Local magnetic inhomogeneities inBa(Fe1−xNix)2

Abstract

We report 75 As NMR measurements in BaFe2As2 doped with Ni. Like Co, Ni doping suppresses the antiferromagnetic and structural phase transitions and gives rise to superconductivity for sufficiently large Ni doping. The spin lattice relaxation rate diverges at TN, with a critical exponent consistent with 3D ordering of local moments. In the ordered state the spectra quickly broaden inhomogeneously with doping. We extract the average size of the ordered moment as a function of doping, and show that a model in which the order remains commensurate but with local amplitude variations in the vicinity of the dopant fully explains our observations. PACS numbers: 76.60. Gv, 71.27.+a, 75.50.Ee, 75.30.Mb The recent discovery of the iron arsenide superconductors has reignited interest in the interplay of superconductivity and magnetism in condensed matter.1,2 There are several iron arsenide families that exhibit superconductivity either under pressure or with chemical doping. 3Each family, however, contains a common structural element consisting of FeAs planes, in which the Fe 3d orbitals hybridize with the As p orbitals, giving rise to multiple bands that cross the Fermi energy.4,5 In the parent state (either undoped or at ambient pressure) the nesting of two of these Fermi surfaces gives rise to a spin density wave instability.6 It is believed that doping (both inplane and out-of plane) and/or pressure tunes the chemical potential and modifies the nesting condition, which consequently suppresses the long range antiferromagnetic order. For sufficiently large electron or hole doping superconductivity emerges.7 Although it appears that the superconductivity may be intimately related to the antiferromagnetic fluctuations present in these systems, details of how these fluctuations emerge and are related to the superconductivity remain clouded.8 Furthermore, it is unclear how the superconducting condensate can remain intact in the presence of a high concentration of inplane impurities.9 Detailed experimental studies of the local effects of dopants therefore are crucial to understand the physics that drive the evolution of long range order in these materials.In order to shed light on the influence of dopants on the antiferromagnetic order, we have conducted 75 As Nuclear Magnetic Resonance (NMR) studies in a series of Ni doped Ba(Fe 1−x Ni x ) 2 As 2 crystals. The advantage of this particular system is that large high quality single crystals are available, and Ni has the greatest effect of any transition metal dopants so that superconductivity is reached at only ∼2%.7,10 We find that the As NMR spectrum in the antiferromagnetic phase broadens inhomogeneously very quickly with doping, reflecting a large distribution of local hyperfine fields. This distribution can be understood by realizing that the local spin polarization surrounding the dopant site is not only reduced on-site (at the Ni), but also extends to several of the surrounding Fe sites. In the paramagnetic state, we find that the FIG. 1. (color online)The local stru...

Cu Substitution Effects on the Local Magnetic Properties ofBa(Fe1−xCux)Takeda1, Imai2, Tachibana3 et al. 2014Phys. Rev. Lett.262320View full textAdd to dashboardCite

“…In x = 0.023, 75 (1/T 1 T ) shows a monotonic increase below ∼150 K due to a gradual growth of short range SDW order toward the SDW transition at T SDW ∼ 83 K. With increasing the Cu concentration, the enhancement of 75 (1/T 1 T ) is progressively suppressed. At a qualitative level, our new results are similar to the concentration dependence previously reported for the Co [14][15][16] or Ni [19,20] substituted BaFe 2 As 2 .…”

Evidence for filamentary superconductivity nucleated at antiphase domain walls in antiferromagnetic CaFe2As2

“…26 Both types of DWs give rise to perturbations of the local hyperfine field and could be responsible for the slow dynamics we observe. 27 The DWs give rise to a low frequency tail in the spectra that is clearly evident in Fig. 1(f).…”

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“…We note that a similar conclusion was obtained from previous NMR experiments on Ni-substituted Ba-122. 60 Further information about the role of the Co-induced disorder in the suppression of the magnetism can be obtained from the comparison with the μSR data on the hole-doped Ba 1−x K x Fe 2 As 2 . 22 The substitution-induced disordering effects should be significantly weaker here since the K ions are incorporated on the Ba sites where they do not directly disturb the iron arsenide layers.…”