Grain boundary characteristics of oxypnictide NdFeAs(O,F) superconductors

Iida, K.; Omura, Tomohiko; Matsumoto, Takuya; Hatano, Takafumi; Ikuta, Hiroshi

doi:10.1088/1361-6668/ab1660

Cited by 19 publications

(20 citation statements)

References 33 publications

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“…The film growth was monitored by reflection high-energy electron diffraction (RHEED). To obtain NdFeAs(O,F) films with different F content, a 20 nm-thick NdOF overlayer was deposited on top of the NdFeAsO layer at various temperatures in the range of 550 22 , where T dep is the deposition temperature. Since it is difficult to determine the fluorine content precisely, both the c-axis lattice parameter and T c have been used as the indicators of the F-content in Nd-FeAs(O,F).…”

Section: Methodsmentioning

confidence: 99%

Anisotropy of the transport properties of NdFeAs(O,F) thin films grown on vicinal substrates

Iida¹,

Matsumoto²,

Kondo³

et al. 2020

Supercond. Sci. Technol.

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NdFeAs(O,F) thin films having different fluorine contents were grown on 5 • or 10 • vicinal cut MgO and CaF 2 single crystalline substrates by molecular beam epitaxy. Structural characterisations by reflection high-energy electron diffraction and x-ray diffraction confirmed the epitaxial growth of NdFeAs(O,F). The resistivities of the ab-plane and along the c-axis (ρ ab and ρ c ) were derived from the resistivity measurements in the longitudinal and transversal directions. The c-axis resistivity was always higher than ρ ab , resulting from the anisotropic electronic structure. The resistivity anisotropy γ ρ = ρ c /ρ ab at 300 K was almost constant in the range of 50 ≤ γ ρ ≤ 90 irrespective of the fluorine content. On the other hand, γ ρ at 56 K showed a strong fluorine dependence: γ ρ was over 200 for the films with optimum fluorine contents (superconducting transition temperature T c around 50 K), whereas γ ρ was around 70 for the films in the under-doped regime (T c between 35 and 45 K). The mass anisotropy γ m = m * c /m * ab (m * c and m * ab are the effective masses along the c-axis and on the ab-plane) close to T c derived from the anisotropic Ginzburg-Landau approach using the angular-dependency of ρ ab was in the range from 2 to 5. On the assumption γ 2 m = γ ρ , those values are small compared to the normal state anisotropy. arXiv:2003.09105v1 [cond-mat.supr-con]

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Section: Methodsmentioning

confidence: 99%

Anisotropy of the transport properties of NdFeAs(O,F) thin films grown on vicinal substrates

Iida¹,

Matsumoto²,

Kondo³

et al. 2020

Supercond. Sci. Technol.

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“…Hence, these HAGBs become serious weak links. 5,6 In our previous study, 7 θ c was evaluated to be 8.5°for NdFeAs(O,F) bicrystals with symmetrical [001]-tilt boundaries, which is close to θ c ∼ 9°for Co-doped BaFe 2 As 2 (Ba-122) 8 and Fe(Se,Te) bicrystals. 9,10 Therefore, strict control of the texture is required to fully utilize its potential advantages in applications.…”

Section: Introductionmentioning

confidence: 53%

“…However, polycrystalline Ln FeAs(O,F) generally does not show the best performance, because J c significantly decreases in the presence of high-angle grain boundaries (HAGB) with misorientation angles larger than the critical angle θ c , for which intergrain J c starts to decrease exponentially. Hence, these HAGBs become serious weak links. , In our previous study, θ c was evaluated to be 8.5° for NdFeAs(O,F) bicrystals with symmetrical [001]-tilt boundaries, which is close to θ c ∼ 9° for Co-doped BaFe 2 As 2 (Ba-122) and Fe(Se,Te) bicrystals. , Therefore, strict control of the texture is required to fully utilize its potential advantages in applications.…”

Section: Introductionmentioning

confidence: 95%

Nanoscale Texture and Microstructure in a NdFeAs(O,F)/IBAD-MgO Superconducting Thin Film with Superior Critical Current Properties

Guo

Gao

Kondo

et al. 2021

ACS Appl. Electron. Mater.

