Journal of Superconductivity and Novel Magnetism最新文献

The structural, electronic, magnetic, and photocatalytic properties of the vacancy-ordered double perovskites K₂MCl₆ (M = Re, W, Ru, Os) were systematically investigated using first-principles calculations within the WC-GGA, TB-mBJ approaches. The optimized lattice constants show excellent agreement with experimental data, with deviations below 0.5%, confirming the high accuracy of the structural description. Energy–volume analyses for NM, FM, and AFM configurations demonstrate that the ferromagnetic phase is the ground state for all compounds, driven by spin polarization of the B-site d electrons. The calculated magnetic moments further support the robust FM ordering. Electronic band structures and DOS reveal mixed behavior: K₂ReCl₆ exhibits a semiconducting character, whereas K₂WCl₆, K₂RuCl₆, and K₂OsCl₆ display half-metallicity, with metallic states in one spin channel and a finite gap in the opposite one. The TB-mBJ band gaps enhance the reliability of the electronic description. Thermodynamic stability is confirmed through negative formation energies and positive cohesive energies. Band-edge positions evaluated through qualitative electronegativity trends and quantitative spin-polarized electronic calculations indicate favorable alignment for photocatalytic reactions, suggesting potential for water splitting and CO₂ reduction. The combined structural, magnetic, and electronic stability highlights K₂MCl₆ compounds as promising candidates for photocatalysis and spintronic applications.

In this paper, flower-like nickel oxide (NiO) nanostructures were synthesized successfully by a green and cost-effective hydrothermal method. The approach therefore constitutes a hydrothermal-like green process using ordinary laboratory glassware instead of conventional autoclaves. The structural characterization confirmed the existence of a cubic phase of NiO with crystallite average sizes ranging from 41.80 to 89.07 nm, depending on the annealing temperature. Morphological analysis by SEM revealed well-aligned nanoflowers at 300–400 °C, while high-temperature heating led to the deterioration of the hierarchical structure. Raman spectra confirmed the vibrational modes of NiO, and magnetic measurements revealed weak ferromagnetic behavior that was consistent with superparamagnetic properties. These findings are indicative of the potential of flower-like NiO for application in environmental remediation and gas sensing devices.

Cobalt-substituted barium hexaferrite (BaCo₂Fe₁₆O₂₇, x = 0) and its cobalt-nickel co-substituted counterparts (BaCo_1.6Ni_0.4Fe₁₆O₂₇, x = 0.4; and BaCo_1.4Ni_0.6Fe₁₆O₂₇, x = 0.6) were successfully synthesized via the sol-gel auto-combustion method. This synthesis route allowed for homogeneous mixing and yielded phase-pure hexaferrite structures with controlled morphology. Structural analysis confirmed the formation of the W-type hexaferrite phase, while the incorporation of Co²⁺ and Ni²⁺ ions resulted in notable changes in lattice parameters and crystallite size. The microstructure displays notable grain size heterogeneity, densely packed grains, indicative of partial sintering and strong interparticle interactions. FTIR spectroscopy showed the Fe–O stretching vibrations associated with tetrahedral sites remained largely unaffected by variations in nickel and cobalt substituting concentrations. In contrast, the Fe–O stretching modes at octahedral sites exhibited a slight but discernible redshift, indicating subtle modifications in the local bonding environment. UV-Vis spectroscopy demonstrated a redshift in the absorption edge and a reduction in optical bandgap with increasing Ni content, attributed to modifications in the electronic structure. Magnetic measurements revealed significant enhancement in coercivity and saturation magnetization due to the substitution effects, particularly at higher Ni concentrations. The saturation magnetization (M_s) initially decreases from 72 emu/g (x = 0.0) to 66.6 emu/g (x = 0.4) due to Fe³⁺ moment dilution by Co/Ni substitution, then recovers to 71.74 emu/g (x = 0.6) as Ni²⁺’s higher moment and improved spin alignment dominate. The magnetic anisotropy constant (K) mirrors this trend declining from 0.361 × 10⁶ erg/cm³ to 0.145 × 10⁶ erg/cm³ before rebounding to 0.339 × 10⁶ erg/cm³ indicating restored anisotropic stability via spin-orbit coupling at higher substitutions. These findings highlight the potential of Co- and Co/Ni-substituted barium hexaferrites for applications in high-frequency devices, microwave absorption, and magneto-optical systems.

