Irreversibility Field Research Articles

Upward shift of the vortex solid phase in high-temperature-superconducting wires through high density nanoparticle addition.

We show a simple and effective way to improve the vortex irreversibility line up to very high magnetic fields (60T) by increasing the density of second phase BaZrO3 nanoparticles. (Y0.77,Gd0.23)Ba2Cu3Oy films were grown on metal substrates with different concentration of BaZrO3 nanoparticles by the metal organic deposition method. We find that upon increase of the BaZrO3 concentration, the nanoparticle size remains constant but the twin-boundary density increases. Up to the highest nanoparticle concentration (n ~ 1.3 × 1022/m3), the irreversibility field (Hirr) continues to increase with no sign of saturation up to 60 T, although the vortices vastly outnumber pinning centers. We find extremely high Hirr, namely Hirr = 30 T (H||45°) and 24 T (H||c) at 65 K and 58 T (H||45°) and 45 T (H||c) at 50K. The difference in pinning landscape shifts the vortex solid-liquid transition upwards, increasing the vortex region useful for power applications, while keeping the upper critical field, critical temperature and electronic mass anisotropy unchanged.

Visual Field Loss in a Case of Recurrent Cystic Craniopharyngioma During Concomitant Treatment With Pegylated Interferon α-2b

A 13-year-old male with suprasellar cystic craniopharyngioma initially controlled with sequential subtotal resections and proton-beam irradiation was later treated with intracystic pegylated interferon α-2b due to progression and a lack of further surgical options. After initial successful control of recurrent cyst enlargement and stabilization of the ophthalmic examination, progressive and irreversible visual field loss ensued. Imaging revealed intracranial leakage from the intracystic catheter, and direct administration of interferon α-2b was discontinued. Given the recent interest in interferon α-2b, oncologists are advised to vigilantly monitor patients for signs of local toxicity that may result from unintended leakage during intracystic delivery.

Evidence of Point Pinning Centers in Un-Doped Mgb2 Wires at 20 K after HIP Process

In this paper we present results of transport critical current density (Jc) at 20 K and 4.2 K, irreversible magnetic field (Birr), upper critical field (Bc2), critical temperature (Tc), pinning force (Fp), scanning pinning force scaling results (Fp/Fpmax and B/Birr) and electron microscope (SEM) images of un-doped MgB2 wires of 0.63 mm diameter. All wires were annealed at pressures ranging from 0.1 MPa to 1 GPa for 15 min between 680°C to 740°C. SEM images show that 1 GPa pressure yields small grains, higher MgB2 material density, and small voids. The results obtained by a physical properties measurement system (PPMS) show that high pressure (1 GPa) and 700°C annealing slightly decreases Tc above 27 K and increases Tc and Birr below 25 K. Un-doped MgB2 wire annealed in 1 GPa for 15 min at 680°C at has a 20 K, 4.5 T Jc of 100 A/mm2 in and a Birr of 7 T. At 4.2 K, this wire has Jc of 100 A/mm2 at 10.5 T. Scaling results show that the dominant pinning mechanism is point pinning for undoped MgB2 wires under 1 GPa pressure and annealed at 680°C (at 20 K).

Mechanical Spectroscopy of MgB2 Containing Sic / Spektroskopia Mechaniczna MgB2 Zawierającego Sic

The compound magnesium diboride (MgB2) has been well-known since the 1950s; however, its superconducting properties were unknown. Intrinsic characteristics of MgB2 make this material a promising candidate for technological applications, although the low value of the irreversibility field and the decrease in critical current density with the increase in the magnetic field considerably reduce its utility. The present work aimed to study the effect of carbon-based doping on anelastic properties of MgB2 as measured by mechanical spectroscopy. The samples were prepared by using the powder-intube method. The samples were made with 5, 7.5, and 10 wt.% of silicon carbide (SiC). The results reveal complex mechanical loss spectra caused by the interaction between point defects and surface defects in the crystalline lattice of MgB2.

