Journal of Superconductivity and Novel Magnetism最新文献

We have investigated the superconductor-insulator transition (SIT) induced by a disorder of a granular system. The effective energy gap model based on localized Cooper pairs and hopping quasiparticles has been used. The “quasi-reentrant” behavior appears when the metal volume fraction is slightly smaller than the percolation threshold. In particular, we have found the double “quasi-reentrant” behavior with the increased superconducting transition width. This result indicates that the double “quasi-reentrant” behavior should have been observed in a granular system with a large transition width. Besides, the “quasi-reentrant” behavior has been flattened when applying the magnetic field. In order to clarify the method is suitable for a granular system, the theoretical result has been compared with the experimental value of Pb(_x)(SiO(_2))(_{1-x}) granular film. It has almost no difference near the minimum value and the differences onset at lower temperatures due to the negative electroresistance effect. The “quasi-reentrant” behavior becomes unobvious due to the Copper pairs destroyed by the magnetic field.

The growth of the high-T(_c) cuprate Bi(_2)Sr(_2)CaCu(_2)O(_{8+delta }) (BSCCO-2212) superconductor is always challenging due to the contamination from alumina crucible or the intergrowth of other phases such as Bi(_2)Sr(_2)CuO(_{6+ delta }) (Bi-2201). In this work, we report the synthesis of single crystals of BSCCO-2212 with the highest reported superconducting transition temperature, T(_c), of 96.6 K, achieved using an improved self-flux technique. The as-grown crystals were characterized by scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and powder x-ray diffraction (p-XRD) measurements. PXRD on single crystals reveals the absence of contamination from the alumina crucible as well as the absence of the intergrowth of Bi-2201 phase. PXRD on crushed single crystals reveals minimal intergrowth of Bi-2201 and Bi-2223 phases. The physical properties were investigated by carrying out temperature-dependent magnetization and resistance measurements. Magnetization measurements revealed the T(_c) of three single crystals of BSCCO-2212 to be 93 K, 94 K, and 96.6 K, higher than the previously reported values in the range from (sim ) 90 to 93 K [1,2,3,4,5]. Such a high T(_c) and the absence of an intergrowth phase indicates a good quality of the grown crystals that can be utilized for fundamental as well as basic research.

We consider a spin valve composed of a superconducting film (S) between two ferromagnetic insulators (FI) on two sides. In the dirty limit, the superconductor is described by Usadel equations. Appropriate boundary conditions were chosen for two S-FI interfaces, which are described via the interface parameter spin mixing angle. By numerically solving the Usadel equations, the density of states (DOS) at different spin mixing angles was obtained. It was shown previously that the critical temperature of such an FI-S-FI structure depends on the mutual alignment of the FI layers’ magnetization. We follow the evolution of DOS at the change of misalignment of ferromagnets magnetization and probe the zero bias peak creation. The DOS characteristic features may give fruitful information about triplet superconducting components creation and interplay inside the S layer.

Recently, Junbao He et al. reported on magnetic ordering in the topological candidate EuZnBi(_2) (He et al. J. Supercond. Nov. Magn. 37, 579 2024) and suggested that there is a spin reorientation near 15 K, below the initial antiferromagnetic transition at T(_N)=20 K. Here we use (^{151})Eu Mössbauer spectroscopy to show that the situation is more complex. The initial ordering involves an incommensurate sinusoidally modulated structure that progressively squares up on further cooling.

In this paper, a microscopic model of anisotropic soft magnetic composites based on three-dimensional homogeneous distribution is established with numerical calculation. The effects of the particle orientation angle and the axial diameter ratio on the equivalent permeability of composites were investigated by the finite element method, and the accuracy of the model was verified by using experimental results in comparison with the simulation results. The results show that the composites have the largest equivalent permeability of 63.944 when the particle orientation angle is 0° (particle size of 75 µm). As the insulating layer is consistent, the larger equivalent permeability of composites is obtained by the larger particle axial diameter ratio. The equivalent permeability of composites is 114.07 when the axial diameter ratio is 100, which is 89.76% larger than that of the composites with an axial diameter ratio of 5. The equivalent permeability of the specimen with the orientation angle of 0° and the particle axial diameter ratio of 42 is 48.579, and the equivalent permeability of the model under the same conditions is 47.276 in the simulation. The comparative analysis found that the simulation and experiment value only have an error of 2.68%, which is within the acceptable range and further proves the accuracy of this model.

