Study of (DNPs)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/CuTl-1223 Nanoparticle-Superconductor Composites

Sol-gel and solid-state reaction methods were used to synthesize diamond nanoparticles (DNPs) and (DNPs) _x /CuTl-1223 (x = 0, 0.25, 0.50, and 1.00 wt.%) nanoparticle-superconductor composites, respectively. Effects of these DNPs on structural, morphological, compositional, and transport properties of CuTl-1223 superconducting phase were investigated by different experimental techniques such as X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy, scanning electron microscopy (SEM), and resistivity versus temperature (R-T) measurements. The unchanged crystal structure and stoichiometry of host CuTl-1223 superconducting matrix with addition of DNPs gave evidence about the dispersion of nanoparticles at the grain boundaries of the host matrix, which may heal up the inter-granular voids and pores resulting in enhanced inter-grain connectivity. Critical transition temperature T _c (0) and hole concentration of CuTl-1223 superconductor were observed to be increased with addition of DNPs up to a certain optimum value (i.e. x = 0.5 wt.%).     

Tuning of Dielectric Parameters of (NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/CuTl-1223 Nano-superconductor Composites by Temperature and Frequency

Nickel ferrite (NiFe₂O₄) nanoparticles and Cu_0.5Tl_0.5Ba₂Ca₂Cu₃O_10?δ (CuTl-1223) superconductor were prepared separately and then mixed in an appropriate ratios at the final stage to obtain (NiFe₂O₄) _x /CuTl-1223 (x = 0, 0.5, and 1.0 wt%) nano-superconductor composites. There was no significant change observed in crystal structure of the host CuTl-1223 superconducting matrix after the addition of NiFe₂O₄ nanoparticles. The value of zero-resistivity critical temperature { T _{c(R = 0)} (K)} was decreased with increasing content of these nanoparticles in these composites. Maximum values of dielectric loss tangent (tanδ) at lowest possible frequency of 40 Hz were increased with the increase of operating temperature, while its values were decreased and become almost zero at higher frequencies for all these samples at all operating temperatures. A peak in dielectric loss tangent was shifted towards lower frequency values with the addition of these nanoparticles in CuTl-1223 superconducting matrix. The dielectric loss tangent peak was also shifted towards lower frequency values in all these samples with increasing operating temperature, which shows the relaxator-like behavior in these samples. The dielectric parameters of these composites can be tuned by frequency, operating temperatures, and nature and content of these nanoparticles.     

Effect of Charge Reservoir Layer on the Structural and Transport Properties of CuTlBa<Subscript>2−Y</Subscript>Sr<Subscript><Emphasis Type="Italic">Y</Emphasis></Subscript>-1223 (<Emphasis Type="Italic">Y</Emphasis> = 0 − 0.25) Superconductor

The bulk superconducting composites Cu_0.5Tl_0.5 Ba_2?YSr _Y Ca₂Cu₃O₁0?δ (Y = 0, 0.15, and 0.25) have been synthesized at an ambient pressure. The techniques used to characterize the samples were X-ray scans, Fourier transform infrared spectroscopy (FTIR), and dc resistivity (ρ) measurements. In CuTl-1223 system, we replaced Sr atom at Ba site and studied the superconducting properties of squeezed charge reservoir layer (CRL). From the XRD analysis, it is confirmed that the samples have orthorhombic structure and the dimensional parameters of the unit cell suppressed with the dopant atom which is most probably due to small in size of Sr atom as compared with Ba. The normal-state resistivity and critical temperatures, i.e., T _c (R = 0) and \(T_{\mathrm {c}}^{\text {onset}}\) are observed to be suppressed. The lower values of critical temperature T _c (R = 0) and activation energy U _o (eV) might be possible due to a weak flux pinning. Accordingly, a reduction of weak links and enhanced insulating nature of inter-grain coupling were observed with the doping of Sr atom in CRL. Moreover, the doping in CRL of Sr atom is also confirmed with the FTIR technique. The intrinsic parameters, i.e., coherence length ξ _c(0), crossover temperatures (T _3D?2D), inter-layer coupling (J), etc. were calculated by fluctuation-induced conductivity (FIC) analysis.     

