High Residual Resistivity Ratio Research Articles

Physical properties optimization of polycrystalline LiFeAs

We present a study of parameter optimization for synthesizing truly stoichiometric polycrystalline LiFeAs. Stoichiometric LiFeAs has been prepared in a very broad range of synthesis temperature (200–900°C) under otherwise exactly the same conditions, and has been characterized by structural, magnetic, transport, nuclear quadrupole resonance (NQR), and specific heat measurements. Our study showed that the LiFeAs phase is formed at 200°C with a large amount of impurity phases. The amount of these impurity phases reduces with increasing synthesis temperature and the clean LiFeAs phase is obtained at a synthesis temperature of 600°C. Magnetic susceptibility and resistivity measurements confirmed that the superconducting properties such as the critical temperature Tc, and the upper critical field Hc2 do not depend on the synthesis temperature (≤700°C), remaining at almost the same value of ∼19K and ∼40T, respectively. However, the width ΔTc of the transition and the NQR line width decrease with increasing the synthesis temperature and reached to minimum value for the synthesis temperature of 600°C. Our careful analysis suggests that the best sample obtained at 600°C is optimal concerning the low resistivity, high residual resistivity ratio (RRR), low ΔTc, high Tc and Hc2, and a small NQR line width with values which are comparable to that reported for LiFeAs single crystals. Specific heat measurements confirmed the bulk superconducting nature of the samples. The Hc2 value estimated from the specific heat is consistent with that of the resistivity measurements. Concisely, 600°C synthesis temperature yields optimal high quality polycrystalline LiFeAs bulk samples. Further improvement of the quality of the sample prepared at 600°C could be obtained by a controlled slow cooling process. Microstructural analysis reveals that the abundance of micro-cracks becomes strongly reduced by the slow cooling process, resulting in an increase in clean and well-connected grain boundaries.

Evaluation of critical current density and residual resistance ratio limits in powder in tube Nb3Sn conductors

High critical current density (Jc) Nb3Sn A15 multifilamentary wires require a large volume fraction of small grain (SG), superconducting A15 phase, as well as Cu stabilizer with high Residual Resistance Ratio (RRR) to provide electromagnetic stabilization and protection. In powder-in-tube (PIT) wires the unreacted Nb7.5 wt%Ta outer layer of the tubular filaments acts as a diffusion barrier and protects the interfilamentary Cu stabilizer from Sn contamination. A high RRR requirement generally imposes a restricted A15 reaction heat treatment to prevent localized full reaction of the filament that could allow Sn to reach the Cu. In this study we investigate recent high quality PIT wires that achieve a Jc (12 T, 4.2 K) up to ∼2500 A mm−2 and find that the minimum diffusion barrier thickness decreases as the filament aspect ratio increases from ∼1 in the inner rings of filaments to 1.3 in the outer filament rings. We found that just 2–3 diffusion barrier breaches can degrade RRR from 300 to 150 or less. Using progressive etching of the Cu we also found that the RRR degradation is localized near the external filaments where deformation is highest. Consequently minimizing filament distortion during strand fabrication is important for reducing RRR degradation. The additional challenge of developing the highest possible Jc must be addressed by forming the maximum fraction of high Jc SG A15 and minimizing low Jc large-grain (LG) A15 morphologies. In one wire we found that 15% of the filaments had a significantly enhanced SG/LG A15 ratio and no residual A15 in the core, a feature that opens a path to substantial Jc improvement.

Results of R&D and Production on Two Types of High-Purity Aluminum-Stabilized Superconducting Cable

Two types of high-purity aluminum-stabilized superconducting conductor were developed and produced for use in large-aperture high-field superconducting solenoids. One (DS1) has transverse rectangular dimensions of 5.27 mm × 20.1 mm, whereas the other (PS) has 5.52 mm × 30.1 mm. The superconductor for both cables is copper-stabilized NbTi strand woven into a Rutherford cable. The stabilizer for the DS1 conductor is 5N aluminum, whereas the stabilizer for PS is an aluminum alloy of 5N aluminum with a 0.1wt% Ni addition. Continuous unit lengths up to 1700 m are required for production. In total, over 18 000 m are required for this contract. A major goal of the R&D is to produce a cable having high shear strength between Cu and Al boundary while maintaining high residual resistivity ratio (RRR) and strength in the Al parts. We report on the cable mechanical properties, RRR of aluminum, sharing stress between Cu and Al, and critical current $(I_{C})$ of the cable, as well as the progress toward the fabrication of long unit lengths for production.

