Journal of Superconductivity and Novel Magnetism最新文献

The resistive transition, under an external magnetic field, was measured for two SmFe_1-xCo_xAsO superconducting films with different quality of intergranular coupling. It is observed that the sample with a low density of intergranular coupling has a semiconducting character and the transition to the long-range superconducting state is controlled by the thermal activated phase slip of the order parameter, described by the Ambegaokar-Halperin model. While in the sample with an intergranular coupling characterized by metallic connectivity, it is the freezing of the vortex lines into a vortex glass phase that controls the transition to the long-range superconducting state. A true superconducting state is observed only in the sample that shows a vortex glass phase. In contrast, in the sample in which the resistivity in the normal state indicates a semiconducting granular coupling, a broad transition that does not reach the zero-resistance state occurs. In fact, in this last sample at high applied fields, the long-range superconducting state is not reached because the intergranular coupling is broken, and the resistive signal associated with the thermal activated flux creep of the bulk vortices becomes dominant.

The structural, mechanical, optoelectronic, magnetic, and thermodynamic properties of the intermetallic compounds GdM₂ (M = Fe, Co, Ni) have been investigated using the full-potential linear muffin-tin orbital (FP-LMTO) method within the local spin density approximation (LSDA) and LSDA + U approaches. In the ferromagnetic phase, both functionals were employed to account for the Coulomb repulsion among electrons of the same atom through the Hubbard U term. These methods were specifically applied to describe the Gd–4f electrons in the electronic and magnetic calculations of ferromagnetic Laves-phase GdM₂ (M = Fe, Co, Ni). Our findings reveal that the LSDA + U approach provides the most stable phase for all the studied compounds. The elastic constant C44 C_{44}C44 indicates that resistance to unidirectional compression is greater than resistance to shear deformation. While LSDA accurately reproduces experimental lattice constants, LSDA + U slightly overestimates them. However, LSDA + U delivers a more precise description of the band structure, density of states, and magnetic moments compared to LSDA. Additionally, a stronger hybridization interaction is observed between Gd-d and Ni-d electrons compared to Gd-d and Co-d, with the weakest interaction occurring between Gd-d and Fe-d electrons. The calculated lattice parameters for GdFe₂, GdCo₂, and GdNi₂ deviate from experimental values by only 0.3%, 0.4%, and 0.2%, respectively, demonstrating a high degree of accuracy. A critical pressure of 10 GPa was found for GdNi₂, indicating a relatively low pressure is needed to transition from the ferromagnetic phase to a non-magnetic state. The calculated total magnetic moments range from approximately 6.5 μB to 7.2 μB per formula unit, with the LSDA + U method providing a more accurate description of the magnetic ordering.

This research focuses on the mechanical, electronic, optical, and superconductive nature of CaRh₂Si₂ and SrCo₂Si₂, which have structures similar to those of the ThCr₂Si₂ compound. The investigation used a first-principles approach based on DFT, with calculations performed via CASTEP. The optimized lattice parameters closely match previously synthesized parameters. These materials possess positive elastic constants, which ensure they are mechanically stable. Additionally, Pugh’s and Poisson’s ratio values suggest that CaRh₂Si₂ and SrCo₂Si₂ exhibit brittle behavior. At the Fermi level, the valence and conduction bands overlap, indicating the metallic characteristics of both compounds. Various optical properties were investigated, with the reflectance parameter demonstrating that CaRh₂Si₂ and SrCo₂Si₂ have the potential to be effective solar reflectors. The photoconductive nature and absorption parameters begin when the photon’s energy is zero, confirming these material’s metallic characteristics. The thermodynamic properties of CaRh₂Si₂ and SrCo₂Si₂ were derived from their elastic stiffness constants, and the Debye temperatures of these two compounds are 576.08 K and 446 K, respectively, based on the elastic constants data. In BCS (Bardeen-Cooper-Schrieffer) theory, superconductivity occurs when electrons form Cooper pairs at low temperatures due to attractive forces mediated by phonons. The superconducting properties suggest that CaRh₂Si₂ and SrCo₂Si₂ moderately coupled superconductors. The outcomes of this study could serve as an adequate basis for further investigations.

The high-temperature superconductor (HTS) has important application prospects in the field of nuclear fusion. In the high-field large magnets, it is inevitable to be affected by the fault current impact. Under such extreme conditions, superconducting magnet systems will face quench problems and generate large AC loss. AC loss can cause the temperature to rise, which worsens the quench environment. Under the inrush currents, the analysis of the temperature rise pattern of the HTS plays a crucial role regarding the safe operation of the magnet system. In this paper, Comsol’s two-dimensional H method and heat transfer module are used to simulate AC loss and the temperature variation of Bi-2223 HTS conductor for different conditions under the impact of fault current. The temperature can reach up to 81.8 K, 85.2 K, and 90.7 K when the inrush current is 10 kA, 15 kA, and 20 kA, respectively, with a constant flat-topping time of 0.2 s and a ramp-up rate of 100 kA/s. The inrush current is 10 kA, and the flat-top time is 0.2 s, 0.4 s, and 0.6 s, and then, the maximum temperature can reach 81.7 K, 82.5 K, and 83.4 K, respectively. The result shows that as the peak value of the inrush current and the duration of the peak hold time increase, the AC loss and temperature of the HTS conductor increase. As the number of conductor slots increases, the temperature decreases significantly.