Complex Electric Modulus Spectroscopy of (MnFe<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/CuTl-1223 Nanoparticles-Superconductor Composites

Manganese ferrite (MnFe₂O₄) nanoparticles and Cu_0.5Tl₀.₅Ba₂Ca₂Cu₃O_10?δ(CuTl-1223) superconducting phase were synthesized by sol-gel and solid-state reaction methods, respectively. Different contents of MnFe₂O₄ nanoparticles were added in CuTl-1223 superconducting matrix to get (MnFe₂O₄)_x/CuTl-1223; x =? 0～2.0 wt% nanoparticles-superconductor composites. Complex electric modulus spectroscopy measurements of (MnFe₂O₄)_x/CuTl-1223 composites were carried out at different test frequencies from 20 Hz to 10 MHz and at different operating temperatures from 78 to 253 K to analyze and interpret the dynamical aspects of electrical transport phenomena (i.e., such as carrier hopping rate, conductivity, and blocking factor). The complex electric modulus spectra showed the effects of both grains and grain-boundaries on electrical properties. The capacitance of grain-boundaries was found higher than that of grains. The capacitive behavior of grains was increased and that of grain-boundaries was decreased with increasing operating temperature for all these samples. Blocking factor of these composites was increased with increasing contents of MnFe₂O₄ nanoparticles. Shifting of peaks in imaginary part of modulus spectra towards lower frequency with increasing contents of these nanoparticles showed non-Debye type relaxation phenomenon in the material.     

Methods of Extracting Current Density Dependence of the Effective Activation Energy <Emphasis Type="Italic">U</Emphasis><Subscript>eff</Subscript>(J) in High <Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> Superconductors

We have reviewed the methods of extracting current density dependence of the effective activation energy U_eff(J) from experimental data, including transport measurements and magnetic relaxations. Then we applied the method proposed by Maley etc. on our single-phase HgBaCaCuO-1223 sample to obtain the effective activation energy. The effective activation energy U_eff(J, H = 1~T) is extracted from the magnetization relaxation data. On the other hand, U_eff(J) can be theoretically estimated for the model of a sinusoidal washboard potential in superconductors. By comparing the two results we believe that the single curve obtained in the former way can be seen as real current density dependence of effective activation energy U_eff(J). In addition, we have analyzed the reasons why the magnetic decay data at various temperatures can be scaled onto a single curve. The pinning mechanism in the measured temperature range does not change, and the activation energy depends separately on the three variables: T, B, and J are thought as two important factors for this. In the temperature close to zero and near T_c, thermally assisted flux motion would no longer valid since other processes predominate.     

Effects of Iron Contents on the Vortex State in Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>0.5</Subscript>Te<Subscript>0.5</Subscript>

We investigate the effects of iron content on the upper critical field (H _c2) and the activation energy U(T) in thermally activated flux flow in Fe _x Se_0.5Te_0.5 near the superconducting transition temperature T _c . The variations in H _c2(T) with temperature are analyzed using Ginzburg-Landau (GL), Werthamer-Helfand-Hohenberg (WHH) models along with the empirical relation (ER). The obtained values of H _c2(0) depend strongly on the model and the criteria used to determine the transition temperature. However, the general trend is that that H _c2(0) increases with the increasing Fe content. The activation energy U(T) is maximum for x =? 1 and rapidly suppressed by excess or deficiency of iron. The low values of U(T) (～10 meV) reflect the low vortex-pinning nature (due to defects, vacancies, etc.) in the Fe _x Se_0.5Te_0.5 superconductor.     

Experimental Study of the Thermodynamic Properties of Diethyl Ether (DEE) at Saturation

