Journal of Superconductivity and Novel Magnetism最新文献

Tetra metal nitrides (M₄N; M = Cr, Fe, Co, Mn, Ni) are a promising spintronic material with an anti-perovskite structure and fascinating magnetic characteristics due to a magneto-volume effect. Though a fully stochiometric Mn₄N or Fe₄N has been achieved, the lattice parameter (LP) of Co₄N was always been found to be significantly lower than the anticipated theoretical values, indicating a sub-stochiometric Co₄N phase. The formation enthalpy of Co₄N is slightly positive resulting in unfavorable thermodynamical conditions and significant out-diffusion of N from Co₄N. In this work, we present a comparative study of undoped and Pd-doped Co₄N thin films synthesized using a reactive nitrogen sputtering. The structural, composition, and magnetic properties have been studied by combining x-ray diffraction, Rutherford backscattering, energy-dispersive x-ray spectroscopy, secondary ion mass spectroscopy, vibrating sample magnetometer, magneto-optical Kerr effect, and polarized neutron reflectivity measurements. It was found that Pd doping of about 5 at.% results in a significant enhancement in the LP of Co₄N signifying a higher amount of N retention without adversely affecting the growth and magnetic properties. It is further suggested that the amount of Pd doping may further be increased to realize a fully stoichiometric Co₄N.

FeNi3@MnO₂/FeNi₃ (FMF) nanocomposite with different weight ratios of FeNi₃ to FeNi₃@MnO₂ was prepared under 100 ℃ for 3 h. Then, these compounds’ structural and microstructural properties were characterized using X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. Also, the electromagnetic properties of the samples were characterized using a vibrating sample magnetometer, and microwave absorption in the 2–18 GHz frequency range. The results show that the magnetic characteristics of the samples increased with the increase in the ratio of FeNi₃ nanoparticles. However, the excessive growth of FeNi₃ nanoparticles decreased the dielectric properties (the dielectric constant) so that the electrical conductivity of the FMF (4) sample is lower than the FMF (3) sample. The reflection loss of the FMF (1) sample was better than the other samples and about − 27 db.

Considering that Bi(_2)Sr(_2)CaCu(_2)O(_{8+delta }) (Bi2212) intrinsic Josephson junctions have a body-centered tetragonal structure, and we adjusted a directional tunneling matrix element in the effective density of states. Using the d-wave Eliashberg equations, tunneling spectra of overdoped Bi2212 intrinsic Josephson junction are well reproduced in a wide temperature range below the critical temperature (T_c). From which we obtained the pairing glue spectral function (alpha ^2F(Omega )), which is composed of a low-energy resonance peak and a high-energy broad spectrum. These two parts have different effects on the tunneling spectra: the low-energy resonance peak controls the entire shape of the tunneling spectra, and the high-energy broad spectrum maintains a certain electron-boson coupling constant (lambda ). Some interesting and important rules in the fitting process are summarized. The dip of tunneling spectra gradually disappears with increasing temperature, which can be fitted using a simple Dynes’ form of density of states, where the energy gap is energy-independent. As the temperature rises to (T_c), the main peak position of the density of states of the sample still remains a finite value, that is, the energy gap is not closed when the superconductivity disappears, which provides an angle to investigate the pseudogap phenomenon in high-(T_c) superconductors.

Magnetocaloric effect (MCE) and critical behavior of polycrystalline GdPO₄ prepared by chemical precipitation and solid-state sintering method were investigated. X-ray diffraction (XRD) patterns indicate that GdPO₄ is crystalized in monazite phase with monoclinic crystal structure. Magnetization measurements confirm the presence of ferromagnetic-paramagnetic phase transition, as well as the presence of ferromagnetic and antiferromagnetic interactions between Gd³⁺ ions in the low-temperature region. Arrott curves indicate that the sample exhibits second-order magnetic phase transition behavior. The maximum value of magnetic entropy change (−∆S_M^max) and relative cooling power (RCP) are 50.1 J/(kg·K) (11.3 J/(kg·K)) and 416.3 J/kg (56.4 J/kg) at 5 T (1 T) magnetic field, respectively. The maximum value of adiabatic temperature change (∆T_ad^max) reaches 36.5 K under an applied magnetic field of 5 T, indicating that GdPO₄ is a good candidate for low-temperature magnetic refrigeration. Critical behavior is analyzed and the existence of short-range ordering (SRO) magnetic exchange interactions in the long-range ordering (LRO) of GdPO₄ is suggested to be responsible for the large magnetocaloric effect.

