Magnetic Critical Current Density Research Articles

Thermal gradient-induced critical current degradation in mesoscopic superconducting thin film

Abstract Superconducting materials inevitably suffer from the sudden change of temperature in localized areas in practical applications, and the concomitant thermal gradient may be detrimental to their performance. Critical current density is a key factor affecting the performance of superconductors. However, the effect of thermal gradient on the critical current density has not been identified. Here, by combining the time-dependent Ginzburg–Landau equations and the heat transfer equation, the thermal gradient and magnetic field dependence of the critical current density are systematically investigated and rationalized by exploring the behavior of vortex and magnetization. For lower magnetic fields, it is found that the thermal gradients strongly reduce the local surface barriers, which inhibits vortex entry and movement, leading to a rapid deterioration of the current-carrying capability. Under moderate magnetic fields, the critical current density corresponding to higher thermal gradients decreases more slowly with increasing magnetic field, which results from the thermal gradient-induced entry and moving of vortices along the current direction. As the magnetic field continues to increase, the variation of the critical current density transitions into a platform period and even slightly rises. The enhanced critical current is primarily attributed to the excess entry of vortices, which increases the surface barrier of the sample. With the further increase in the magnetic field, the critical current density continues to decrease due to increased magnetic field penetration. These results unveil the fundamental interplay between thermal gradients, external magnetic field, vortex, magnetization and critical current density, and provide a theoretical basis for understanding the heat-induced quenching of mesoscopic superconducting thin films in practical applications.

Effect of nano-Ag addition on the superconducting properties of MWCNT doped magnesium diboride superconductor

Bulk MgB2 superconductor added with nano silver (Ag), multiwalled carbon nanotube (MWCNT) and both the added (i.e., nano-Ag and MWCNT) samples were synthesized. The influence of nano-Ag and MWCNT addition on the structural and superconducting properties were studied. The XRD analysis confirms the formation of hexagonal MgB2 superconductor. Microstructural studies of all the synthesized samples are carried out by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The superconducting properties such as critical temperature (Tc), magnetic critical current density (Jc), irreversibility field (Birr) and flux pinning properties of all the synthesized samples were investigated at 10 K and field up to 7 T. Nano Ag added MWCNT doped MgB2 shows maximum enhancement in the critical current density i.e., 5.83 × 104 A/cm2 at 5 T and 10 K.

The influence of the HIP process on the microstructure, critical temperature and critical current density of double-doped (with Dy2O3 + C, and Tb2O3 + C) MgB2 bulk compounds

ABSTRACT We present the results of the irreversibility field (B irr ), critical temperature (T c), and magnetic critical current density (J cm) for cylindrical samples encapsulated in carbon and doped with 1-3 at% dysprosium and terbium oxides. All samples were analyzed microstructurally by using a scanning electron microscope (SEM) and X-ray diffraction (XRD). The influence of annealing temperature and time and high isostatic pressure on J cm was investigated. Our results show that double-doped (C + Tb2O3 and C + Dy2O3) and low isostatic pressure heat treatment (0.1 MPa) decrease the T c by about 0.5 K compared to the undoped MgB2 (0.1 MPa). The magnetic measurements show that double-doped and heat treatment under high isostatic pressure (HIP - 1 GPa) leads to a decrease in T c of about 2.5 K compared to the undoped MgB2 sample after the HIP process. Our results show that the HIP process slightly increases J cm at 4.2 K and 20 K, and improves J cm at 30 K in undoped MgB2 samples. Furthermore, our results show that the HIP process and double-doped significantly increase J cm at 4.2K and decrease J cm at 30K. Our results indicate that the HIP process can produce high field and low-temperature pinning centers in doubly doped MgB2 materials.