High field superconducting properties of Ba(Fe1−xCox)2As2 thin films

In general, the critical current density, Jc, of type II superconductors and its anisotropy with respect to magnetic field orientation is determined by intrinsic and extrinsic properties. The Fe-based superconductors of the ‘122’ family with their moderate electronic anisotropies and high yet accessible critical fields (Hc2 and Hirr) are a good model system to study this interplay. In this paper, we explore the vortex matter of optimally Co-doped BaFe2As2 thin films with extended planar and c-axis correlated defects. The temperature and angular dependence of the upper critical field is well explained by a two-band model in the clean limit. The dirty band scenario, however, cannot be ruled out completely. Above the irreversibility field, the flux motion is thermally activated, where the activation energy U0 is going to zero at the extrapolated zero-kelvin Hirr value. The anisotropy of the critical current density Jc is both influenced by the Hc2 anisotropy (and therefore by multi-band effects) as well as the extended planar and columnar defects present in the sample.

Magnetization of underdopedYBa2Cu3Oyabove the irreversibility field

Torque magnetization measurements on YBa$_2$Cu$_3$O$_{y}$ (YBCO) at doping $y=6.67$($p=0.12$), in DC fields ($B$) up to 33 T and temperatures down to 4.5 K, show that weak diamagnetism persists above the extrapolated irreversibility field $H_{\rm irr} (T=0) \approx 24$ T. The differential susceptibility $dM/dB$, however, is more rapidly suppressed for $B\gtrsim 16$ T than expected from the properties of the low field superconducting state, and saturates at a low value for fields $B \gtrsim 24$ T. In addition, torque measurements on a $p=0.11$ YBCO crystal in pulsed field up to 65 T and temperatures down to 8 K show similar behaviour, with no additional features at higher fields. We discuss several candidate scenarios to explain these observations: (a) superconductivity survives but is heavily suppressed at high field by competition with CDW order; (b) static superconductivity disappears near 24 T and is followed by a region of fluctuating superconductivity, which causes $dM/dB$ to saturate at high field; (c) the stronger 3D ordered CDW that sets in above 15 T may suppress the normal state spin susceptibility sufficiently to give an apparent diamagnetism of the magnitude observed.

Comparison of flux pinning in Si- and SiCl4-doped MgB2 superconductors: evidence for coexistence of different pinning mechanisms

The transport and magnetic properties of 5 wt% nano-Si- and 10 wt% SiCl4-doped MgB2 have been studied by measuring the resistivity, ρ, critical current density, Jc, irreversibility field, Hirr, and upper critical field, Hc2. Similar scattering mechanisms have been found for both dopants, which are supported by the results for the critical temperature and the upper critical field. The critical current density and the irreversibility field results indicate that there are different pinning mechanisms for nano-Si- and SiCl4-doped MgB2. Pinning mechanisms are studied in terms of the different pinning models. It was found that a variety of pinning mechanisms, e.g. normal point pinning, normal surface pinning, and normal volume pinning mechanisms coexist in both types of doped MgB2. The results show that the contributions of the pinning mechanisms are dependent on the temperature and magnetic field.

The influence of heating rate on superconducting characteristics of MgB2 obtained by spark plasma sintering technique

Superconducting bulks of MgB2 were obtained by the Spark Plasma Sintering (SPS) technique. Different heating rates of 20, 100, 235, 355, and 475 °C/min were used. Samples have high density, above 95%. The onset critical temperature Tc, is about 38.8 K. There is an optimum heating rate of ∼100 °C/min to maximize the critical current density Jc0, the irreversibility field Hirr, the product (Jc0 x μ0Hirr), and to partially avoid formation of undesirable flux jumps at low temperatures. Significant microstructure differences were revealed for samples processed with low and high heating rates in respect to grain boundaries.

Vortex Dynamics and Diamagnetic Torque Signals in Two Dimensional Organic Superconductor λ-(BETS)2GaCl4

Resistance and magnetic torque measurements have been performed for a two dimensional organic superconductor λ-(BETS)2GaCl4, where BETS stands for bis(ethylenedithio)tetraselenafulvalene. In magnetic fields along the c-axis (parallel to the conducting layers), the interlayer resistance shows a peak at ∼7 T for large currents in the superconducting state. The peak disappears when the magnetic field is tilted from the layers only by 0.2°, which shows that the peak is caused by the dynamics of the Josephson vortices penetrating in the insulating layers. The current–voltage characteristics show that the depinning current of the Josephson vortices is consistently suppressed at around 7 T. The diamagnetic signal, irreversibility field, and depinning current rapidly decrease with increasing field above ∼10 T at low temperatures. The results suggest that there exists a phase boundary in the superconducting state, likely a transition to a Fulde–Ferrell–Larkin–Ovchinnikov phase.