We examine the temperature dependencies of critical parameters and the diode effect in narrow superconducting bridges with disordered boundary layers. The Ginzburg-Landau theory is used to analyze how variations of the temperature and the external magnetic field affect the diode efficiency of the structure. In the considered temperature range ((T ge 0.8 T_C)), the diode efficiency changes slightly with temperature in low magnetic fields. Moreover, with increasing magnetic field, the diode efficiency increases with decreasing temperature and reaches a maximum of 12% at the largest of the considered magnetic fields. Additionally, further disordering of the surface layer, for example via atomic implantation with low-energy argon atoms, could allow the diode efficiency to be increased to 17%.

In this work, the modulation of the magnetic and magnetocaloric properties of the classical La_0.65Ca_0.35MnO₃ system through B-site doping with the non-magnetic element Al has been investigated. Structural studies of La_0.65Ca_0.35Mn_1-xAl_xO₃ (x = 0.0 and 0.1) reveal that the substitution of Mn³⁺ by the small-ion-radius Al³⁺ leads to a decrease in the unit cell volume. Based on density functional theory (DFT), the total density of states (TDOS) and the partial density of states (PDOS) of the system are calculated using the Vienna ab initio Simulation Package (VASP). It is found that the doped system weakens the hybridization between O-2p orbitals and Mn-3d orbitals, which, in turn, directly affects the double-exchange interactions within the system. The Curie temperature (T_C) is effectively tuned from 261 to 98 K. Studies on the magnetocaloric effect show that the doped samples exhibit a wider full width at half-maximum temperature region (∆T_FWHM), which increases from 30.89 K to 50.65 K (under a 5 T magnetic field). The doped sample demonstrates superior relative cooling power (RCP = 239.44 J·kg⁻¹ for x = 0.0, μ₀H = 5 T; 281.45 J·kg⁻¹ for x = 0.1, μ₀H = 5 T) and refrigerant capacity (RC = 186.82 J·kg⁻¹ for x = 0.0, μ₀H = 5 T; 229.76 J·kg⁻¹ for x = 0.1, μ₀H = 5 T).Based on Landau theory and the magnetocaloric effect, it is found that this series of materials belongs to the type of first-order phase transition, and the continuity of the phase transition is enhanced after doping. The B-site doping of Al has been demonstrated to optimize the magnetocaloric properties of the La_0.65Ca_0.35MnO₃ system.

Barium hexaferrites (BaFe_12-xO₁₉) with iron deficiency (x = 0, 0.4, 0.8, 1.2, 1.6, 2) were synthesized by sol–gel method and then sintered at 1050 °C for 4 h. X-ray diffraction and Rietveld refinement analysis confirms single-phase hexagonal structure belonging to (P{6}_{3}/mmc) space group. Iron (Fe) ion deficiency introduced in the barium hexaferrite causes no structural distortion; however, the grain size was found to increase from 196 to 433 nm. X-ray photoelectron spectroscopy studies show presence of Fe²⁺ ions with Fe³⁺ along with oxygen defects, and the presence of metal–oxygen bonds was confirmed using Fourier transform infrared spectroscopy. Highest saturation magnetization and coercivity were observed for iron deficiency with x = 0.4 and x = 2. Deficiency in iron ion sites results in change in Fe²⁺/Fe³⁺ ratio, causing possible variations in the site occupation and disruptions in the local exchange interactions, which induce easy rotation of spins attributing to decrease of coercivity with sustained magnetization.

The density functional theory (DFT) was applied to examine the structure, elastic, thermoelectric, optical and electronic response of NiYX (where X = P, As. Sb) half-Heusler (H-H) alloys. The structural optimization was executed through the framework of Perdew Burke Ernzerhof (PBE)-generalized gradient approximation (GGA). Formation energy for the three compounds is negative, suggesting possible experimental synthesis. The phonon and elastic properties evaluation reveal that all the materials are dynamically and mechanically stable and are brittle in nature. The NiYP, NiYAs and NiYSb revealed indirect band gap with both the GGA-PBE and modified Becke-Johnson (mBJ) approximations. The estimated density of states (DOS) indicate that Ni-d and Y-d states donate mainly to the conduction and valence bands while the p-states of Sb, As and P contributes minorly to both bands. The optical property analysis unveils that these materials are optically active and appropriate for optoelectronic applications. The thermoelectric characteristics of the considered materials were analyzed and their figure of merit and power factors at room temperature and higher temperatures signals their suitability in thermoelectric device applications.