Impact of rare earth oxides metal (Dy, Tb, Er, Eu, Sm) admixtures on the structure, morphology, pinning centers and critical parameters of MgB2 materials manufactured by ambient pressure sintering and hot isostatic pressure processes

In our paper, we present the results of the structural, morphological and critical parameters of MgB2 cylindrical materials doped with Dy2O3, Tb2O3, Er2O3, Eu2O3 and Sm2O3. The materials were annealed under low and high isostatic pressures at 800 °C for 1 h in an argon atmosphere. Our results show that the HIP process significantly increases the density of MgB2 materials modified by the RE2O3 additive from 1.05 to 2.4 g/cm3. Scanning electron microscope (SEM) images indicate that the HIP process removes voids from the MgB2 material. The results indicate that the HIP process slightly affects the formation of REB4 and REB6 and does not affect the process of RE substitution for Mg. Further SEM results show that the morphologies of samples annealed at low isostatic pressure with Dy2O3, Tb2O3 and Er2O3 doping are completely different from those of samples with Eu2O3 and Sm2O3 doping. The magnetic measurements indicate that RE2O3 doping and annealing under low isostatic pressure slightly increases the critical temperature (Tc) and decreases the irreversible magnetic field (Birr). On the other hand, thermal treatment under high isostatic pressure with an admixture of RE2O3 lead to a greater increase in Tc and a decrease in Birr. Further magnetic measurements show that RE2O3 doping and annealing under low isostatic pressure mainly increase the magnetic critical current density (Jmc) at 4.2 K–30 K in low magnetic fields. Additional results indicate that high isostatic-pressure annealing of the RE2O3-doped MgB2 material significantly increases Jmc in low magnetic fields and decreases Jmc in high magnetic fields. Our results indicate that the Eu2O3 admixture most effectively increases Tc and Jmc and creates the large amount of the REB6 phase. It is as the only admixture that increases Birr after the HIP process and yields the smallest MgB2 grains.

Defect-induced weak collective pinning in superconducting YB6 crystals

In a previous study (2017 Phys. Rev. B 96 144501), a strong variation in the superconducting transition temperature T c of YB6 differing by a factor of two has been explained by a change in the density of yttrium and boron vacancies tuning the electron–phonon interaction. Here, by using an array of miniature Hall probes, we address the penetration of the magnetic field, pinning, and critical current density on a series of YB6 single crystals with T c variation between 4.25 and 7.35 K. The analysis of the superconducting and normal-state specific heat characteristics allowed us to determine T c and the stoichiometry of our samples. We observed almost no pinning in the most stoichiometric YB6 crystal with the lowest T c. Upon increasing the number of vacancies weak pinning appears, and the critical current density is enhanced following the increased transition temperature in a linear variation. We argue that such an increase is, within weak collective pinning theory, consistent with the increasing number of vacancies that serve as pinning centers.

Influence of bending and heat treatment under high isostatic pressure on synthesis reaction and transport critical current density in isotopic IMD Mg11B2 'wind and react' coils

We show for the first time the results for small coils, which were wound with unreacted magnesium diboride (MgB2) wires with nano-amorphous isotopic boron (11B) by using combined internal magnesium (Mg) diffusion (IMD) and hot isostatic pressing (HIP) techniques. The small coils with a diameter of 20 mm or 10 mm were annealed under low (0.1 MPa) and high (1.1 GPa) isostatic pressure. The 10 mm coils annealed under high isostatic pressure of 1.1 GPa had significantly increased critical temperature (Tc), irreversible magnetic field (Birr) and transport critical current density (Jtc), with significantly accelerated Mg diffusion and improved homogeneity of the Mg11B2 material. Moreover, our findings show for the first time that the bending of the unreacted IMD Mg11B2 wires significantly affects the synthesis reaction under the low and high isostatic pressure. Our findings also show for the first time that diffusion of Mg during heat treatment under high isostatic pressure is completely different than during heating under low isostatic pressure. Mg diffusion under low isostatic pressure annealing is mainly dependent on the heat treatment temperature, bending diameter and 11B layer density. This Mg diffusion is related to the simultaneous formation of the superconducting phase. However, the diffusion of Mg under high isostatic pressure heat treatment is mainly dependent on the isostatic pressure, 11B layer density and Mg state (liquid or solid). Our results indicate that liquid Mg is first pushed into the 11B layer by high isostatic pressure and then a superconducting phase is formed.