Single crystal growth and characterization of URu2Si2

We review recent progress in single crystal growth and study of electronic properties in . Czocharalski pulling, using purified uranium metal and subsequent annealing under ultra-high vacuum, is successfully applied to this compound, and it yields the highest residual resistivity ratio. These high-quality single crystals allow us to investigate Fermi surfaces using quantum oscillation and to make detailed transport measurements at low temperature.

Single-Crystal like MgB2thin films grown on c-cut sapphire substrates

Abstract Single-crystal like MgB 2 thin film was grown on (000) Al 2 O l3 substrate by using hybrid physical-chemical vapor deposition (HPCVD) system. Single crystal were properties studied by X-ray diffraction (XRD)and the full width half maximum at (FWHM) of the (0001) MgB 2 peak is 0.15°, which is very close to that has been reported for MgB 2 single-crystal. It indicates that the crystalline quality of thin film is good. Temperature dependence on resistivity was investigated by physical property measurement system (PPMS) in various applied fields from 0 to 9 T. The upper critical field ( H c2 ) and irreversibility field ( H irr ) were determined from PPMS data, the estimated values arand e comparable with that of MgB 2 single-crystals. The thin film shows a high critical temperature ( T c ) of 40.4 K with a sharp superconducting transition width of 0.2 K, and a high residual resistivity ratio (RRR=21), it reflects that MgB 2 thin film has a pure phase structure. Keywords

Physical properties of epitaxial ZrN/MgO(001) layers grown by reactive magnetron sputtering

Single-crystal ZrN films, 830 nm thick, are grown on MgO(001) at 450 °C by magnetically unbalanced reactive magnetron sputtering. The combination of high-resolution x-ray diffraction reciprocal lattice maps, high-resolution cross-sectional transmission electron microscopy, and selected-area electron diffraction shows that ZrN grows epitaxially on MgO(001) with a cube-on-cube orientational relationship, (001)ZrN‖(001)MgO and [100]ZrN‖[100]MgO. The layers are essentially fully relaxed with a lattice parameter of 0.4575 nm, in good agreement with reported results for bulk ZrN crystals. X-ray reflectivity results reveal that the films are completely dense with smooth surfaces (roughness = 1.3 nm, consistent with atomic-force microscopy analyses). Based on temperature-dependent electronic transport measurements, epitaxial ZrN/MgO(001) layers have a room-temperature resistivity ρ300K of 12.0 μΩ-cm, a temperature coefficient of resistivity between 100 and 300 K of 5.6 × 10−8 Ω-cm K−1, a residual resistivity ρo below 30 K of 0.78 μΩ-cm (corresponding to a residual resistivity ratio ρ300Κ/ρ15K = 15), and the layers exhibit a superconducting transition temperature of 10.4 K. The relatively high residual resistivity ratio, combined with long in-plane and out-of-plane x-ray coherence lengths, ξ‖ = 18 nm and ξ⊥ = 161 nm, indicates high crystalline quality with low mosaicity. The reflectance of ZrN(001), as determined by variable-angle spectroscopic ellipsometry, decreases slowly from 95% at 1 eV to 90% at 2 eV with a reflectance edge at 3.04 eV. Interband transitions dominate the dielectric response above 2 eV. The ZrN(001) nanoindentation hardness and modulus are 22.7 ± 1.7 and 450 ± 25 GPa.

Thermodynamic and transport properties of RSn2 (R=Tb–Tm, Lu, Y) single crystals

We report the anisotropic magnetization, heat capacity and resistivity measurements for isostructural RSn2 (R=Tb–Tm, Lu, Y) single crystals. The R=Tb–Tm compounds exhibit long range antiferromagnetic ordering with TN ranging from 26.3K to 3.6K. An additional incommensurate-to-commensurate transition at a lower critical temperature T1 is observed for R=Tb–Ho. Field-induced metamagnetic transitions have also been observed for all magnetic rare earth compounds. The resistivity has a typical metallic character with a relatively high residual resistivity ratio. Both the AFM ordering temperature and spin disordered resistivity scale with the de Gennes factor. The magnetic heat capacity data indicate partially lifted ground state degeneracies associated with crystal electric field splitting.