The analysis of ferromagnetic resonance (FMR) through micromagnetic simulations is a powerful tool used to gain insight into the effects of shape on magnetization dynamics. Mathematical models incorporated into simulation programs have greatly facilitated the understanding of resulting magnetic behaviors. The present study aimed to investigate the influence of shape on the magnetization dynamics of Permalloy discs and stadium-like geometries of finite dimensions. The finite element numerical method was used to perform simulations, employing the Nmag simulator. To simulate the FMR spectra, the ringdown method was utilized, a common technique in computational micromagnetism. Resonance modes were identified using Fourier transform analysis. The novelty of this work is to demonstrate that in a film with a thickness of 50 nm and lateral dimensions less than 1000 nm, it cannot be considered as an infinite film. The results of the calculations showed that, for an in-plane field in a Py disk with these dimensions, there are resonance fields affected when the geometry is slightly stretched. This phenomenon was observed applying a field parallel to the plane of a stadium-shaped geometry. The effects of shape anisotropy remain relevant even in a 50 nm film with lateral dimensions exceeding 1000 nm. The aim of the study is to review results that are normally used for work with thin films. Assuming infinite dimensions and disregarding shape effects always requires care, since depending on the study in question, artifacts due to the shape effect may arise and be interpreted incorrectly.

The influence of thermal and thermomagnetic treatment on the magnetic properties of iron—cobalt oxides compacts fabricated by powder metallurgy is studied. The influence of magnetic pulse processing (MPP) on the formation of the phase composition and magnetic properties of nanocrystalline α-Fe (50%) + Fe₂O₃ (50%); α-Fe (50%) + Fe₂O₃ (40%) + Co₃O₄ (10%) and α-Fe (50%) + Fe₂O₃ (30%) + Co₃O₄ (20%) pressed powder compacts during synthesis in a high-energy mill and subsequent annealing have been investigated. According to the X-ray diffraction analysis, annealing α-Fe (50%) + Fe₂O₃ (50%) pressed samples at 250 ℃ in air, promotes the oxidation of α-Fe and FeO to magnetite (Fe₃O₄). Additional annealing of the compact in vacuum at 250 ℃ increases its remnant magnetization and magnetic anisotropy. Whereas, increasing the concentration of Co₃O₄ oxide has no strong effect on the coercivity and residual magnetization of the compacts. Eventually, thermomagnetic treatment of the α-Fe (50%) + Fe₂O₃ (30%) + Co₃O₄ (20%) system does not improve its magnetic properties.

Decomposition of superconductors sometimes becomes crucial when studying essential physical properties of the superconductors. For example, the cuprate superconductor YBa₂Cu₃O_7-d decomposes by long-time air exposure. In this study, we investigate the aging effects on superconducting properties of BiS₂-based superconductors Bi₄O₄S₃ and LaO_0.5F_0.5BiS₂; both were first synthesized in 2012, using their polycrystalline samples synthesized several years ago. We find that 12-year-old Bi₄O₄S₃ samples exhibit bulk superconductivity with a slight degradation of the superconducting transition temperature (T_c) of 0.2 K. For a high-pressure-synthesized LaO_0.5F_0.5BiS₂ sample, clear decrease in T_c is observed, which suggests that high-pressure strain is reduced by aging.

The features of the charge subsystem of the Pr_0.5Nd_0.5BaMn₂O₆ single crystal with partial A-site ordering are investigated. The manganite is a mixture of a phase with a tetragonal structure (43%) with a high degree of A-site ordering and a disordered phase with a cubic structure (57%). Anisotropy of the optical properties and electrical resistance in the crystallographic ab plane and along the c axis was found. The anisotropy manifests itself as a metal–insulator transition near T≈T_C = 150 K in the ab plane and a semiconductor character of the resistance along c above and below T_C. The magnetoresistance effect was revealed, which in a field of 1 T reaches 65% in the ab plane and 45% along the c axis near T_C and about 40% at low temperatures.

In this manuscript we study the magnetic MAOX phases (M = Mn, Cr; A = Ga, Al, X = C) obtained by the replacement of the A-layer in the parent MAX phase by the AO₂ layer. The screening analysis of the magnetic and electronic properties of Mn- and Cr-based MAOX phases is performed using DFT calculations. All MAOX are thermodynamically stable. It was found that in MAOX phases Cr magnetic moments are pronounced increased in compare to corresponding MAX phase. Moreover, drastically changes in the electronic structure arise in Cr₂AlO₂C and Cr₂GaO₂C MAOX phases. The metal behavior in Cr₂GaC MAX phase changes for the near to half-metallic behavior with 90% spin polarization at the Fermi energy in Cr₂GaO₂C MAOX phases. We have found that in Cr₂AlO₂C, the change in the electronic structure leads to the formation of the spin-gapless semiconductor state under slight extension in the ab plane. The obtained results make Cr₂GaO₂C and especially Cr₂AlO₂C prospective candidates for application as functional elements of electronics and spintronics.