Enhancement of critical current of SiC and malic acid codoped MgB2∕Fe wires

The influences of microstructure, connectivity, and disorder on the critical current density, [Formula: see text], are discussed to clarify the different mechanisms of [Formula: see text] in different magnetic field ranges for in situ and combined in situ/ex situ [Formula: see text] wires with nano SiC and malic acid codoping. Sintering temperature plays a very important role in the electromagnetic properties at different temperatures and under various magnetic fields. Connectivity, upper critical field, [Formula: see text], and irreversibility field, [Formula: see text], are studied to demonstrate the mechanism of [Formula: see text] dependence on magnetic field. The combined in situ/ex situ process is proved to be a promising technique for fabrication of practical [Formula: see text] wires.

Study of the Irreversibility Line of GdBa 2Cu 3 O 7 − δ Added with Nanosized Ferrite CoFe 2 O 4

(CoFe 2O4) xGdBa 2Cu 3O7−δ superconducting samples were synthesized using a conventional solid-state reaction technique with 0≤x≤0.1 wt%. Characterization of the prepared samples was carried out using X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The effect of adding CoFe 2O4 to the Gd-123 phase, which acts as a flux pinning center, was investigated by measuring the ac magnetic susceptibility at different applied dc magnetic fields. Both the superconducting transition temperature Tc and the critical current density Jc increased up to x=0.01 wt%, and then, they decreased with further increase in x. The temperature dependence of the irreversibility line was investigated by the logarithmic plot of the irreversibility field Hirr versus (1−Tirr/Tc(0)). This plot shows a crossover at a field of 1000 Oe, indicating 3D to 2D flux creep fluctuations. Furthermore, the Hirr–T curves are well fitted according to a theoretical model proposed by Matsushita, based on the depinning mechanism caused by thermally activated flux creep.

Evidence for enhancement of vortex matching field above 5 T and oxygen-deficient annuli around barium-niobate nanorods

We report the dependence of critical temperature (Tc), the irreversibility line, the microstructure of nanorods with perovskite-based barium niobates (BNOs), and the c-axis length of the YBa2Cu3Oy (Y123) superconducting matrix on the BNO-doping level and growth temperature for Y123 thin films with BNO nanorods. The characteristic field (Bcr) determined from the vortex-Bose-glass-like irreversibility lines in Y123+BNO films is strongly correlated to the BNO nanorod density and the growth temperature. Despite a monotonic decrease in Tc with increasing Bcr and nanorod density, the irreversibility fields (Birr) were enhanced up to Bcr ∼ 5 T. From the Bcr value and the mean diameter of the BNO nanorod (∼10 nm), we estimate that a lower Tc matrix annulus with a diameter of 12–14 nm exists around each BNO nanorod due to the strong interface strains. Our present study suggests that generation of this lower Tc region around each BNO nanorod increases the vortex-pinning strength significantly and, moreover, may offer a new way of enhancing Birr for REBa2Cu3Oy film with nanorods.

Vortex pinning at low temperature under high magnetic field in SmBa2Cu3Oy superconducting films with high number density and small size of BaHfO3 nano-rods

We have fabricated 5.6 vol.% BaHfO3 (BHO)-doped SmBa2Cu3Oy (SmBCO) films at a low substrate temperature of 750 °C by using the low temperature growth (LTG) technique with a seed layer. Using the LTG technique, we can obtain a purely c-axis oriented SmBCO film even at low growth temperatures. The LTG technique also greatly increases the number density of BHO nanorods. The size and matching field of the BHO nanorods in the films were 5.4 nm and 9.9 T. Transport measurements were carried out, and a high irreversibility field (Birr) and strong flux pinning force density (Fp) were obtained. The Birr was 15.1 T at 77 K for B//c. The maximum Fp of 407 GN m−3 in 10 T at 40 K and 770 GN m−3 from 9 to 17 T at 20 K have been measured. We believe that the LTG technique can be considered as important for improving of flux pinning performance in BaMO3 (BMO: M = Zr, Sn and Hf)-doped SmBCO coated conductors.

