Journal of Superconductivity and Novel Magnetism最新文献

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.

The solid-state approach was successfully used to develop Bi_1 − xYₓFe_1 − xTiₓO₃ ceramics with doping concentrations up to x = 0.24. Rietveld refinement of the XRD data showed a structural phase transition from the rhombohedral R3c phase (for x ≤ 0.16) to an orthorhombic Pnma phase (for x = 0.24), indicating doping-induced structural distortion. X-ray diffraction (XRD) patterns verified the formation of a single-phase structure across all samples. Temperature-dependent dielectric result showed abnormalities at around 375 °C. Magnetic transitions are indicated by a high-temperature anomaly close to 370 °C that is ascribed to the Néel temperature (T_N). Dielectric properties improved with co-doping confirm from ferroelectric response. Magnetization measurements (M–H loops) showed magnetization increases with increasing x, likely due to the partial suppression of the Fe–O–Fe spin cycloid by Ti doping, which enhances weak ferromagnetism. FT-IR spectroscopy revealed a broad absorption band one at 554 cm^− 1 and other 430 cm^− 1, which became sharper with increasing x, indicating changes in Fe–O vibrational modes due to doping. FT-IR spectra display broad absorption bands attributed to vibrations of Fe-O and Bi-O in the FeO₆ octahedral within the doped ceramics. Optical absorption spectra showed light absorption between 300 nm and 700 nm, corresponding to an optical band gap within the visible range, making these materials potentially suitable for photovoltaic or optoelectronic applications.

The present study investigates the effect of Y³⁺ and Cr³⁺ co-doping on the crystal structure, vibrational, elastic, morphological and magnetic properties of CoFe_2−xY_x/2Cr_x/2O₄ (x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10.) nanoparticles synthesized via cost-effective sol-gel auto-combustion method ensuring high purity. The nanoparticles were synthesized in nano-sized dimension using sol-gel auto-combustion method. All the synthesized nanoparticles show single phase nature associated with cubic spinel structure which was well supported by Rietveld analysis. The Rietveld refinements exhibited a better goodness of fit (χ²) between 2.54 and 2.64. The lattice constant decreases from 8.387 Å to 8.380 Å on doping Y³⁺ and Cr³⁺ ions. The crystallite size obtained from Scherrer’s equation is in the range of 23 nm to 18 nm. Cation distribution obtained from XRD method and Rietveld method suggests occupancy of Y³⁺, Cr³⁺, Co²⁺ ions at octahedral [B] site while Fe³⁺ ions get distributed over tetrahedral (A) and octahedral [B] sites. FTIR spectra show the absorption bands near 400 cm^− 1 to 550 cm^− 1 characterizing the spinel nature. The vibrational and mechanical properties like Debye temperature, stiffness constant, Young’s modulus, bulk modulus, modulus of rigidity increases marginally on doping of Y³⁺ and Cr³⁺ ions. Raman spectra characterizes the formation of spinel ferrite structure. The surface morphology was viewed through Scanning Electron Microscopy (SEM) technique which show dense morphology with grain size in the range of 21 nm to 25 nm. The saturation magnetization (Ms) linked with the A-B superexchange interaction decreased with Y³⁺ and Cr³⁺ ions. The coercivity (Hc) initially increases and then decreases with Y³⁺ and Cr³⁺ ions within the doping level of x = 0.00 to 0.10.

Manganese ferrite nanoparticles have attracted considerable interest in the field of magnetic resonance imaging as contrast agents due to their exceptional magnetic properties. Structural features, such as size polydispersity and random cation distribution, can vary simultaneously during the preparation of these nanoparticles, which significantly influences their magnetic properties and, consequently, their transverse relaxivity. The theoretical prediction of this influence therefore deserves special attention. In this context, molecular dynamics and density functional theory calculations were extensively used. Our simulation results showed that the smaller the nanoparticle size, the higher the surface area to volume ratio. However, the variation of this ratio leads to heterogeneity of the crystal lattice constant across the nanoparticle. This, combined with the random distribution of cations, leads to variations in magnetization, and consequently, in transverse relaxivity. More interestingly, transverse relaxivity was found to increase with size, and beyond 10 nm, a notable dependence on cation distribution appeared. The lower the degree of inversion, the higher the relaxivity. Such a critical size is actually smaller than those of conventional ferrite-based contrast agents. Our study therefore has the merit of guiding the experimental operator in order to better predict the evolution of relaxivity as a function of the size and distribution of cations, which opens the way to better control of the effectiveness of ferrite manganese-based contrast agents.