Effect of thickness on the superconducting properties of the thin films of FeTe0.55Se0.45 deposited on YSZ substrates using Pulsed laser deposition (PLD) technique

We report the successful fabrication of thin films of thickness ∼26 nm, 59 nm, 76 nm, and 158 nm of the composition FeTe0.55Se0.45 on the (100)-oriented Yttria-stabilized zirconia (YSZ) substrate using the pulsed laser deposition (PLD) technique. The effect of thickness on the structure and superconducting properties of the thin films has been investigated. X-ray diffraction study reveals the growth of thin films along the c-axis. The temperature-dependent resistivity measurements confirm superconductivity in all the fabricated thin films, and the highest ∼ 16.2 K has been observed for the thin film of thickness ∼59 nm, which is higher than the corresponding value of the polycrystalline target sample. The superconducting parameters such as upper critical magnetic field coherence length thermally activated energy (TAE) U0, and vortex phase state have been measured by magnetotransport measurements near in the different applied magnetic fields up to 12 T. The TAE values determined from the conventional and modified thermally activated flux flow (TAFF) models are discussed and compared. A crossover from 2-dimensional (2D) vortex to 3-dimensional (3D) vortex behaviour has been observed. The vortex phase diagrams are used to observe the transition from the vortex liquid to the vortex glass state in the grown thin films. Moreover, magnetoresistance (MR) measurements show non-saturating linear magnetoresistance (LMR) in the high field region, indicating the presence of the possible topological state in the grown thin films. The values of magnetic field dependent transport critical current density, have been evaluated from the current-voltage (I-V) curves. Furthermore, the normalised pinning force density has been used to characterize the pinning processes using the Dew Hughes’ scaling rule which reveals that the grown thin films contain δl-surface pinning centers.

Phase Transition of Nb3Sn during the Heat Treatment of Precursors after Mechanical Alloying

The phase transition process of Nb3Sn during heat treatment exerts important influences on Nb3Sn formation and the superconducting characteristics of Nb3Sn superconductors. A simple method for quickly preparing Nb3Sn was studied. First, Nb, Sn, and Cu powders were mechanically alloyed to prepare the precursor. Then, the precursor was heat treated at different times to form Nb3Sn. During the first stage, the morphology and crystal structure of the products were analyzed after different milling times. The results of the transmission electron microscopy showed the poor crystallinity of the products compared with the original materials. During the second stage, heat treatment was performed at different temperatures ranging from room temperature to 1073 K. After treatment, the products were studied via X-ray diffraction analysis to determine how the structure changed with increasing temperature. Only the Nb diffraction peaks in the precursor were observed after high-energy ball milling for more than 3 h. When the heat treatment temperature was above 773 K and heat treatment time was 15 min, Nb3Sn began to form. When the temperature was above 973 K, some impurities, such as Nb2O5, appeared. After 5 h of ball milling, the precursor was heat treated at different times in a vacuum heat treatment furnace. The crystal structure of the product exhibited evident diffraction peaks of Nb3Sn. The critical temperatures of the samples that were heat treated at different times were between 17 K and 18 K. The magnetic critical current density of the sample versus the applied magnetic field at 4.2 K indicated that the magnetic Jc was approximately 30,000 A/cm2.

Effectiveness of Two Pinning Profiles in Controlling Magnetic Critical Current Density in YBCO + YCMO Composites

Effectiveness of Two Pinning Profiles in Controlling Magnetic Critical Current Density in YBCO + YCMO Composites

Superconducting FeSe membrane synthesized by etching of water-soluble Sr3Al2O6 layer

