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

Abstract

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.

New Scaling Laws for Pinning Force Density in Superconductors

“…In the results, both research groups [43,44] reported that the point pinning dominates at 0.5 T T c , while at T T c 0.5, the dominant mechanism is the surface pinning. Because the comprehensive analysis of → F p (B, T, θ = 0 • ) for NdFeAs(O,F) thin films was reported in [43,44], here, we only presented an analysis for the…”

“…In this section, the analysis was applied for two NdFeAs(O,F) thin films for which raw transport critical current density data, 𝐽 (𝐵, 𝑇, 𝜃 = 0°), were reported by Tarantini et al [43] and Guo et al [44]. Both research groups found that the 𝐹 ⃗ (𝐵, 𝑇, 𝜃 = 0°) data fit to Equation (3) generates unphysical 𝑝(𝑇) and 𝑞(𝑇) values, which was interpreted as a → F p (B, T, θ = 0 • ) curves and data fits to Equation (13) for a MgB 2 thin film deposited on a 6H-SiC single-crystal substrate for the temperature range of 4.2-30 K (a-f).…”

“…In this section, the analysis was applied for two NdFeAs(O,F) thin films for which raw transport critical current density data, J c (B, T, θ = 0 • ), were reported by Tarantini et al [43] and Guo et al [44]. Both research groups found that the → F p (B, T, θ = 0 • ) data fit to Equation (3) generates unphysical p(T) and q(T) values, which was interpreted as a demonstration of a superposition of two different pinning mechanisms.…”

“…A slightly different approach was proposed by Guo et al [44], who considered that two summation parts in Equation ( 26) have q(T) ≡ 2.0, and thus, these authors proposed to use the following fitting function:…”

Recent progress on epitaxial growth of Fe-based superconducting thin films

“…Ga has been used to remove excess fluorine during deposition. Although fine tuning is necessary, parent compound NdFeAsO has been fabricated on a large variety of oxide and fluorine substrates [67], as well as technical substrates [68,69]. Typical growth temperatures are around 800 • C. The growth rate is approximately 0.8-1 nm min −1 .…”