The isochoric heat capacities \({({C_{V1}^{\prime}} ,{C_{V1}^{\prime\prime}},{C_{V2}^{\prime}},{C_{V2}^{\prime\prime}})}\), saturation densities (\({\rho _{\rm S}^{\prime}}\) and \(({\rho_{\rm S}^{\prime\prime})}\)), vapor pressures (P _S), thermal-pressure coefficients \({\gamma_V=\left({\partial P/\partial T}\right)_V}\), and first temperature derivatives of the vapor pressure γ _S = (dP _S/dT) of diethyl ether (DEE) on the liquid–gas coexistence curve near the critical point have been measured with a high-temperature and high-pressure nearly constant-volume adiabatic piezo-calorimeter. The measurements of \({({C_{V1}^{\prime}} ,{C_{V1}^{\prime\prime}},{C_{V2}^{\prime}},{C_{V2}^{\prime\prime}})}\) were made in the liquid and vapor one- and two-phase regions along the coexistence curve. The calorimeter was additionally supplied with a calibrated extensometer to accurately and simultaneously measure the PVT, C _V VT, and thermal-pressure coefficient, γ _V , along the saturation curve. The measurements were carried out in the temperature range from 416 K to 466.845 K (the critical temperature) for 17 liquid and vapor densities from 212.6 kg · m^?3 to 534.6 kg · m^?3. The quasi-static thermo- (reading of PRT, T ? τ plot) and baro-gram (readings of the tensotransducer, P ? τ plot) techniques were used to accurately measure the phase-transition parameters (P _S ,ρ _S ,T _S) and γ _V . The total experimental uncertainty of density (ρ _S), pressure (P _S), temperature (T _S), isochoric heat capacities \({({C_{V1}^{\prime}} ,{C_{V1}^{\prime\prime}},{C_{V2}^{\prime}},{C_{V2}^{\prime\prime}})}\), and thermal-pressure coefficient, γ _V , were estimated to be 0.02 % to 0.05 %, 0.05 %, 15 mK, 2 % to 3 %, and 0.12 % to 1.5 %, respectively. The measured values of saturated caloric \({({C_{V1}^{\prime}} ,{C_{V1}^{\prime\prime}},{C_{V2}^{\prime}},{C_{V2}^{\prime\prime}})}\) and saturated thermal (P _S, ρ _S, T _S) properties were used to calculate other derived thermodynamic properties C _P , C _S, W, K _T , P _int, ΔH _vap, and \({\left({\partial V/\partial T}\right)_P^{\prime}}\) of DEE near the critical point. The second temperature derivatives of the vapor pressure, (d² P _S/dT ²), and chemical potential, (d² μ/dT ²), were also calculated directly from the measured one- and two-phase liquid and vapor isochoric heat capacities \({({C_{V1}^{\prime}} ,{C_{V1}^{\prime\prime}},{C_{V2}^{\prime}},{C_{V2}^{\prime\prime}})}\) near the critical point. The derived values of (d² P _S/dT ²) from calorimetric measurements were compared with values calculated from vapor–pressure equations. The measured and derived thermodynamic properties of DEE near the critical point were interpreted in terms of the “complete scaling” theory of critical phenomena. In particular, the effect of a Yang–Yang anomaly of strength R _μ on the coexistence-curve diameter behavior near the critical point was studied. Extended scaling-type equations for the measured properties P _S (T), ρ _S (T), and \({({C_{V1}^{\prime}} ,{C_{V1}^{\prime\prime}},{C_{V2}^{\prime}},{C_{V2}^{\prime\prime}})}\) as a function of temperature were developed.     

X-ray Photoelectron Spectroscopy,Magnetotransport and Magnetisation Study of Nb<Subscript>2</Subscript>PdS<Subscript>5</Subscript> Superconductor

In the present report, we investigate various properties of the Nb₂PdS₅ superconductor. Scanning electron microscopy displayed slabs like laminar growth of Nb₂PdS₅ while X-ray photoelectron spectroscopy exhibited the hybridisation of sulphur (2p) with both palladium (3d) and niobium (3d). High-field (140 kOe) magneto-transport measurements revealed that superconductivity (\(T_{c}^{\text {onset}} =?7\) K and T cρ=?0 = 6.2 K) of the studied Nb₂PdS₅ material is quite robust against magnetic field with the upper critical field (H _c2) outside the Pauli paramagnetic limit. Thermally activated flux flow (TAFF) of the compound showed that resistivity curves follow Arrhenius behaviour. The activation energy for Nb₂PdS₅ is found to decrease from 15.15 meV at 10 kOe to 2.35 meV at 140 kOe. Seemingly, the single vortex pinning is dominant in low-field regions, while collective pinning is dominant in high-field region. The temperature dependence of AC susceptibility confirmed the T _c at 6 K, further varying amplitude and frequency, showed well-coupled granular nature of superconductivity. The lower critical field (H _c1) is extracted from DC magnetisation measurements at various T below T _c. It is found that H _c1(T) of Nb₂PdS₅ material seemingly follows the multiband nature of superconductivity.     