The development of the epitaxial lift-off technique using a pseudoperovskite Sr3Al2O6 sacrificial layer has unleashed latent physical properties emerging in freestanding membranes, mainly composed of lattice-matched perovskite-type complex oxides. Here, we report the superconductivity in a freestanding single-crystalline FeSe membrane prepared using a SrTiO3 capped water-soluble Sr3Al2O6 sacrificial layer, which serves as an ex situ growth template. The FeSe membrane is synthesized by etching the sacrificial layer and transferred on a SiO2/Si substrate. X-ray diffraction pattern and scanning transmission electron microscopy reveal that the FeSe membrane is fully relaxed with minimum degradation of its structural properties during the lift-off process. A superconductivity with zero resistance below 4.2 K is exhibited in the freestanding FeSe membrane, while it is not observed in a compressed thin-film form by an in-plane tensile strain. In addition, critical magnetic field and critical current density of the FeSe membrane are comparable to those of the bulk single crystal. Our demonstration of the superconducting FeSe membrane ensures a high utility of the epitaxial lift-off technique for various thin-film materials grown on SrTiO3. This study paves the way for functional applications using ex situ thin-film growth and lift-off technique with an expanded selection from an inventory of various materials.

Fabrication of multi-filament(Ba,A)Fe2As2 (A: Na, K) HIP round wires and a small superconducting coil

We report the fabrication and systematic characterizations of multi-filament (Ba,A)Fe2As2 (A: Na, K) round wires processed under high pressure. The evaluated transport and magnetic critical current density show clear core-number dependence. This trend originates from degradation of the superconducting core during the wire-drawing process, degradation of the concentric texturing of the core, and the enhanced sausaging effect of the core (revealed by x-ray tomography) as the number of cores increases. Details of the characterizations and suggestions for the improvement of the fabrication process are discussed. We constructed a superconducting magnet using seven-filament (Ba,K)Fe2As2 hot-isostatic pressed round wires. The generated magnetic field and critical current (I c) of the coils at 4.2 K are 1.0 kOe and 18 A, respectively. Measurements of the I c of short segments picked up from the coil show that there is enough room for improvements.

Superconducting properties and intergranular coupling of high-strength Sr1-xKxFe2As2/Ag/Nb/Cu composite conductors

The mechanical properties of composite metal sheaths have large difference, resulting in the easy breakage of superconducting wires. Here, a roll-die drawing method is proposed to fabricate 19-filamentary Sr1-xKxFe2As2/Ag/Nb/Cu composite tapes with long length and uniform shape. It is found that there is no reaction layer between Ag matrix and superconducting core; Nb sheath provides high mechanical strength and prevents the diffusion of K element; and outer Cu sheath has good mechanical ductility. Consequently, the magnetic critical current density Jc of multi-core Sr1-xKxFe2As2 tape reaches up to 4.7 × 104 A/cm2 at 5 K and 5 T. Meanwhile, the magnetic Jc still remains 1.1 × 104 A/cm2 even at 20 K and 5 T. These remarkable magnetic Jc values are probably attributed to both high mass density of superconducting core and high magnetic onset Tc (∼35.2 K). While, the transport Jc of multi-core tape is greatly decreased into about 2.0 × 103 A/cm2 at 4.2 K and 10 T. Besides the Tp1 peak associated with the intragranular critical current, an additional broad Tp2 peak in the imaginary χ''(T) parts of AC susceptibility indicates the weak intergranular coupling in this tape. These results can provide a prime reference for the structure design and coupling performance of multi-core pnictide conductors.

Drastic enhancement of magnetic critical current density in Zn doped Bi-2212

At several temperatures (T) we have studied magnetization (M) as a function of magnetic field (H) in (i) the pure Bi-2212 and (ii) the Zn doped Bi-2212 below respective critical temperatures, Tc. An enhancement in ΔM(H) in Zn doped Bi-2212 has been observed. Based on the criterion, ΔM = constant the HT diagram is found. Shifting in the irreversibility line (IL) has also been investigated. An enormous enhancement of magnetic critical current density (Jcm) in the Zn doped Bi-2212 superconductor is observed. We have analysed Jcm(H) to understand the microscopic nature of vortex pinning. A possible crossover from the collective pinning regime at lower magnetic field to a single vortex pinning with the increase in H at several T is observed in both samples having almost similar granular nature. Pinning of pancake vortices is strongly affected by the localised magnetic moment induced by nonmagnetic Zn in Cu sites. Change in the pinning force density in doped Bi-2212 has been explained.

