High Residual Resistivity Ratio Research Articles

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.

Development of Internal Tin ${\hbox{Nb}}_{3}{\hbox{Sn}}$ Conductor for Fusion and Particle Accelerator Applications

Performance improvements are needed for large-scale applications of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn, such as ITER or LHC upgrades. The highest critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) values are achieved in distributed-barrier strand made by the Restacked Rod Process, which can reach 12 T, 4.2 K J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values of 3000 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , with high residual resistivity ratio (RRR) values. For purposes of accelerator magnet stability, it is desirable to combine high J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> with a small effective filament diameter ( D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ) . Initial experiments show reducing D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> from 80 mum to 40 mum leads to a 10% reduction in J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . For fusion applications, a single-barrier design with well-spaced filaments is used to achieve the low hysteresis losses that are required. The status of our fusion strand development program is presented, including results for strand made using Nb-47 wt%Ti rods to supply Ti dopant. Such strands can reach 12 T, 4.2 K J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> > 1000 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , with losses < 1000 mJ/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> .

Titanium Based Transition Edge Microcalorimeters for Optical Photon Measurements

Transition edge sensor microcalorimeters can be used in many optical quantum measurements because of its low dark counts, high quantum efficiency, and high resolving power of a photon number in weak light pulses. In order to increase count rates up to a few MHz, we have developed a titanium transition edge sensor for the optical measurements, and its performances were analysed. Titanium is one of the ideal superconductor because of its higher transition temperature and lower optical reflectance at 1.5 wavelength. Our titanium film was fabricated with electron-beam evaporation, and showed high residual resistance ratio of 3.5. The sharp superconducting transition also was found at 359 mK, which is close to the critical temperature in bulk. The fabricated device showed a fast response to pulsed laser illumination of 1.5 wavelength with the fall time constant of 300 ns. These features are very promising for high-speed single photon detection in many quantum optical measurements.

Multiple magnetic transitions and magnon gaplike characteristics in the high purity TbB4 single crystal

We synthesized the high purity TbB4 single crystal, which shows quite low resistivity at T=2K, ρ(T=2K)=0.0397μΩcm, resulting in a high residual resistivity ratio RRR≡ρ(300K)∕ρ(2K)=361.9. Because of the geometry of the grown single crystals, temperature-dependent magnetization M(T) was measured with applied magnetic field parallel to (010) and (111) planes. M(T) was found to be anisotropic at high temperature and revealed two magnetic transitions: one is antiferromagnetic transition at TN=44K and the other one at Tm≈23K for both field orientations. These transitions were also manifested by the slope change of temperature-dependent resistivity, i.e., dρ(T)∕dT. The second transition temperature Tm was found to decrease and the transition width broadened with increasing applied magnetic field, indicating that the transition at Tm is suppressed by applied magnetic field. The Fermi liquid and antiferromagnetic (AF) magnon gaplike increase of ρ(T) was found at low temperatures of T⩽20K. The AF magnon gap Δ=15.6K at H=0T was also found to decrease with applying magnetic fields. We will discuss the origin of the second transition Tm in terms of the interaction between magnetic and quadrupolar orbital fluctuations.

Deposition of superconducting niobium films for RF cavities by means of UHV cathodic Arc

The Ultra-High-Vacuum (UHV) arc technology was proposed as an alternative for depositing thin superconducting films of pure niobium on the internal surfaces of RF cavities for particle accelerators. The paper describes status of research on the deposition of such films for the RF accelerating cavities. UHV arc-based devices, equipped with planar- or cylindrical-cathodes, are described. The main results of experiments and some characteristics of the arc-deposited thin Nb-films, as well as results obtained recently with the formation of such films, are also presented. The critical temperature T c of the deposited Nb-films appeared to be very close to that of pure bulk niobium ( T c=9.26 K) and the transition to the superconducting state was very narrow. The deposited Nb-films had higher residual resistivity ratio (RRR) values (up to 80) and larger grains sizes, as compared with those sputtered at the same temperature. The paper also presents recent results of the Cu-cavity coating by means of an UHV linear (cylindrical) arc, operated at IPJ in Poland.

Correlation between strand stability and magnet performance

Magnet programs at BNL, LBNL and FNAL have observed instabilities in high J/sub c/ Nb/sub 3/Sn strands and magnets made from these strands. This paper correlates the strand stability determined from a short sample-strand test to the observed magnet performance. It has been observed that strands that carry high currents at high fields (greater than 10 T) cannot sustain these same currents at low fields (1-3 T) when the sample current is fixed and the magnetic field is ramped. This suggests that the present generation of strand is susceptible to flux jumps (FJ). To prevent flux jumps from limiting stand performance, one must accommodate the energy released during a flux jump. To better understand FJ this work has focused on wire with a given sub-element diameter and shows that one can significantly improve stability by increasing the copper conductivity (higher residual resistivity ratio, RRR, of the Cu). This increased stability significantly improves the conductor performance and permits it to carry more current.