Ab Initio Study of Structural,Electronic, and Magnetic Properties of $\mathrm {A}_{1-x}^{\text {III}}$MnxBVI: In1−xMnxS-Diluted Magnetic Semiconductor

First-principles density functional calculations on the new class of diluted magnetic semiconductor \(A_{1-x}^{III}{Mn}_{x}B^{VI}\) In_1?x Mn _x S for x =?0.25 and 0.5 are investigated to study the structural, electronic, and magnetic properties, employing the full-potential linearized augmented plane wave method. Electronic band structures and density of states revealed a half-metallic character of In_1?x Mn _x S and show the stability of anti-ferromagnetic states as compared with ferromagnetic states. The calculated exchange constants J _dd are in good agreement with experimental and theoretical results on magnetic properties of single crystalline \(\mathrm {A}_{1-x}^{\text {III}}{\text {Mn}}_{x}\mathrm {B}^{\text {VI}}\) in the anti-ferromagnetic case. Our predicated calculations on the s,p-d exchange constants N ₀ α and N ₀ β show that they are lower than in \(\mathrm {A}_{1-x}^{\text {II}}{\text {Mn}}_{x}\mathrm {B}^{\text {VI}}\) DMS. The local environment is found tetrahedral as in the II–VI DMS and other (III,Mn) VI compounds. The total magnetic moment for In_1?x Mn _x S for different concentrations is in accordance with the exact value 5 μ _B and comes mainly from impurity Mn. The local magnetic moments of Mn ions are reduced from their free space charges values due to the p-d hybridization which produces small magnetic moments on the nonmagnetic In and S sites. The Curie temperature of In_1?x Mn _x S is calculated within the mean field approximation and compared with other DMS systems.     

Inducement of Itinerant Electron Transport in Charge-Ordered Pr<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> by Ba Doping

The effects of Ba ²⁺ doping on the electrical and magnetic properties of charge-ordered Pr_0.6Ca_0.4MnO₃ were investigated through electrical resistivity and AC susceptibility measurements. X-ray diffraction data analysis showed an increase in unit cell volume with increasing Ba ²⁺ content indicating the possibility of substituting Ba ²⁺ for the Ca-site. Electrical resistivity measurements showed insulating behavior and a resistivity anomaly at around 220 K. This anomaly is attributed to the existence of charge ordering transition temperature, \(T^{\mathrm {R}}_{\text {CO}}\) for the x = 0 sample. The Ba-substituted samples exhibited metallic to insulator transition (MI) behavior, with transition temperature, T _MI, increasing from ～98 K (x = 0.1) to ～122 K (x = 0.3). AC susceptibility measurements showed ferromagnetic to paramagnetic (FM-PM) transition for Ba-substituted samples with FM-PM transition temperature, T _c, increasing from ～121 K (x = 0.1) to ～170 K (x = 0.3), while for x = 0, an antiferromagnetic to paramagnetic transition behavior with transition temperature, T _N, ～170 K was observed. In addition, inverse susceptibility versus T plot showed a deviation from the Curie–Weiss behavior above T _c, indicating the existence of the Griffiths phase with deviation temperature, T _G, increasing from 160 K (x = 0.1) to 206 K (x = 0.3). Magnetoresistance, MR, behavior indicates intrinsic MR mechanism for x = 0.1 which changed to extrinsic MR for x > 0.2 as a result of Ba substitution. The weakening of charge ordering and inducement of ferromagnetic metallic (FMM) state as well as increase in both T _c and T _MI are suggested to be related to the increase of tolerance factor, τ, and increase of e _g ?electron bandwidth as average ionic radius at A-site, <r _A> increased with Ba substitution. The substitution may have reduced MnO₆ octahedral distortion and changed the Mn–O–Mn angle which, in turn, promotes itinerancy of charge carrier and enhanced double exchange mechanism. On the other hand, increase in A-site disorder, which is indicated by the increase in σ ² is suggested to be responsible for the widening of the difference between T _c and T _MI.     

