Stripe Antiferromagnetic Spin Fluctuations inSrCo2As2

Abstract

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...

Anomalous Composition-Induced Crossover in the Magnetic Properties of the Itinerant-Electron AntiferromagnetCa1−xSrxCo2−yAs2Sangeetha1, Smetana2, Mudring3 et al. 2017Phys. Rev. Lett.213540View full textAdd to dashboardCite

“…However, single-crystal χ(T ) data for both c-axis and ab-plane magnetic field alignments show broad maxima at T ≈ 115 K, suggesting the presence of dynamic AFM correlations. Indeed, inelastic neutron scattering measurements on single crystals revealed strong AFM correlations with the same stripe wave vector as seen in the parent and doped Fe-based AFe 2 As 2 compounds [33].In view of the A-type AFM of CaCo 2−y As 2 where strong FM correlations dominate and the contrasting strong AFM correlations in paramagnetic (PM) SrCo 2 As 2 detected by neutron scattering, studies of the magnetic properties of Ca 1−x Sr x Co 2−y As 2 crystals have the potential to reveal additional interesting physics. This system is metallic with the ThCr 2 Si 2 -type structure over the entire composition range 0 ≤ x ≤ 1 [22].…”

Antiferromagnetic order and spin dynamics in iron-based superconductors

“…If this scenario is correct for all electron-doping levels, one would expect transversely elongated spin excitations in heavily Co-doped BaFe 2 As 2 or SrFe 2 As 2 . Figure 26(c) and 26(d) shows wave vector dependence of spin excitations in SrCo 2 As 2 (Jayasekara et al, 2013). Although spin excitations still appear at the same wave vector positions as those of BaFe 2 As 2 , they are longitudinally elongated.…”

Effective One-Dimensional Coupling in the Highly Frustrated Square-Lattice Itinerant Magnet CaCo2−yAs2

“…INS experiments on SrCo 2 As 2 observe stripe-type AFM spin fluctuations peaked at Q stripe similar to AFe 2 As 2 [19]. The anisotropy of the spin fluctuations gives η ≈ −0.6 [19], and indicates a moderate frustration.Here, we report inelastic neutron scattering measurements of magnetic excitations in CaCo 2−y As 2 at T = 8 K, below the AFM ordering temperature T N = 52 K [15,20], that reveal planes of magnetic scattering in reciprocal space perpendicular to the Co square-lattice planes. This is in sharp contrast to rods of scattering expected perpendicular to quasi-two-dimensional planes of spins but similar to the expectation for quasi-one-dimensional spin chains.…”

“…For example, INS measurements for the parent and doped compositions of AFe 2 As 2 compounds find η in the range from 0.3-0.7 [17,18], suggesting the presence of a moderate degree of frustration. INS experiments on SrCo 2 As 2 observe stripe-type AFM spin fluctuations peaked at Q stripe similar to AFe 2 As 2 [19]. The anisotropy of the spin fluctuations gives η ≈ −0.6 [19], and indicates a moderate frustration.…”