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

Inelastic neutron scattering measurements of paramagnetic SrCo2As2 at T = 5 K reveal antiferromagnetic (AFM) spin fluctuations that are peaked at a wavevector of Q AFM = (1/2, 1/2, 1) and possess a large energy scale. These stripe spin fluctuations are similar to those found in AFe2As2 compounds, where spin-density wave AFM is driven by Fermi surface nesting between electron and hole pockets separated by Q AFM . SrCo2As2 has a more complex Fermi surface and band structure calculations indicate a potential instability towards either a ferromagnetic or stripe AFM ground state. The results suggest that stripe AFM magnetism is a general feature of both iron and cobaltbased arsenides and the search for spin fluctuation-induced unconventional superconductivity should be expanded to include cobalt-based compounds.The AFe 2 As 2 compounds (A = Ca, Sr, Ba) are itinerant antiferromagnets (AFMs) where spin-density wave ordering is driven by Fermi surface nesting between electron and hole pockets [1]. The in-plane nesting vector Q AFM = (1/2, 1/2) describes a stripe AFM structure consisting of ferromagnetic (FM) chains of spins extending along the [1,1] direction with AFM alignment along [1, 1] [see Fig. 1(f)]. In Ba(Fe 1−x Co x ) 2 As 2 , electrondoping by the substitution of Co for Fe destabilizes the stripe AFM ordering by shrinking (enlarging) the hole (electron) pockets and detuning the nesting condition. Ultimately, the suppression of stripe AFM ordering upon Co substitutions of a few percent allows a superconducting ground state to appear in the presence of substantial spin fluctuations at Q AFM . Further Co substitutions (x > 12%) lead to a complete suppression of both stripe spin fluctuations [2,3] and superconductivity [4][5][6].The ACo 2 As 2 compounds with a full replacement of Fe by Co have garnered little attention. Initial experiments on BaCo 2 As 2 [7] and SrCo 2 As 2 [8] describe these materials as metals with enhanced paramagnetic susceptibility and no magnetic ordering or superconductivity down to 2 K. Band structure calculations find a large density-ofstates at the Fermi energy that is proposed to drive a ferromagnetic instability and enhanced paramagnetism [7,8]. Recent angle-resolved photoemission spectroscopy (ARPES) data on BaCo 2 As 2 [9, 10] and SrCo 2 As 2 [8] reveal a complex multi-band Fermi surface and, unlike the iron arsenides, no clear nesting features exist that might suggest an instability towards AFM ordering.In this Letter, we report the remarkable discovery that SrCo 2 As 2 is near an instability to stripe AFM order, not ferromagnetism, i.e. it adopts the same magnetic state as found in the tetragonal phase of iron arsenide-based parent and superconducting compounds. Inelastic neutron scattering (INS) was used to measure steeply dispersing and quasi-two-dimensional (2-D) paramagnetic excitations near Q AFM = (1/2, 1/2, 1) that share many similarities to AFe 2 As 2 , despite their dissimilar Fermi surface topologies. One notable difference is the opposite anisotropy of the longitudinal a...

The magnetic excitations in the paramagnetic-tetragonal phase of underdoped Ba(Fe 0.953 Co 0.047 ) 2 As 2 , as measured by inelastic neutron scattering, can be well described by a phenomenological model with purely diffusive spin dynamics. At low energies, the spectrum around the magnetic ordering vector Q AFM consists of a single peak with elliptical shape in momentum space. At high energies, this inelastic peak is split into two peaks across the direction perpendicular to Q AFM . We use our fittings to argue that such a splitting is not due to incommensurability or propagating spin-wave excitations, but is rather a consequence of the anisotropies in the Landau damping and in the magnetic correlation length, both of which are allowed by the tetragonal symmetry of the system. We also measure the magnetic spectrum deep inside the magnetically ordered phase, and find that it is remarkably similar to the spectrum of the paramagnetic phase, revealing the strongly overdamped character of the magnetic excitations.