Enhanced critical current density of ex situ MgB2 via control of Mg assisted sintering

In this work, ex situ MgB2 was mixed with 0.5 mol of Mg and sintered. The sintering conditions were varied over a temperature range of 600–1000 °C for 1, 3, and 7 h, respectively. The addition of Mg during the sintering increased the partial pressure of Mg and thus suppressed the decomposition of MgB2. Onset of critical temperature, Tc, was retained at ∼ 38 K even after the addition of Mg. By increasing the sintering temperature, magnetic critical current density, Jc at self-field, and 20 K of the ex situ samples increased consistently. With the addition of Mg for 1 h sintering, self-field Jc (20 K) was enhanced more than 20 times to 104 A cm−2 as the sintering temperature was increased. Such significant enhancement in the Jc is mainly due to the improved grain coupling aided by Mg during the short sintering.

Enhanced critical current density in K-doped Ba122 polycrystalline bulk superconductors via fast densification

SummaryIron-based superconductors are expected to be used in strong magnet applications owing to their excellent superconducting properties. The process of sintering a mechanically alloyed precursor powder is effective in achieving a high upper critical field and critical current density in BaFe2As2 (Ba122) polycrystalline bulk materials. However, when this process is applied to K-doped Ba122, which shows the highest critical temperature in the Ba122 family, suppressing the vaporization of potassium is challenging. In this study, spark plasma sintering (SPS) method was applied to K-doped Ba122 to achieve fast densification. In contrast to the conventional synthesis method, which requires several tens of hours, optimally K-doped bulks with near theoretical density were obtained after only 5 min of SPS, and the magnetic critical current density reached 105 A/cm2 at 5 K. The demonstrated superconducting properties suggest that this fast densification technique is a useful tool for applying K-doped Ba122 to bulk trapped field magnets.

Ionization and Displacement Damage on Nanostructure of Spin–Orbit Torque Magnetic Tunnel Junction

Spin–orbit torque magnetic tunnel junction (SOT-MTJ) is the memory cell of magnetic random access memory (MRAM), which is a promising candidate for upper level cache in the harsh environment due to nonvolatility, high speed, and high endurance. Therefore, it is significant to research the mechanism of irradiation effects on the SOT-MTJ before its integration onboard. This article investigates the ionization and displacement damage of in-plane magnetic anisotropy SOT-MTJ, including 1-MeV electron and 17.2-keV He <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co gamma irradiations with various fluences and doses, respectively. The devices are resilient to tunnel magnetoresistance (TMR), whereas the magnetic properties are changed after irradiation. The 100-Mrad (Si) gamma and 17.2-keV He <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> irradiations generate similar damages on the device performances, in the aspects of the magnetic anisotropy and critical current density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J_{\mathrm {c}}$ </tex-math></inline-formula> ). However, the displacement damage originating from the electron irradiation does not have a significant influence on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$J_{\mathrm {c}}$ </tex-math></inline-formula> . In addition, the saturation magnetization and exchange bias field are changed by irradiations without regularity. Some possible mechanisms are discussed to explain the experimental results through the simulation of stopping and range of ions in matter (SRIM) and microstructural tests.

Dependence of thermomagnetic instability on strong nonlinear &lt;i&gt;E&lt;/i&gt;-&lt;i&gt;J&lt;/i&gt; models in superconducting films