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This paper reports the nanoscale texture and microstructure of a high-performance NdFeAs(O,F) superconducting thin film grown by molecular beam epitaxy on a textured MgO/Y 2 O 3 /Hastelloy substrate. The NdFeAs(O,F) film forms a highly textured columnar grain structure by epitaxial growth on the MgO template. Although the film contains stacking faults along the abplane as well as grain boundaries perpendicular to the ab-plane, good superconducting properties are measured: a critical temperature, T c , of 46 K and a self-field critical current density, J c , of 2 × 10 6 A/cm 2 at 4.2 K. Automated crystal orientation mapping by scanning precession electron diffraction in transmission electron microscope is employed to analyze the misorientation angles between adjacent grains in a large ensemble (247 grains), and 99% of the grain boundaries show in-plane misorientation angles (Δγ) less than the critical angle θ c , which satisfies one of the necessary conditions for the high J c . Comparing the columnar grain size distribution with the mean distance of the flux line lattice, the triple junctions of low-angle grain boundaries are found to be effective pinning centers, even at high temperatures (≥35 K) and/or low magnetic fields.

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“…Moreover, the formation of Ba-O and Fe-Co-As enriched phases as the secondary phases in the aggregates was observed. Several previous studies have reported similar phases in Ba-122 bulks that were fabricated in a glove box [10,25]. Therefore, even at a low temperature of 600 °C, phase separation may have occurred owing to the loss of Ba atoms from the grain surface.…”

Section: Discussionmentioning

confidence: 81%

“…Iron-based superconductors also have inherently weak links [20][21][22]. Previous studies have shown that the transport critical current in Fe (Se,Te) decreases to approximately 1/5th-1/4th at a misorientation angle of 15° compared to that at 0° and further decreases by an order of magnitude at 25° [21][22][23][24][25]. In our previous study, we found a correlation between the improvement in critical current characteristics and the reduction in grain size by microstructural analysis using electron microscopy [26].…”

Section: Introductionmentioning

confidence: 99%

3D microstructure and critical current properties of ultra-fine-grain Ba(Fe,Co)2As2 bulk superconductors

Shimada

Tokuta

Yamanaka

et al. 2021

Preprint

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In iron-based superconductors, randomly oriented grain boundaries have a strong influence on the transport properties via intrinsic weak-link and flux pinning mechanisms. Herein we report the critical current density (Jc) and the three-dimensional microstructure of polycrystalline bulk Co-doped Ba122 superconductors, with highly dense grain boundaries (grain size smaller than 50 nm), produced by high-energy milling. Three-dimensional electron microscopy revealed that the anomalous growth of secondary particles (aggregation) and the inter-aggregation structures were significantly different in the samples with finer grains, which may have extrinsically limited Jc. These important microstructural features were quantified as two parameters—local thickness and total pore length—by reconstructing the three-dimensional structure of the superconducting phase using the adaptive thresholding method. The results obtained in this study suggest that understanding and controlling the microstructural formation process by sintering are instrumental for improving the Jc properties of 122 polycrystalline materials consisting of ultrafine grains.

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Grain boundary characteristics of oxypnictide NdFeAs(O,F) superconductors

Cited by 19 publications

References 33 publications

Anisotropy of the transport properties of NdFeAs(O,F) thin films grown on vicinal substrates

Anisotropy of the transport properties of NdFeAs(O,F) thin films grown on vicinal substrates

Nanoscale Texture and Microstructure in a NdFeAs(O,F)/IBAD-MgO Superconducting Thin Film with Superior Critical Current Properties

3D microstructure and critical current properties of ultra-fine-grain Ba(Fe,Co)2As2 bulk superconductors

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