Helical magnetic ordering in Sr(Co1−xNix)2As2

Abstract

SrCo2As2 is a peculiar itinerant magnetic system that does not order magnetically, but inelastic neutronscattering experiments observe the same stripe-type antiferromagnetic (AF) fluctuations found in many of the Fe-based superconductors along with evidence of magnetic frustration. Here we present results from neutron diffraction measurements on single crystals of Sr(Co1-xNix)(2)As-2 that show the development of long-range AF order with Ni doping. However, the AF order is not stripe type. Rather, the magnetic structure consists of ferromagnetically aligned (FM) layers (with moments laying in the layer) that are AF arranged along c with an incommensurate propagation vector of (0 0 tau), i.e., a helix. Using high-energy x-ray diffraction, we find no evidence for a temperature-induced structural phase transition that would indicate a collinear AF order. This finding supports a picture of competing FM and AF interactions within the square transition-metal layers due to flat-band magnetic instabilities. However, the composition dependence of the propagation vector suggests that far more subtle Fermi surface and orbital effects control the interlayer magnetic correlations. Disciplines Disciplines Condensed Matter Physics Comments Comments This article is published as Wilde,

Non-Fermi-liquid types of behavior associated with a magnetic quantum critical point in Sr(Co1−xNix)2As2Sangeetha1, Wang2, Smirnov3 et al. 2019Phys. Rev. BSelf Cite278430View full textAdd to dashboardCite

“…We calculated the turn angle kd between adjacent ab-plane ordered moments along the helix c axis within MFT from the ratio χ ab (T → 0)/χ (T N ) for the crystals showing AFM ordering and found that the helix turn angle varies with Ni concentration. Magnetic neutron diffraction measurements from two groups [63,64] confirm the helical magnetic structure with turn angles of the same order as those estimated here.…”

“…The above MFT discussion of helical ordering may be expected to be of only qualitative significance, since the MFT was formulated for local-moment AFMs instead of itinerant AFMs. However, magnetic neutron diffraction measurements [63,64] found that the magnetic structure is indeed helical, with helix turn angles that decrease from 104 • to 94 • on increasing x from x = 0.06 to 0.15, which have the same trend and are of the same order as we infer in Fig. 12 from Eq.…”

Ferromagnetic cluster-glass phase in Ca(Co1−xIrx)2−yAs2Pakhira1, Sangeetha2, Smetana3 et al. 2020Phys. Rev. BSelf Cite14062View full textAdd to dashboardCite

“…Recently, non-Fermi-liquid types of behavior associated with a composition-induced magnetic quantum-critical point in Sr(Co 1−x Ni x ) 2 As 2 crystals near x = 0.3 has been reported [37]. In addition, crystals of Sr(Co 1−x Ni x ) 2 As 2 with 0 < x < 0.3 exhibit c-axis helical or spin-density-wave AFM ordering [37][38][39], which is quite unusual in itinerant antiferromagnets. A small amount (2.5%) of La doping has been reported to cause FM ordering in Sr 1−x La x Co 2 As 2 [40,41].…”

Suppression of antiferromagnetic order and strong ferromagnetic spin fluctuations in Ni-doped CaCo2As2 single crystals

“…Additional scans along (00L), (10L), and ( 1 2 0L) at T = 4.8 and 20 K (not shown) do not show evidence of new emerging Bragg reflections that may indicate magnetic ordering that is different from A-type AFM ordering. For instance, the absence of magnetic reflections along (00L) eliminates the helical structure recently discovered in the related Sr(Co 1−x Ni x ) 2−y As 2 system in which adjacent FM layers (moments aligned in the plane) are rotated at a finite angle as they stack along the c axis [29]. We note that examination of nuclear Bragg reflections do not show increased intensity as the temperature is lowered from 20 K to base temperature thus excluding FM ordering of the whole system assuming that the ordered moment is not larger than ∼ 0.1 µ B .…”

“…In contrast to 122-type iron-arsenide compounds, none of the parent cobalt arsenides exhibit superconductivity even with doping but are still found to be rich in physics associated with itinerant magnetism of the conduction electrons [14][15][16][17][18][19][20][21][22][23][24]. Interesting physical properties are observed including non-Fermi-liquid behavior associated with a quantum critical point, spin-glass behavior, and helical magnetic ordering in electron-or hole-doped 122-type CoAs-materials [25][26][27][28][29][30][31][32]. For example, metallic SrCo 2 As 2 is a Stoner-enhanced paramagnet and does not exhibit any magnetic ordering down to a temperature T of 0.05 K; however, stripe-type AFM fluctuations were revealed via inelastic neutron-scattering (INS) measurements [16][17][18].…”