Effect of CdTe Addition on the Electrical Properties and AC Susceptibility of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor

The effect of CdTe addition on YBa₂Cu₃O_7?δ (CdTe) _x (x = 0–0.10) has been studied. XRD patterns showed the presence of impurities including CdTe for x ≥ 0.05 wt%. The resistance versus temperature curves showed metallic behavior for all samples. The onset temperature T _{c onset} for all samples is between 90 and 92 K. The superconducting transition width, ΔT _c = 4 K for all samples except for x = 0.1 wt% where ΔT _c = 6 K. The superconducting transition determined by AC susceptibility measurement also showed little change in \(T_{\mathrm {c}\chi ^{\prime }}\) (between 89 and 92 K). However, the peak temperature T _p of the imaginary part of the susceptibility χ″, showed a drastic decrease for samples with x > 0.05 wt%. This indicated that the superconducting grains were strongly decoupled for x > 0.05 wt% due to the impurities as observed in the XRD pattern. The intergrain critical current density, J _c at T _p for the x = 0 sample is J _c (82 K) = 17 A cm^?2. These results can be useful in the fabrication of semiconductor/YBCO superconductor hybrid systems.     

Influence of coronene addition on some superconducting properties of bulk MgB<Subscript>2</Subscript>

This study reports the effect of coronene (C₂₄H₁₂) addition on some superconducting properties such as critical temperature (T_c), critical current density (J_c), flux pinning force density (F_p), irreversibility field (H_irr), upper critical magnetic field (H_c2), and activation energy (U₀), of bulk MgB₂ superconductor by means of magnetisation and magnetoresistivity measurements. Disk-shaped polycrystalline MgB₂ samples with varying C₂₄H₁₂ contents of 0, 2, 4, 6, 8, 10 wt%, were produced at 850 °C in Ar atmosphere. The obtained results show an increase in field-J_c values at 10 and 20 K resulting from the strengthened flux pinning, and a decrease in critical temperature (T_c) because of C substitution into MgB₂ lattice, with increasing amount of C₂₄H₁₂ powder. The H_c2(0) and H_irr(0) values are respectively found as 144, 181, 172 kOe, and 128, 161, 145 kOe for pure, 4 wt% and 10 wt% C₂₄H₁₂ added samples. The U₀ depending on the magnetic field curves were plotted using thermally activated flux flow model. The maximum U₀ values are respectively obtained as 0.20, 0.23 and 0.12 eV at 30 kOe for pure, 4 wt% and 10 wt% C₂₄H₁₂ added samples. As a result, the superconducting properties of bulk MgB₂ at high fields was improved using C₂₄H₁₂, active carbon source addition, because of the presence of uniformly dispersed C particles with nanometer order of magnitude, and acting as effective pinning centres in MgB₂ structure.     

Low Magnetic Field Magnetocaloric Effect in $\text {Gd}_{\mathrm {5-}_{x}}$EuxGe4

The magnetocaloric effect is investigated for \(\text {Gd}_{\mathrm {5-}_{x}}\)Eu _x Ge₄ (0.25 ≤ x ≤ 2) system near a phase transition from a ferromagnetic to a paramagnetic state as a function of temperature with low external magnetic field change of 100 Oe. The sample with x = 1 has the smallest value of maximum magnetic entropy change and the specific heat change, and highest values of full-width at half-maximum and relative cooling power. The results indicate that the \(\text {Gd}_{\mathrm {5-}_{x}}\)Eu _x Ge₄ system has a prospective application for magnetic refrigerant in an extended high temperature range. Consequently, \(\text {Gd}_{\mathrm {5-}_{x}}\)Eu _x Ge₄ compounds are very attractive candidates for magnetic refrigeration applications, especially nitrogen liquefier.     