We have grown large (8–10 mm in dimension) single crystals of K(_{0.8})Fe(_{1.7})(Se(_{0.73})S(_{0.27}))(_2) composition with partial isovalent (Se,S) substitution related to the 122-Se family of iron-chalcogenide superconductors. High quality and homogeneity of the superconducting properties below (T_c approx 26) K were confirmed using X-ray diffraction, electron microscopy, energy dispersive X-ray spectroscopy, transport, and magnetic probes. Using detailed magnetotransport R(T) probes in magnetic fields up to 16 T in two field orientations (in-plane (H parallel ab) and out-of-plane (H parallel c)), we determined temperature dependences of the upper critical field (mu _0H_{c2}^{ab}(T)) and (mu _0H_{c2}^c(T)), magnetic anisotropy (gamma approx 3.5) in the vicinity of (T_c), and estimate zero-temperature values (mu _0H_{c2}^{ab}(0) approx 99) T, (mu _0H_{c2}^c(0) approx 28) T.

A systematic investigation of the magnetic properties using AC susceptibility measurements has been performed in (Dy(_{0.6})Gd(_{0.4}))(_{5})Pd(_{2}) compound. The (chi ') curve showed a peak (T_{g1}approx ) 59 K and a weak hump-like behavior at low temperatures. Interestingly, the (chi '') curve displayed two frequency-dependent distinct peaks, one at (T_{g1}) and other at (T_{g2}approx ) 16 K. An obtained value of the relative shift in freezing temperatures (delta T_{f1}approx ) 0.017 and (delta T_{f2}approx ) 0.07 are obtained from the AC susceptibility data reflects the formation of double cluster-glass states. The frequency dependence of T(_f) is also analyzed within the framework of dynamic scaling laws such as power law and Vogel-Fulcher law. The analysis using power law yields a characteristic time constant for a single spin flip is (tau ^*=) (1.55 times 10^{-9}) s and critical exponent (znu '=) 3.53 for the temperature (T_{g1}). Whereas (tau ^*=) (3.98 times 10^{-6}) s and critical exponent (znu '=) 1.89 for the temperature (T_{g2}). Further evidence of cluster-glass behavior comes from the frequency dependence of the freezing temperature fitted with the Vogel-Fulcher law. Values of fitting parameters are, Vogel-Fulcher temperature T(_0 approx 54.38) K, an activation energy ( E_a / k_Bapprox 69.18 ) K and ( E_a / k_BT_0approx 1.27 > 1 ) for (T_{g1}); T(_0 approx 14.24) K, ( E_a / k_Bapprox 14.71 ) K, and ( E_a / k_BT_0approx 1.03 > 1 ) for (T_{g2}). The nonzero values of T(_0) and ( E_a / k_BT_0 ) further support the evidence for the cluster-glass behavior. The magnetic contribution to the specific heat follows a ( T^{3/2} ) temperature dependence below the cluster-glass freezing temperature, also supporting the evidence for cluster-glass behavior.

In the “crossing lattices” domain of cuprate superconductors with extreme anisotropies, scanning Hall probe microscopy (SHPM) has shown how Josephson vortex lattice (JV) interacts with pancake vortices (PV) in single Bi₂Sr₂CaCu₂O_8+δ (2212) crystals under enormous in-plane fields. Using SHPM, researchers have examined vortex formations in the regime of interacting crossing lattices subjected to in- and out-the-plane magnetic fields. We establish a rich vortex phase diagram that depends on both in- and out-the-plane magnetic fields and is dominated by a field-driven change in the underlying Josephson vortex lattice structure, while the evolution of vortex chain structures at small in-plane fields as the out-of-plane field increases, and find especially stable composite structures made up of chains divided by one or more rows of free pancake vortex stacks. We are also able to follow the interchain distance is inversely proportional to the in-plane field. Our results are these chains provide light on the superconducting regime of crossing vortex lattices in very anisotropic cuprates.

Nb₃Sn-based superconductors are used in high-field magnets of various research projects, including magnetic systems of Tokamak thermonuclear fusion and accelerators for high-energy physics. An important task aimed at improving the current-carrying characteristics of the wire is to study the effect of design features and alloying of multifilamentary strands and the heat treatment modes on the formation of superconducting layers with an optimal structure, namely, with fine equiaxed grains of Nb₃Sn phase with close to stoichiometry composition. This study presents the results of scanning and transmission electron microscopy examination of the structure and morphology of Nb₃Sn layers in multifilamentary superconducting bronze-processed strands, with coupled Nb filaments doped with Ti, after various heat treatment (diffusion annealing) modes. The study used single-stage and two-stage annealing with different temperatures and durations.

In this work, 1-D nickel nanowires with different diameters were synthesized. Magnetic properties of the products were investigated by using first-order reversal curves (FORC) experimental testing, and magnetization reversal processes were theoretically simulated based on the Object Oriented Micro Magnetic Framework (OOMMF) software. The experimental results show different shapes of FORC patterns, indicating the influence of the diameter on the magnetization switching processes of the nanowires. Micromagnetic simulations were carried out on a model of two parallel cylindrical nanowire systems, and the simulation results indicated that both the shape anisotropy and magnetostatic interactions played important roles in identifying the magnetization reversal processes. Furthermore, by means of micromagnetic slicing, the real time distribution status of magnetic moments in the two parallel nickel nanowires had been illustrated visually, which showed that the magnetization reversal process in the two parallel nanowires were different especially in the x and y directions, although of the same objective condition.