Growth features of RENi 2B 2C ( RE=Y, Ho, Tb) single crystals

Features characterizing the perfection of high-quality YNi 2B 2C, HoNi 2B 2C and TbNi 2B 2C single crystals grown by the floating zone technique with optical heating in the TSFZ regime are investigated. The grown crystals are crack free, single phase and single crystalline in the whole cross-section. Various origins of small fractions of minority phases in several crystals impurities, phase diagram features and limits of growth parameters are revealed. For the crystals a subgrain misorientation <0.15° was determined by neutron diffraction. The YNi 2B 2C crystals show an extremely narrow superconducting transition width 0.1 K and high residual resistivity ratios up to 48. Longitudinal and radial element segregation phenomena with particular respect to the light elements are related to the various modifications of superconducting properties and T c variations in HoNi 2B 2C single crystals. The interface shape and radial segregation suggest a noticeable convection flow in HoNi 2B 2C melts.

Low-temperature properties ofYbAl2

Measurements on the intermediate-valent intermetallic compound $\mathrm{Yb}{\mathrm{Al}}_{2}$ are presented. We discuss magnetization, resistivity, magnetoresistance and specific heat in the temperature range between 1.5 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in magnetic field up to $14\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Our samples, with a high residual resistivity ratio of nearly 50, display a ${T}^{3.5}$ temperature dependence of the resistivity and a strong magnetoresistance, reaching $\ensuremath{\Delta}\ensuremath{\rho}∕{\ensuremath{\rho}}_{0}\ensuremath{\approx}1.5$ at $12\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ and $3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The low-temperature specific heat and magnetization of $\mathrm{Yb}{\mathrm{Al}}_{2}$ remain unchanged in high fields, as expected for this intermediate-valent compound with a high fluctuation temperature.

Super-conducting niobium films produced by means of UHV arc

The paper shows that high-quality super-conducting Nb films can be deposited by means of arc discharges under ultra-high vacuum conditions. The critical temperatureT c of Nb-films obtained with a planar cathodic arc Ultra High Vacuum (UHV) system, is very close to that of pure bulk niobiumT c=9.26 K and the transition to super-conducting state is very narrow. The films have higher Residual Resistivity Ratio (RRR) values (up to 80) and larger grain sizes, as compared with sputtered Nb-films deposited at the same temperature.

High-quality single crystal growth of UGe2 and URhGe

Abstract We searched for an experimental condition of crystal growth for a ferromagnetic superconductors of UGe 2 and URhGe. Samples were prepared for both polycrystalline and single crystals, which were arc-melted and grown by the Czochralski-pulling method in a tetra-arc furnace, respectively. Annealing is an important process to enhance sample-quality, namely the residual resistivity ratio. We tried to anneal the samples under various temperatures, which were wrapped by the Ta-foil and vacuum-sealed in the quartz ample, together with the solid state electrotransport method in ultra-high vacuum. The highest residual resistivity ratio was 900 for UGe 2 and 41 for URhGe.

Magnetic Properties of Single Crystalline Mn4Si7.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Magnetic properties of single crystalline Mn 4Si 7

A single crystal of MnSi 2− x was obtained by a modified Czochralski pulling technique in a cold copper crucible. The quality and the nature of the sample were checked by an accurate crystal structure determination, which revealed the sample to be Mn 4Si 7. Resistivity and magnetic measurements were performed on this sample in the temperature range between 2 K and room temperature and in magnetic fields up to 7.5 T. Mn 4Si 7 shows a metallic behaviour and the good crystal quality was revealed by the high residual resistance ratio of 360. For the magnetic susceptibility we observed a Curie–Weiss law above about 40 K with a low effective moment of p eff=0.365 μ B/Mn. Below this temperature, moments order in an anisotropic helical state, and in fields above 1 T, they align with a saturation moment of p sat=0.012 μ B/Mn. Mn 4Si 7 is a weak itinerant magnetic system that could be a good candidate for the observation of the critical quantum fluctuations expected for marginal Fermi liquids.

Superconducting MgB2 thin films on silicon carbide substrates by hybrid physical–chemical vapor deposition

We have used two polytypes of silicon carbide single crystals, 4H-SiC and 6H-SiC, as the substrates for MgB2 thin films grown by hybrid physical-chemical vapor deposition (HPCVD). The c-cut surface of both polytypes has a hexagonal lattice that matches closely with that of MgB2. Thermodynamic calculations indicate that SiC is chemically stable under the in situ deposition conditions for MgB2 using HPCVD. The MgB2 films on both polytypes show high-quality epitaxy with a Rutherford backscattering channeling yield of 12%. They have Tc above 40 K, low resistivities, high residual resistivity ratios, and high critical current densities. The results demonstrate that SiC is an ideal substrate for MgB2 thin films.