Perturbation Sensitivity of Magnetothermal Instability

The magnetothermal instability (MTI) is an issue in the high-Jc Nb3Sn superconducting strands for accelerator magnets. The first kind of MTI is magnetization instability, which may be significantly reduced by using small filaments and with a high copper residual resistivity ratio (RRR) ≥ 100. However, theoretical and experimental evidence suggests that there is a second kind of MTI, which is self-field instability, which seems to be highly sensitive to external perturbations and less sensitive to RRRs. Recently at the European Organization for Nuclear Research (CERN), for studying this effect, a new measurement setup based on a Q-switched laser has been developed. By using the setup, it is possible to provide a local and controlled perturbation in nanosecond timescale from 0.5 to 26 μJ , which is absorbed by the strand at cryogenic temperature. In this paper, we report the results obtained by testing high-Jc Nb3Sn strands with different RRRs. It is shown that the self-field instability is sensitive to the trigger energy in moderately high magnetic fields. Therefore, the stability is highly dependent on the perturbation spectrum, i.e., the conventional stability measurements might not be fully representative of the strand stability in magnets wound with high-Jc Nb3Sn strands.

Coherent growth of superconducting TiN thin films by plasma enhanced molecular beam epitaxy

We have investigated the formation of titanium nitride (TiN) thin films on (001) MgO substrates by molecular beam epitaxy and radio frequency acitvated nitrogen plasma. Although cubic TiN is stabile over a wide temperature range, superconducting TiN films are exclusively obtained when the substrate temperature exceeds 710 °C. TiN films grown at 720 °C show a high residual resistivity ratio of approximately 11 and the superconducting transition temperature (Tc) is well above 5 K. Superconductivity has been confirmed also by magnetiztion measurements. In addition, we determined the upper critical magnetic field (μ0Hc2) as well as the corresponding coherence length (ξGL) by transport measurements under high magnetic fields. High-resolution transmission electron microscopy data revealed full in plane coherency to the substrate as well as a low defect density in the film, in agreement with a mean-free path length ℓ ≈ 106 nm, which is estimated from the residual resistivity value. The observations of reflection high energy electron diffraction intensity oscillations during the growth, distinct Laue fringes around the main Bragg peaks, and higher order diffraction spots in the reciprocal space map suggest the full controlability of the thickness of high quality superconducting TiN thin films.

ChemInform Abstract: Preparation and Characterization of Extra-Low-Oxygen Titanium.

Abstract Removal of oxygen in titanium by reaction with chemically active calcium dissolved in CaCl2 was examined between 1273 and 1473 K with the purpose of obtaining extra low-oxygen titanium. CaCl2 was used as a flux to facilitate the reaction by decreasing the activity of the by-product CaO. Titanium wires and small pieces of titanium were deoxidized to 20–60 mass ppm oxygen by use of calcium-saturated CaCl2 at a temperature of 1273 K. Trace element analysis (e.g. glow discharge mass spectrometry), micro Vickers hardness measurements, and electrical resistivity measurements were carried out to characterize the deoxidized titanium. The deoxidation of electrolytically refined titanium wire produced titanium with a high residual resistivity ratio ( ga 298 /α 4.2 ⋍100 ). The “ideal resistivities”, or hypothetical resistivities of pure titanium, at 77 and 298 K were determined to be 40 and 440 nΩm respectively. The influence of oxygen on resistivity at 4.2 K was also measured by using titanium containing 30 and 500 mass ppm O, and was determined to be 88 nΩm (mol.% O)−1.

