nature of the magnetic ordering, strong but short-ranged antiferromagnetic ͑AFM͒ correlations are clearly observed. These correlations, which consist of quasielastic scattering centered at the wave vector Q AFM of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer ͑ប Ͼ 60 meV͒. The L dependence of the scattering implies rather weak interlayer coupling in the tetragonal c direction corresponding to nearly two-dimensional fluctuations in the ͑ab͒ plane. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. Similar to the cobalt-doped superconducting BaFe 2 As 2 compounds, these experimental features can be adequately reproduced by a scattering model that describes short-ranged and anisotropic spin correlations with overdamped dynamics.

Inelastic neutron scattering measurements on the low energy spin waves in CaFe 2 As 2 show that the magnetic exchange interactions in the Fe layers are exceptionally large and similar to the cuprates. However, the exchange between layers is ~10% of the coupling in the layers and the magnetism is more appropriately categorized as anisotropic threedimensional, in contrast to the two-dimensional cuprates. Band structure calculations of the spin dynamics and magnetic exchange interactions are in good agreement with the experimental data. PACS: 75.30.Ds, 78.70.Nx, 75.30.Et, However, the parent phases of the iron-arsenides are not insulators. Rather, they are metallic and, for the AFe 2 As 2 compounds, the AF ordering is strongly coupled to a structural transition from a high-temperature tetragonal structure to a low temperature orthorhombic structure.[12] One other notable difference between the cuprates and iron arsenides concerns the conditions necessary for SC. While doping charge carriers does indeed suppress AF and lead to superconductivity in both systems, it has recently been shown that pressure alone can destroy the AF state in CaFe 2 As 2 and lead to SC. [13,14] 3 Despite these differences, the energy scale and dimensionality (or anisotropy) of the magnetic interactions may actually be quite similar, possibly leading to a common origin for SC in these two families of compounds. In order to move beyond qualitative comparisons and address the relevance of magnetic interactions to SC in the ironarsenides, direct measurements of the energy scale and anisotropy of the magnetic interactions are necessary. Here we report results from inelastic neutron scattering from CaFe 2 As 2 , both below and above the AF ordering temperature, and demonstrate that the magnetic exchange interactions are exceptionally large, with a similar energy scale as the cuprates. Although the magnetic exchange between the Fe layers is relatively small (> ~10% of the in-plane exchange), it is substantially larger than that found for the cuprates (~0.001%). This anisotropic 3D magnetism is supported by theoretical calculations of the spin dynamics. Despite the first-order magnetostructural transition observed in CaFe 2 As 2 , spin correlations are observed to persist above the AF ordering temperature, attesting to the strength of the magnetism and supportive of a model of frustrated magnetism in the high-temperature tetragonal phase.CaFe 2 As 2 is a non-superconducting parent compound that becomes superconducting by either doping [15] or the application of pressure.[14] CaFe 2 As 2 orders into a columnartype AF structure (as shown in Fig 1a)) with a simultaneous structural transition from a tetragonal (I4/mmm) to an orthorhombic (Fmmm) crystal structure below T s = 172 K with a = 5.51 Å, b = 5.45 Å, and c = 11.66 Å.[12] For the inelastic neutron scattering study, single crystals of CaFe 2 As 2 were grown out of Sn flux using conventional high temperature solution growth techniques described previously. scattering plane (in orthorhombic...

Abstract. Neutron diffraction studies of Ba(Fe 1-x Co x ) 2 As 2 reveal that commensurate antiferromagnetic order gives way to incommensurate magnetic order for Co compositions between 0.056 < x < 0.06. The incommensurability has the form of a small transverse splitting (0, ±ε, 0) from the nominal commensurate antiferromagnetic propagation vector Q AFM = (1, 0, 1) (in orthorhombic notation) where ε ≈ 0.02 − 0.03 and is composition dependent. The results are consistent with the formation of a spin-density wave driven by Fermi surface nesting of electron and hole pockets and confirm the itinerant nature of magnetism in the iron arsenide superconductors. 2Unconventional superconductivity is often associated with the pairing of electrons via spin fluctuations that appear close to a magnetic ordering instability. In this respect, the nature and origin of the magnetic instability itself is an important ingredient of any theory of superconductivity. In the iron arsenide compounds, the magnetism has been discussed from two limits; an itinerant and a local moment limit. The parent AEFe 2 As 2 -based superconductors (AE = Ca, Sr, Ba) are antiferromagnetic (AFM) metals, which suggests that an itinerant description is an appropriate starting point. AFM order is observed with a commensurate magnetic propagation vector Q AFM = (1, 0, 1) (expressed in orthorhombic notation) in a variety of iron arsenide compounds by neutron and x-ray resonant magnetic diffraction. [1][2][3][4][5][6][7][8][9] The small ordered moments measured in these systems (< 1 µ B ) also favor an itinerant description. In principle, the propagation vector of the AFM order itself, Q AFM , should further strengthen the case for itinerant magnetism, as both band structure calculations [10,11] and angle-resolved photoemission data [12][13][14] display Fermi surface nesting between electron and hole pockets with a nesting vector close to Q AFM . Here we define an itinerant spin-density wave (SDW) as magnetic order resulting from an instability due to Fermi surface nesting, with the best known example being the incommensurate (IC) SDW order observed in Cr metal.[15] However, the commensurate (C) AFM order observed at Q AFM can also be described within a local moment picture that may become relevant in the presence of moderately large electronic correlations and can be quantified, for example, in terms of the J 1 -J 2 Heisenberg model whereDetailed band structure calculations of the magnetic susceptibility in the iron arsenides predict that the Fermi surface nesting condition can result in either C-SDW order at Q AFM , or IC-SDW order with a propagation vector τ = Q AFM + ε where ε is a small incommesurability. [17,18] Although the observation of IC magnetic order with a propagation vector similar to that predicted by band structure calculations would clearly favor an itinerant SDW description of the AEFe 2 As 2 system, detailed magnetic diffraction studies have observed only C-AFM order with a propagation vector Q AFM in several

Large single crystals of fcc iron (y phase) were grown in situ and used to study the lattice dynamics of this phase of Fe by standard inelastic neutron scattering techniques. The phonon dispersion curves were measured along the [00$], [g'0], and [gg] symmetry directions at 1428 K. A selected number of phonon frequencies were also obtained at 1227 and 1640 K. We find that the measured dispersion curves are qualitatively similar to those of Ni and Ni03Feo 7 The 1428-K data were used to evaluate the elastic constants, the phonon density of states, and the lattice specific heat of y-Fe.