Tin oxide (SnO₂) nanoparticles were synthesized using a hydrothermal method. In this study, we used two different methodologies. The concentration and medium variation technique was utilized to precisely manipulate morphology and microstructure. The concentration of the Tin precursor (SnCl₂) was systematically increased from 0.08 M to 0.14 M in increments of 0.02 M, while the ethanol content in a 36 ml bath volume was adjusted across particular quantities (0, 8, 18, 24, and 36 ml). Structural investigation utilizing the Scherrer, Halder-Wagner, and Size-Strain plot methods found that the crystallite size dependence was non-linear with the precursor concentration, peaking at 0.1 M and reaching a maximum value of around 14.7 nm. Similarly, during solvent variation, the highest crystallite size was seen when the ethanol content in the aqueous bath reached 18 ml, with subsequent increases up to 36 ml resulting in a decrease in size. Microstrain, on the other hand, fluctuated very slightly when the concentration of the Sn precursor was changed, but increased steadily as the ethanol content increased. Furthermore, this tuning successfully engineered the morphology, transforming the irregularly shaped, globular agglomerations observed at lower concentrations into structures resembling nanoneedles; these nanoneedles, with a length of around 150 nm, were easily visible when the ethanol concentration reached 36 ml. Furthermore, this work lays the groundwork for future uses, such as gas detection and other optoelectronic applications.

The quaternary Heusler alloys CoFeMnAl and CoFeMnGe have been thoroughly investigated using density functional theory to investigate their structural, electronic, magnetic, and mechanical properties. The results show that both materials are nearly half-metallic at equilibrium conditions, which evolve into pronounced half-metallic behavior under hydrostatic pressure. Specifically, CoFeMnAl transitions from metallic to half-metallic about 8 GPa, while CoFeMnGe does the same around 23 GPa. With increased pressure, the upper bands in the spin-down channel move deeper below the Fermi level, indicating the gradual opening of a half-metallic gap. Despite these modifications, the total magnetic moment stays steady at around 3 µ_B for CoFeMnAl and 4 µ_B for CoFeMnGe. Furthermore, mechanical stability is maintained at up to 12 GPa for CoFeMnAl and 30 GPa for CoFeMnGe, highlighting their promise for pressure-tolerant spintronic device applications.

In the Review we discuss anomalous aspects of superconductivity (SC) and normal state, as well as formation of inhomogeneous (droplet-like or cluster-like) states in electron systems with attraction. We consider both the models with the retardation (Eliashberg mechanism of SC for strong electron-phonon interaction in metallic hydrogen) and without retardation (but with local onsite attraction). We concentrate on the mechanism of the BCS-BEC crossover for the Hubbard model with local attraction and diagonal disorder for the two-dimensional films of the dirty metal. In 2D Hubbard model in the framework of the Bogoliubov-De Gennes (BdG) approximation for strong interaction and strong diagonal disorder at low electron densities the inhomogeneous states are realized in the system with the droplets of the order parameter in the matrix of unpaired states as well as the percolating insulator-superconductor phase transition when we increase electron density. We analyze also the model of the inhomogeneous space-separated Fermi-Bose mixture for the bismuth oxides BaKBiO, which contains the paired clusters of bosonic states as well as unpaired fermionic clusters. This model explains the unconventional phase diagram of the system containing the anomalous phases of bosonic insulator, bosonic semiconductor and bosonic metal. Superconductivity is realized in this system due to local pairs tunneling from one bosonic cluster to the neighboring one via the fermionic barrier. For metallic hydrogen and metallic hydrides, we calculate the critical temperature and discuss important possibility for practical applications how to increase the temperature by decreasing pressure in the framework of the generalized Eliashberg approach. We advocate also interesting analogies with the quantum (vortex) crystal for long-living low-dimensional metastable phases of metallic hydrogen including filamentous phase with proton chains embedded in 3D electron Fermi liquid and planar phase with proton plains. We formulate the concept of two Bose-condensates in SC electron and superfluid (SF) ion subsystems and provide the estimate for the lifetime of the long-living metastable phases at normal pressure. The estimate is connected with the formation and growth of the critical seeds of the new (molecular) phase in the process of quantum under-barrier tunneling.