Charge transport and magnetic ordering in laser ablated Co2FeSi thin films epitaxially grown on (1 0 0) SrTiO3

We have successfully deposited epitaxial thin films of the Heusler alloy Co2FeSi on (1 0 0) cut SrTiO3 single crystal substrates by pulsed laser deposition. X-ray diffraction reveals the L21 cubic crystal structure for the films grown at 400 and 600 °C, with a better crystallographic quality at the higher growth temperature. The magnetic moment in saturation of the 600 °C grown film is 5.0 µB/fu at 5 K with the easy axis of magnetization in the (1 0 0) plane. Inplane hysteresis measurements suggest a soft magnetic state with a coercive field HC ≈ 250 Oe at 5 K for the 600 °C film which decreases on lowering the growth temperature. The normalized magnetization of thin films follows the well-known Bloch T3/2 law due to spin wave excitation. Electrical resistivity measurements on the films with the L21 crystal structure show a pronounced metallic behaviour down to the lowest temperature of measurement (≈5 K), and enhanced metallicity in the 600 °C film as indicated by its lower residual resistivity (≈0.16 µΩ cm) and higher residual resistivity ratio (RRR ≈ 1280). These results suggest that films grown at the higher temperature may be better suited for the fabrication of magnetic tunnel junctions.

Ferromagnetic quantum phase transition in Sr1−xCaxRuO3 thin films

AbstractThe ferromagnetic (FM) quantum phase transition in the perovskite ruthenate Sr1−xCaxRuO3 is studied by low‐temperature magnetization and electrical resistivity measurements on thin films. The films were grown epitaxially on SrTiO3 substrates using metalorganic aerosol deposition and characterized by X‐ray diffraction and room temperature scanning tunneling microscopy. High residual resistivity ratios of 29 and 16 for $x = 0$ and $1$, respectively, prove the high quality of the investigated samples. We observe a continuous suppression of the FM Curie temperature from $T_{\rm C} = 160\,{\rm K}$ at $x = 0$ towards $T_{\rm C} \to 0$ at $x_{\rm c} \approx 0.8$. The analysis of the electrical resistivity between 2 and 10 K reveals T2 and T3/2 behavior at $x \leq 0.6$ and $x \geq 0.7$, respectively. For undoped CaRuO3, the measurement has been extended down to 60 mK, revealing a crossover to T2 behavior around 2 K, which suggests a Fermi‐liquid ground state in this system.

On the Residual Resistivity Ratio in $\hbox{MgB}_{2}$ Superconductors

It is found that there exist two different but related parameters that control the value of the residual resistivity ratio (RRR) in MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , namely, the stoichiometry of the starting powder and the synthesis parameters. The origin of the high RRR in the polycrystalline MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> samples has been attributed to the enhanced crystallinity of the samples due to higher Mg concentration in the starting composition. Scanning-electron-microscope images show the improved crystalline nature of the tape samples synthesized with the excess Mg in the starting powder mixture.

Rare earth magnetism in CeFeAsO: a single crystal study

Single crystals of CeFeAsO, large enough to study the anisotropy of the magnetic properties, were grown by an optimized Sn-flux technique. The high quality of our single crystals is apparent from the highest residual resistivity ratio (RRR)≈12, reported among undoped RFeAsO compounds (R=rare earth) as well as sharp anomalies in resistivity, specific heat, C(T), and thermal expansion at the different phase transitions. The magnetic susceptibility χ(T) presents a large easy-plane anisotropy consistent with the lowest crystal electric field doublet having a dominant Γ6 character. Curie–Weiss-like susceptibilities for magnetic field parallel and perpendicular to the crystallographic c-axis do not reveal an influence of a staggered field on the Ce site induced by magnetic ordering of the Fe. Furthermore, the standard signatures for antiferromagnetic order of Ce at TN4f=3.7 K observed in χ(T) and C(T) are incompatible with a Zeeman splitting Δ≈10 K of the CEF ground state doublet at low temperature due to the Fe-magnetic order as previously proposed. Our results can be reconciled with the earlier observation by assuming a comparatively stronger effect of the Ce–Ce exchange leading to a reduction of this Zeeman splitting below 15 K.