Superconducting properties of SiC-buffered-MgB2tapes

Production of <TEX>$MgB_2$</TEX> film on metallic Hastelloy with SiC as the buffer layer was achieved by means of hybrid physical-chemical vapor deposition technique, whereas SiC buffer layers with varied thickness of 170 and 250 nm were fabricated inside a pulsed laser deposition chamber. Superconducting transition temperature and critical current density were verified by transport and magnetic measurement, respectively. With SiC buffer layer, the reduced delaminated area at the interface of <TEX>$MgB_2$</TEX>-Hastelloy and the slightly increased <TEX>$T_c$</TEX> of <TEX>$MgB_2$</TEX> tapes were clearly noticed. It was found that the upper critical field, the irreversibility field and the critical current density were reduced when <TEX>$MgB_2$</TEX> tapes were buffered with SiC buffer layer. Clarifying the mechanism of SiC buffer layer in <TEX>$MgB_2$</TEX> tape in affecting the superconducting properties is considerably important for practical applications.

Demonstration of an iron-pnictide bulk superconducting magnet capable of trapping over 1 T

A trapped field of over 1 T at 5 K and 0.5 T at 20 K has been measured between a stack of magnetized cylinders of bulk polycrystalline Ba0.6K0.4Fe2As2 superconductors 10 mm in diameter and 18 mm in combined thickness. The trapped field showed a low magnetic creep rate (∼3% after 24 h at 5 K), while magneto-optical imaging revealed a trapped field distribution corresponding to uniform macroscopic current loops circulating through the sample. The superconductors were manufactured by hot isostatic pressing of pre-reacted powders using the scalable powder-in-tube technique. A high Vickers hardness of ∼3.5 GPa and a reasonable fracture toughness of ∼2.35 MPa m0.5 were measured. Given the untextured polycrystalline nature of the cylinders and their large irreversibility field (>90 T), it is expected that larger bulks could trap fields in excess of 10 T.

BaHfO3 artificial pinning centres in TFA-MOD-derived YBCO and GdBCO thin films

Chemical solution deposition (CSD) is a promising way to realize REBa2Cu3O7−x (REBCO; RE = rare earth (here Y, Gd))-coated conductors with high performance in applied magnetic fields. However, the preparation process contains numerous parameters which need to be tuned to achieve high-quality films. Therefore, we investigated the growth of REBCO thin films containing nanometre-scale BaHfO3 (BHO) particles as pinning centres for magnetic flux lines, with emphasis on the influence of crystallization temperature and substrate on the microstructure and superconductivity. Conductivity, microscopy and x-ray investigations show an enhanced performance of BHO nano-composites in comparison to pristine REBCO. Further, those measurements reveal the superiority of GdBCO to YBCO—e.g. by inductive critical current densities, Jc, at self-field and 77 K. YBCO is outperformed by more than 1 MA cm−2 with Jc values of up to 5.0 MA cm−2 for 265 nm thick layers of GdBCO(BHO) on lanthanum aluminate. Transport in-field Jc measurements demonstrate high pinning force maxima of around 4 GN m−3 for YBCO(BHO) and GdBCO(BHO). However, the irreversibility fields are appreciably higher for GdBCO. The critical temperature was not significantly reduced upon BHO addition to both YBCO and GdBCO, indicating a low tendency for Hf diffusion into the REBCO matrix. Angular-dependent Jc measurements show a reduction of the anisotropy in the same order of magnitude for both REBCO compounds. Theoretical models suggest that more than one sort of pinning centre is active in all CSD films.

The electric field jump—detection of damaged Nb barrier in MgB2 wires annealed under pressure

In this paper we present the results of critical current (Ic) measurements of MgB2 wires made with two different set-ups of the four-point probe method: current sweep type—constant magnetic field and increasing current, and field sweep type—constant current and rapidly increasing magnetic field. Results from magnet field sweep type measurements can be interpreted by a new physical concept—a jump of the electric field in low magnetic fields. This physical concept can be correlated with damages in the Nb-barrier existing in the MgB2 wire and be employed as a detection scheme. The damage in Nb barrier reduces critical current density (Jc) and complicates the study on critical temperature (Tc), upper critical field (Bc2), irreversible magnetic field (Birr), pinning force (Fp), and pinning centers in superconducting MgB2 wires. Our proposed method to detect damages in Nb barrier would benefit efforts in development and applications of MgB2 wires.