The &lt;inline-formula&gt;&lt;tex-math id="M4"&gt;\begin{document}$E\text{-}J$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220285_M4.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220285_M4.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; relationship in conventional conductor generally satisfies the linear Ohm's law. However, the &lt;inline-formula&gt;&lt;tex-math id="M5"&gt;\begin{document}$E\text{-}J$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220285_M5.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220285_M5.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; model in superconductors presents strong nonlinear characteristics, which is significantly different from that of the conventional conductor. According to the nonlinear &lt;inline-formula&gt;&lt;tex-math id="M6"&gt;\begin{document}$E\text{-}J$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220285_M6.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220285_M6.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; power law of superconducting materials, we quantitatively investigate the relationship between the magnetic-thermal stability and the nonlinear constitutive characteristic of superconducting films at different temperatures, magnetic field ramp rates, and critical current densities by using the fast Fourier transform method (FFT). We find that the strong nonlinear electromagnetic constitutive model plays a crucial role responsible for the onset and morphology (tree-like and finger-like) of the magneto-thermal instability of superconducting thin films. In addtion, the reason why similar magneto-thermal instabilities cannot be observed in conventional conductors is also explained. It can be found that the magnetic field on the border of the superconducting film increases rapidly for a larger creep exponent due to the enhancement of diamagnetism, which results in a large magnetic pressure and easily triggering off flux avalanches. Therefore, the threshold field of flux avalanches in the superconducting film decreases with flux creep exponent increasing. Finally, we present the curves that can clearly divide the &lt;inline-formula&gt;&lt;tex-math id="M7"&gt;\begin{document}$n_0\text{-}j_{c0}$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220285_M7.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220285_M7.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; plane and &lt;inline-formula&gt;&lt;tex-math id="M8"&gt;\begin{document}$n_0\text{-}\dot {H}_a$\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220285_M8.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20220285_M8.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; plane into magneto-thermal stability region and magneto-thermal instability region for superconducting thin film with different levels of nonlinearity.

Characterization of the (Cu,C)Ba2Ca3Cu4O single crystals grown under high pressure

By using a high pressure and high temperature (3.7 GPa, 1120 ∘C) synthesis technique, we have grown (Cu,C)Ba2Ca3Cu4O single crystals. X-ray diffraction, scanning electron microscopy, resistivity, and magnetization measurements are carried out and all show that the samples have good quality. The single crystal has onset and zero-resistance transition temperatures of about 111 and 109.6 K, indicating a very narrow transition width, which is consistent with a rather sharp magnetization transition. Magnetization hysteresis loops (MHLs) are also measured, showing a pronounced second peak effect in the intermediate temperature region. The magnetic critical current density calculated from the MHLs at 77 K and 1.5 T is about 6.4 × 104 A cm−2. By using a criterion of 1 normal state resistivity, we have determined the irreversibility line which exhibits an irreversibility field of about 8 T at 77 K. Compared with other layered systems, it is easy to find that the irreversibility line is rather high and could be further improved with the optimized transition temperature of about 118 K as previously discovered in polycrystalline samples.

Upper critical field and peak effect in (Li0.8Fe0.2)OHFeSe single crystal

The transport properties, magnetization and dynamic magnetization relaxation rate measurements of (Li0.8Fe0.2)OHFeSe single crystals are presented at various magnetic fields and temperatures. The temperature dependence of upper critical field Hc2 exhibits an upward curvature. The Hc2 has been determined based on the orbital limiting and the Pauli limiting effect. The peak effect occurs in both the magnetic hysteresis loops and critical current density, which associates with the vortex motion. The field dependence of ∆M measured at different field sweeping rates is distinguished, which indicates a giant vortex creep. The magnetic relaxation rate Q has been investigated, indicating a transition between the two different vortices motion with increasing field. Our results suggest that the peak effect originates from the vortex transition from the elastic motion to the plastic motion.

Improvement of flux pinning properties in Fe3O4 nanoparticle-doped Bi1.6Pb0.4Sr2Ca2Cu3O10+δ superconductors

The flux pinning properties of polycrystalline (Bi1.6Pb0.4Sr2Ca2Cu3O10+δ)1−x(Fe3O4)x, with x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05 superconductors were investigated. The collective pinning theory was successfully used to describe the magnetization critical current density (Jc) of all samples. The value of Jc was enhanced by the proper doping contents of Fe3O4 nanoparticles. Possible origins for these achievements were attributed to the increases in both flux pinning force and activation energy. The appearance of the additional point-like pinning centers was confirmed via Dew–Hughes model fitting analysis.