Magneto-transport properties of (Cu)x/CuTl-1223 nanoparticles-superconductor composites

Copper (Cu) nanoparticles and Cu_0.5Tl_0.5Ba₂Ca₂Cu₃O_10−δ (CuTl-1223) superconducting phase were synthesized by sol-gel and solid-state reaction, respectively. These metallic Cu nanoparticles were added in CuTl-1223 superconducting matrix to get (Cu)_x/CuTl-1223; x = 0–4.0 wt% nanoparticles-superconductor composites and their temperature dependent magneto-transport properties were studied. The zero-field-cooled (ZFC) and field-cooled (FC) temperature dependent magnetization (M-T) measurements of (Cu)_x/CuTl-1223 samples showed an increase in transition temperature and in amplitude of diamagnetic signal after the inclusion of Cu nanoparticles in the host CuTl-1223 matrix. The improvement in these magneto-transport properties can be attributed to the increase in number of efficient pinning centres in CuTl-1223 matrix after addition of Cu nanoparticles. Magnetization hysteresis (M-H) loops were obtained at various operating temperatures from which the magnetization critical current density (J_c) was estimated using Bean's critical state model. M-H loops indicated the combined superconducting and ferromagnetic behaviour up to 90 K in all (Cu)_x/CuTl-1223 samples. Improvement in J_c could also be due to increase in number of pinning centres with addition of Cu nanoparticles in CuTl-1223 matrix. Maximum improvement in magneto-transport properties of (Cu)_x/CuTl-1223 samples was observed for x = 1.0 wt%, which had specified the optimum content level of Cu nanoparticles in CuTl-1223 phase.     

AC Susceptibility and Superconducting Properties of Graphene Added YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">d</Emphasis></Subscript>

The effect of graphene (G) addition on YBa₂Cu₃O_7?δ(G)_x (x =?0 ? 0.03 wt%) has been studied using the X-ray diffraction method, scanning electron microscopy, electrical resistance versus temperature, transport critical current density J_c, and AC susceptibility measurements. XRD patterns showed single-phase YBa₂Cu₃O_7?δ (YBCO) for all samples. SEM micrographs showed filling of the voids between YBa₂Cu₃O_7?δ grains as graphene was added. The temperature-dependent electrical resistance curves showed metallic normal state behavior and onset transition temperature T_conset between 90 and 92 K for all samples. AC susceptibility measurement showed transition temperature \(T_{\mathrm {c} \chi ^{\prime }}\) between 90 and 93 K. \(T_{\mathrm {c} \chi ^{\prime }}\) was maintained or improved slightly as graphene was added. The x =?0.001 wt% showed the highest J_c, i.e., 2750 A cm^?2 at 77 K and 5570 mA cm^?2 at 30 K which was 13 and 40 times higher than that of the non-added YBCO, respectively. The peak temperature T_p of the imaginary part of the susceptibility χ^″ was around 78–82 K for all samples indicating grain coupling was not weakened as graphene was added.     

Study of the Magnetic Properties of Ni/Ag Superlattice

Thin multilayer films of alternating ultrathin Ni and Ag layers (L(Ni)=11,15,30 Å, bulk and \(L(\mathrm{Ag})=50~{\AA})\) have been prepared by evaporation in ultrahigh vacuum under controlled conditions and have been studied by the magnetic measurements. The critical temperature T _C is studied as a function of the surface exchange interaction (J _S). The dependence of T _C on the thickness L of the film has been investigated. A critical value of the surface exchange interaction in the film, above which the surface magnetism appears, is obtained. The shift of the critical temperature T _C(L) from the bulk value \([\frac{T_{\mathrm{C}}(\infty )}{T_{\mathrm{C}}(L)}-1]\) can be described by a power law L ^?λ , where \(\lambda =\frac{1}{\nu_{\mathrm{b}}}\) is the inverse of the correlation length’s exponent. The effective critical exponent associated with the magnetization M(β) is deduced for different thicknesses of Ni layers, and the thickness L(Ag) was being kept constant at 50 Å.     