Novel phases in quantum materials arise when quantum fluctuations couple with frustration, geometries, and lattice dimensionalities. In continuation, frustrated magnetism combined with lattice degree of freedom, magnetoelastic coupling and magnetic anisotropy may result in exotic magnetic properties in a system. Here, we have studied a rare earth orthovanadate ErVO₄, where the presence of magnetoelastic coupling along easy c-axis of magnetization results in the evolution of different magnetic phases. The second nearest neighbour Er spins in this compound form a corner-sharing tetrahedra and the spins lie along the easy c-axis, in contradiction to other members of RVO₄ (R = Rare-earth) family. Our DC field superimposed AC susceptibility and heat capacity measurements show the emergence of a magnetic anomaly which is believed to originate due to the presence of large magnetoelastic coupling along the c-axis. Further, above 10 kOe, a magnetic phase associated with positive fifth order susceptibility is observed. In this region, the Zeeman energy linked with magnetic anisotropy stabilizes the interactions among the higher order moments. This observation is further supported by a theoretical model.

In this work, two polycrystalline samples were prepared using the solid-state reaction technique. The X-ray diffraction patterns revealed that the samples A and B are a mixture of the superconducting phases (Bi, Pb)-2212 (Bi_1.6Pb_0.4Sr₂CaCu₂O₈) and (Bi, Pb)-2223 (Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀). The (T_c) was measured using electron paramagnetic resonance (EPR) in absorption mode: 97 K for the sample A and 101 K for sample B. The EPR spectra were measured (derivative of the absorption) and showed a signal in the low-field region only present in the superconducting state. The study of this signal revealed the coexistence of antiferromagnetism and diamagnetism in the sample A, for (T < T_{c}). The same was observed in sample B after a light grinding. The effective penetration depth was estimated from the peak-to-peak linewidth of the EPR signal (at T = 92 K): (lambda )(92 K) = 479 nm for sample A, and 440 nm for sample B. These results are in agreement with the values obtained using techniques such as (mu )-spin rotation and magnetization measurements. With the aid of the two-fluid model and the BCS approximation (lambda ) was also estimated at 0 K. Using these values and the Ginzburg-Landau theory, the lower critical field was calculated at T = 92 K and 0 K.

The critical current density (J_c) of Y_0.5Gd_0.5Ba₂Cu₃O_7-x (YGdBCO)-coated conductors (CCs) could be improved via producing the periodic micro-hole arrays on the surface of CCs. In this work, the influence of micro-hole arrays on the transport AC losses of YGdBCO CCs was studied. The micro-hole arrays were produced in YGdBCO CCs via laser drilling, which were distributed with different spacings (l) between two neighboring micro-holes along longitudinal and transverse directions. With l ≥ 0.7 × 0.7 mm, the critical current (I_c) values of drilled samples were nearly identical with that of virgin one. However, a considerable nonlinear decrease of I_c value was displayed in the drilled samples with l < 0.7 × 0.7 mm. In addition, compared to the virgin one, the AC losses of samples drilled with various transverse and longitudinal spacings were increased more than one order of magnitude under low frequency (31 Hz ~ 177 Hz). There was almost no effect on the AC losses of micro-holes spacing for drilled CCs. A 3D H-formulation simulation model was established to analyze the experimental results. The investigations revealed the magnetic flux densities around micro-holes were altered, which could potentially lead to an increase of AC loss.

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.