Flux free growth of large FeSe1/2Te1/2 superconducting single crystals by an easy high temperature melt and slow cooling method

We report successful growth of flux free large single crystals of superconducting FeSe1/2Te1/2 with typical dimensions of up to few cm. The AC and DC magnetic measurements revealed the superconducting transition temperature (Tc) value of around 11.5K and the isothermal MH showed typical type-II superconducting behavior. The lower critical field (Hc1) being estimated by measuring the low field isothermal magnetization in superconducting regime is found to be above 200Oe at 0K. The temperature dependent electrical resistivity ρ(T ) showed the Tc (onset) to be 14K and the Tc(ρ = 0) at 11.5K. The electrical resistivity under various magnetic fields i.e., ρ(T)H for H//ab and H//c demonstrated the difference in the width of Tc with applied field of 14Tesla to be nearly 2K, confirming the anisotropic nature of superconductivity. The upper critical and irreversibility fields at absolute zero temperature i.e., Hc2(0) and Hirr(0) being determined by the conventional one-band Werthamer–Helfand–Hohenberg (WHH) equation for the criteria of normal state resistivity (ρn) falling to 90% (onset), and 10% (offset) is 76.9Tesla, and 37.45Tesla respectively, for H//c and 135.4Tesla, and 71.41Tesla respectively, for H//ab. The coherence length at the zero temperature is estimated to be above 20Å ´ by using the Ginsburg-Landau theory. The activation energy for the FeSe1/2Te1/2 in both directions H//c and H//ab is determined by using Thermally Activation Flux Flow (TAFF) model.

Magnetic nanoparticles in MgB2: Vortex pinning, pair breaking and connectivity

The results indicating magnetic flux pinning in MgB2 wires doped with three types of magnetic nanoparticles (MNP) are reported. The magnetic state of MNPs, both as-prepared and inside the MgB2 core, was determined with magnetization and ac susceptibility measurements. The competition between detrimental influence of doping (reduced connectivity, pair breaking) and enhanced flux pinning leads to deterioration of electromagnetic properties of doped wires at high MNP content, whereas light doping causes an enhancement of critical current density, Jc, and/or irreversibility field, Birr, for all our MNPs. For Ni and dextrin coated NiFe2O4 MNPs the enhancement of Jc was comparable to that achieved with the best nonmagnetic dopands. Detailed analysis indicates the contribution of magnetic flux pinning including the matching effects in flux pinning on MNPs.

Ti-doping effects on magnetic properties of dense MgB2 bulk superconductors

We have studied the effects of Ti doping on the magnetic properties of MgB2 superconducting bulks. The trapped field, which was obtained by field-cooled magnetization, , was about 3.6 T at 13 K at the surface of the single Ti5–20% doped MgB2 bulks, which was about 1.3 times larger than that of the non-doped bulk. The of 4.6 T was achieved at 14.1 K in the centre of the doubly stacked Ti-doped MgB2 bulks. The remanent magnetic flux density, which corresponds to the trapped field by zero-field cooled magnetization, , was comparable with the absolute value of coercive force with a very small vortex creep rate of about 2% over 40 h. These results suggested that the MgB2 bulk was an excellent ‘quasi-permanent’ magnet. The critical current density, , under magnetic field was also enhanced by Ti doping; under 3 T at 20 K, the of A cm−2 for the pristine sample was enhanced to that of 1.6–1.8 A cm−2 for the Ti-doped samples. The irreversibility field exceeded 5 T at 20 K for the Ti-doped samples. The existence of nanometric unreacted B and strongly Mg-deficient Mg-B particles and TiB2 layers at the periphery of Ti precipitates was suggested in the Ti-doped bulks by microscopic observation. The improvement of the vortex pinning properties in Ti-doped MgB2 originated from the creation of the nanometric nonsuperconducting particles and TiB2 layers acting as strong vortex pinning centres.