Enhancement of Pinning Role by Nonstoichiometry in YBa2Cu3O7−y Superconductors

Considerably improved flux pinning and critical current density Jc values have been achieved in Y-deficient Y-123 superconductors by directional solidification in air. In comparison with the regular Y-123 composition, Y-deficient one also has an orthorhombic structure and Y-123 main crystal phase remains in it. Whereas with the shortage of Y, Y_1?xBa₂Cu₃O_7?y can be regarded as (YBa₂Cu₃)_1?xO_7?t(Ba₂Cu₃) _x O_t?y or (YBa₂Cu₃O_7?z)(□ _x ^YO_z?y), so there may develop several kinds of microstructure defects as pinning sites in the system, such as highly dense, fine-scale, and faultlike defects, as well as localized superstructure, which are able to induce the increasing in flux pinning and Jc values in higher external magnetic fields. This kind of simple nonstoichiometric route could lead to a commercial technique for flux-pinning enhancement in Y-123 bulk materials.     

Investigation of Magnetic,Magnetocaloric, and Critical Properties of La<Subscript>0.5</Subscript>Ba<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> Manganite

We present an extensive study of the magnetic properties of a novel La_0.5Ba_0.5MnO₃ perovskite material prepared by the hydrothermal method. The explored sample was structurally studied by the x-ray diffraction (XRD) method which confirms the formation of a pure cubic phase of a perovskite structure with Pm3m space group. The magnetic properties were probed by employing temperature M (T) and external magnetic field M (μ_oH) dependence of magnetization measurements. A magnetic phase transition from ferromagnetic to paramagnetic phase occurs at 339 K in this sample. The maximum magnetic entropy change (\(\left | {{\Delta } S}_{M}^{\max } \right |\)) took a value of 1.4 J kg^??1 K^??1 at the applied magnetic field of 4.0 T for the explored sample and has also been found to occur at Curie temperature (T_C). This large entropy change might be instigated from the abrupt reduction of magnetization at T_C. The magnetocaloric effect (MCE) is maximum at T_C as represented by M (μ_oH) isotherms. The relative cooling power (RCP) is 243.2 J kg^??1 at μ_oH =?4.0 T. Moreover, the critical properties near T_C have been probed from magnetic data. The critical exponents δ, β, and γ with values 3.82, 0.42, and 1.2 are close to the values predicted by the 3D Ising model. Additionally, the authenticity of the critical exponents has been confirmed by the scaling equation of state and all data fall on two separate branches, one for T < T_C and the other for T > T_C, signifying that the critical exponents obtained in this work are accurate.     

Vortex Structure and Anisotropic Superconducting Gaps in Ba[Fe(Ni)]<Subscript>2</Subscript>As<Subscript>2</Subscript>

We studied nearly optimally Ni-substituted BaFe_2?x Ni _x As₂ (BFNA) single crystals with T _C ≈ 18.5 K. In irreversible magnetization measurements, we determined the field dependence of the critical current density and discuss the nature of observed strong bulk pinning. Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we directly determine two distinct superconducting gaps and resolve their moderate anisotropy in the momentum space. The BCS-ratio for the large gap 2Δ _L /k _B T _C > 4.1 evidences for a strong coupling in the Δ _L -bands.     

Large Magnetocaloric Effect in Fe-B-Mn-Zr-Nb Amorphous Alloys Near Room Temperature

In this paper, we report on the magnetic and magnetocaloric properties in amorphous Fe _86?x B _x Mn ₄Zr ₈Nb ₂ (x = 4, 8, 12, 16 and 20) alloys. The Fe-based alloys were prepared by induction melting and melt-spinning techniques. The X-ray diffraction patterns indicate that the studied alloys are amorphous. With an increase of x from 4 to 20, the Curie temperature increases almost in a linear way from 237 to 328 K and the average saturation magnetic moment per Fe atom decreases monotonously. For a field change of 3 T, the absolute values of maximum isothermal entropy change \(\left | {\Delta S_{T}^{\max }} \right |\) are found to be 2.19, 1.97, 1.63, 1.58, and 1.38 J kg ^?1 K ^?1 at 237, 264, 289, 318, and 330 K for x = 4, 8, 12, 16, and 20, respectively. The large magnetocaloric effect near room temperature suggests that these Fe-based amorphous alloys can be considered as magnetic refrigerants in the range of 230–330 K. The ΔS _T exhibits a potential law with a magnetic field exponent ～0.72 at the Curie temperature. The similar values of n for Fe _86?x B _x Mn ₄Zr ₈Nb ₂ indicate that the ΔS _T curves of these alloys follow a universal class.