High-strength and high-RRR Al-Ni alloy for aluminum-stabilized superconductor

The precipitation type aluminum alloys have excellent performance as the increasing rate in electric resistivity with additives in the precipitation state is considerably low, compared to that of the aluminum alloy with additives in the solid-solution state. It is possible to enhance the mechanical strength without remarkable degradation in residual resistivity ratio (RRR) by increasing content of selected additive elements. Nickel is the suitable additive element because it has very low solubility in aluminum and low increasing rate in electric resistivity, and furthermore, nickel and aluminum form intermetallic compounds which effectively resist the motion of dislocations. First, Al-0.1wt%Ni alloy was developed for the ATLAS thin superconducting solenoid. This alloy achieved high yield strength of 79 MPa (R.T.) and 117 MPa (4.2 K) with high RRR of 490 after cold working of 21% in area reduction. These highly balanced properties could not be achieved with previously developed solid-solution aluminum alloys. In order to achieve higher strength than the above, Al-Ni alloys of up to 2.0 wt% Ni content were investigated. Al-2.0wt%Ni alloy achieved yield strength of 120 MPa (R.T.) and 167 MPa (4.2 K) with RRR of 170 after cold working of 20% in area reduction.

Unusual electrical resistivity anomalies of thulium thin films at low temperatures

High precision electrical resistance measurements were performed in the temperature range from 4.2 K up to room temperature on a pure thulium bulk sample with a high residual resistivity ratio, RRR=146, and on thulium thin films in the thickness interval from 25 to 115 nm. The observed size-effect of the electrical resistivity is in accordance with theory and it is without resistivity vs. thickness anomalies. The typical “hump-backed” anomaly in the resistivity vs. temperature dependence was observed for the Tm bulk sample near the Néel temperature. In contrast, unexpected resistivity vs. temperature anomalies were observed for thulium thin films in the low temperature region below ∼60 K and near ∼170 K. A crystal structure study using X-ray diffraction revealed that these thin films were single phase with the Tm hcp crystal structure and with preferential orientation of crystallites.

R&amp;D of superconductors for 70 MW class generators

Super-GM has been developing 70 MW class superconducting generators (model machines) for a 200 MW class pilot generator. Three types of NbTi conductors were developed for each of three model machines. Regarding the high stability type conductors, residual resistance value of 6.5 /spl mu//spl Omega//m was achieved by adopting an Al stabilizer. Regarding the high current density type, Ic of 13.8 kA at 5 T was realized by reducing void fraction in conductors and maintaining high residual resistivity ratio. For the low AC loss type, high Jc and low AC loss (8.1 kW/m/sup 3/) have been succeeded by applying strand insulation and two stage cabling. To confirm superconducting characteristics concerning long-term reliability, Ic and AC loss of each conductor were measured under 10000 times repeated compressive force of 30 MPa that is supposed in daily start and stop operation of a generator for 30 years. In parallel to the R and D of NbTi conductors, R and D of Nb/sub 3/Sn conductors were also conducted for enhancement of critical current density and critical magnetic field.

Angular dependence of the magnetoresistance of TiSi 2 single crystals

We measured the transverse magnetoresistance Δϱ ϱ of good quality TiSi 2 single crystals at low temperatures (4.2≤ T≤112 K) in magnetic fields up to 7.8 Tesla. Single crystals were produced by a modified Czochralski pulling technique and they have low residual resistivity (typically ϱ(4.2 K) = 0.15 μΩ· cm ) and high residual resistance ratio (typically RRR > 50). The angular dependence of magnetoresistance shows either minima or maxima when the magnetic field is parallel to the principal crystallographic axes. At high fields ( B > 1 T). we found that the magnetoresistance has a magnetic field dependence weaker than the B 2 law expected for compensated metals. At 7.8 T, the values of ω c τ obtained are of the order of unity. The Kohler scaling rule is observed within three orders of magnitude of the reduced parameter B ϱ (where ϱ is the zero field resistivity measured between 4.2 and 112 K).

Electrical resistivity and superconductivity of LaB6 and LuB12

The electrical resistivity of LaB6 and LuB12 single crystals with high residual resistivity ratios has been measured down to 1.6 K. The temperature dependence below 30 K, caused by phonon scattering, shows for both materials a Tn behaviour with n = 4.2 ± 0.1 for LaB6 and n = 5.2 ± 0.1 for LuB12. From susceptibility measurements the superconducting transition temperature of LuB12 was determined to be 0.44 K, for the LaB6 sample with residual resistivity ratio of 160 no transition into the superconducting state was observed down to 5 mK.

Resistance peak at the resistive transition in high-Tc superconductors.

The resistance peak, which is occasionally observed near the resistive transition in high-[ital T][sub [ital c]] superconductors, is shown by numerical analysis to arise from a slight inclination of the sample face from the CuO[sub 2] planes. This inclination combined with large conductivity anisotropy causes a significant distribution of the current density and reduces the voltage drop between voltage probes, resulting in a very small apparent resistivity and high residual resistivity ratio. In the resistive transition region, on the other hand, the anisotropy abruptly decreases, and the apparent resistance increases and approaches the true value before the resistivity vanishes, resulting in an apparent peak. The resistance peak is therefore a manifestation of the fact that the resistive transition along the [ital c] axis is much sharper than that in the CuO[sub 2] planes.

Preparation and characterization of extra-low-oxygen titanium

Removal of oxygen in titanium by reaction with chemically active calcium dissolved in CaCl 2 was examined between 1273 and 1473 K with the purpose of obtaining extra low-oxygen titanium. CaCl 2 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 CaCl 2 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.