Self Field Instability in High-${\rm J}_{\rm c}$${\rm Nb}_{3}{\rm Sn}$ Strands With High Copper Residual Resistivity Ratio

High critical current density (Jc) Nb3Sn conductor is the best candidate for next generation high field (>10 T) accelerator magnets. Although very promising, state of the art high-Jc Nb3Sn strands suffer magneto-thermal instabilities that can severely limit the strand performance. Recently it has been shown that at 1.9 K the self field instability is the dominating mechanism that limits the performance of strands with a low (<10) residual resistivity ratio (RRR) of the stabilizing copper. In this paper the self-field instability is investigated in high-Jc Nb3Sn strands with high RRR. At CERN several state of the art rod re-stack process (RRP) and powder in tube (PIT) Nb3Sn strands have been tested at 4.2 K and 1.9 K to study the effects on strand stability of: RRR, strand diameter and, strand impregnation with stycast. The experimental results are reported and discussed. A new 2-D finite element model for simulating magneto-thermal instabilities and its preliminary results are also presented. The model, which describes the whole development of the flux jump in the strand cross section taking into account the heat and current diffusion in the stabilizing copper, is in good agreement with the experimental data.

Epitaxial c-axis oriented LuNi2B2C thin films on MgO(110)

Epitaxial thin films of LuNi2B2C have been fabricated on MgO(110) substrates using pulsed laser deposition. XRD measurements show almost perfect c-axis texture, a sharply textured Lu2O3 interface and a small amount of a secondary phase, which is assumed to contain Ni and B. The films show good homogeneity, very high in-plane and out-of-plane order and high residual resistivity ratios up to 15 at film thicknesses of 200 – 250 nm. The quality of these films compared to single crystals was tested using a sample in the as-grown state for Hc2(T) measurements. Upper critical field values in the <001> crystallograpic direction are higher than in single crystals and show a less pronounced S-shape of the Hc2(T) behaviour for H || <001>.

Applying evolutionary yield function to predicting the deformation of microstructure-sensitive high-RRR niobium

High-purity high residual resistivity ratio niobium is used to fabricate particle accelerator cavities for advanced superconducting accelerators. This material has an unstable texture that causes different r values in different inplane directions, which evolve with deformation, making prediction of forming characteristics difficult. Thus, an evolutionary yield function having coefficients that evolve with plastic deformation is introduced to account for gradients in microstructure and texture that evolve differently in different layers of the material. With such a model, failure modes and locations are predicted accurately in more complex deformation conditions.

Clean epitaxial MgB2 films fabricated by the ex situ annealing of chemical vapour deposition-grown B films in Mgvapour

Low residual resistivity in superconducting thin films is required for theirapplications in radio frequency (RF) cavities. Here we report on clean epitaxialMgB2 films fabricated by the ex situ annealing of B films, grown by chemical vapour deposition, inMg vapour. The films show sharp superconducting transitions at about 40 K and a highresidual resistivity ratio of about 10. The result indicates that a clean precursor Bfilm and a contamination-free annealing procedure are important for a viableMgB2 film fabrication process for RF cavity applications.

Growth of CuS platelet single crystals by the high-temperature solution growth technique

Millimeter size CuS single crystals with a dark indigo blue color and a plate hexagonal shape have been successfully grown by the high-temperature solution growth technique using the KCl–LiCl eutectic as solvent. Surface microtopographic studies of the crystals indicated that the growth is made by the lateral spreading of the layers. Electrical resistivity measurements clearly show an anomaly at T∼55 K, related with the low-temperature structural transition, a high residual resistivity ratio of ∼400 and a sharp superconducting transition at T≈1.7 K confirming the very good quality of the crystals.

Enhanced Curie temperature and spin polarization in Mn4FeGe3

Intermetallic ferromagnetic compound Mn5Ge3, with Curie temperature TC=296K, a spin polarization of P=42%, and a good lattice match to semiconductors, is a potential spin injector for spintronics. We report the enhanced magnetic properties and spin polarization of Mn4FeGe3. By replacing one Mn atom in Mn5Ge3 with Fe, the TC of Mn4FeGe3 has been enhanced to 319.50K, while still maintaining the same crystal structure, high electrical conductivity, and a high residual resistivity ratio of 5.33. More importantly, the spin polarization in excess of 60% of Mn4FeGe3, much higher than that of Mn5Ge3, has been realized.