Showing papers in "Journal of Superconductivity and Novel Magnetism in 2014"

A Novel Synthesis, Structural, Morphological, and Opto-magnetic Characterizations of Magnetically Separable Spinel Co x Mn 1−x Fe 2 O 4 (0 ≤ x ≤ 1) Nano-catalysts

Abstract: Spinel Co x Mn1−x Fe2O4 (0≤x≤1) nano- crystals were successfully synthesized by a simple microwave-assisted combustion method. High-resolution scanning electron microscopy (HR-SEM) and transmission electron microscopy (HR-TEM) analysis was used to study the morphological variations and found the particle-like nanocrystal morphologies. Energy dispersive X-ray (EDX) results showed that the composition of the elements were relevant as expected from the combustion synthesis. Powder X-ray diffraction (XRD) analysis showed that all composition was found to have cubic spinel-type structure. Average crystallite size of the samples was found to be in the range of 10.36–21.16 nm. The lattice parameter decreased from 8.478 to 8.432 A with increasing Co2+ content. Fourier transform infrared (FT-IR) spectra showed two strong absorption peaks observed at lower frequency (∼435 to ∼800 cm−1), which can be assigned to the M–O (Mn, Co, and Fe) bonds. UV-Visible diffuse reflectance spectroscopy (DRS) shows that the energy band gap of pure MnFe2O4 is 1.78 eV and with increase in the Co2+ ion, it increases from 1.87 to 2.33 eV. Addition of Co2+ in MnFe2O4 reduces the particle size, which can be confirmed by the blue shift in the photoluminescence (PL) spectra. Vibrating sample magnetometer (VSM) results that confirmed a weak ferromagnetic behavior for all composition with saturation magnetization values in the range of 50.05 ±04 to 67.09 °03 emu/g. All composition of spinel Co x Mn1−x Fe2O4 nano-crystals were successfully tested as catalyst for the oxidation of benzyl alcohol to benzaldehyde, which has resulted 87.32 and 94.28 % conversion efficiency of MnFe2O4 and Co0.6Mn0.4Fe2O4, respectively.

X-Ray Diffraction and Cation Distribution Studies in Zinc-Substituted Nickel Ferrite Nanoparticles

Abstract: Structural and cation distribution studies on Ni1−xZnxFe2O4 (with x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) ferrite nanoparticles by using X-ray diffraction analysis are reported. In this work the Nickel-Zinc ferrites nanoparticles are synthesized by sol-gel auto combustion using respec- tive metal nitrates and citric acid as fuel for the auto com- bustion reaction. Formation of ferrite nanoparticles having single-phase spinel structure is evident from the obtained X-ray diffraction patterns. Lattice constant values of the Ni1−xZnxFe2O4 ferrite system are found to increase with in- crease of zinc substitution x. Broad and intense XRD peaks in the patterns indicate the nanocrystalline nature of the produced ferrite samples. Average particle size calculated from most intense Bragg's reflection (311) using Debye- Scherrer's formula is found to be 30 nm. The particle size is found to decrease with increase in zinc substitution x. Observed X-ray density is found to decrease with increase in zinc substitution x. Bulk density, porosity, and unit cell volume are also calculated from the XRD data. Distribu- tion of metal cations in the spinel structure estimated from X-ray diffraction data show that along with Ni 2+ ions most of the Zn 2+ ions also occupy the octahedral (B) sites, which are attributed to nanosize dimensions of the ferrite samples.

Microwave Absorption Properties of Radar Absorbing Nanosized Cobalt Ferrites for High Frequency Applications

Abstract: Radar absorbing materials of nickel-cobalt ferrites (NCF) of general formula Ni(1−x)Co(x)Fe2O4 (0.0≤x≤0.5, in step of 0.1) were synthesized by the coprecipitation route. X-ray diffraction studies confirmed that all the samples exhibit the single-phase cubic spinel structure. The average particle size of as obtained samples has been found in the range of 38–41 nm. The structural morphology of the prepared samples was carried out using SEM. SEM images indicated that the final product consists of nanorods with a diameter of about 80 nm and length up to about 150 nm, and their chemical compositions were measured using the energy dispersive spectroscopy (EDS) technique. The infrared spectra are measured in the frequency range 700–350 cm−1. Furthermore, the influence of Co on the magnetic and microwave absorbance characteristics by using VSM and network analyzer of Ni(1−x)Co(x)Fe2O4 (0.0≤x≤0.5 nanoparticles has been investigated respectively in detail. Our experimental results show that the low loss Ni substituted Co ferrites is an excellent magnetic material for VHF (very high frequency: 30–300 MHz) miniature antenna applications.

Study of Defect Modes in 1d Photonic Crystal Structure Containing High and Low T c Superconductor as a Defect Layer

Abstract: The present paper describes the study of defect modes in a one-dimensional photonic crystal (1d-PC) containing a high and low temperature superconductor as a defect layer at different temperatures below the superconducting transition temperature (T c ). Since the refractive index of the superconducting material is dependent on the penetration depth, which depends on the temperature of the superconducting material, hence by changing the temperature of the superconducting material its refractive index can also be changed. Analysis of the transmission spectra of defect modes in the reflection band of 1d-PC structure shows a shift in the wavelength peak of the defect mode. The shift in peak is different for different superconducting materials and it increases with the increase in temperature whether the defect layer is high T c or low T c superconductors. We also study the presence two defect layers in a 1d-PC structure, one with high T c and other with a low T c superconductor. Further, the effect of variation in the thickness of the defect layer on the defect modes of the PC structure has also been studied In order to obtain the transmission (reflection) spectra of a 1d-PC structure with a defect, we employ the transfer matrix method (TMM). This property of the defective PC structure can be exploited in designing the temperature sensor and narrow optical filters. Further, this tunable feature of superconductor photonic crystal has technical use in the superconducting electronics and photonics.

Structural and Magnetic Studies of Rare-Earth Substituted Nickel Ferrites

Abstract: Polycrystalline ferrites NiFe2O4 and NiFe1.99R0.01O4 (R=Sm, Gd, Eu, and La) samples were prepared by usual ceramic method. The structural and the magnetic properties of the samples were characterized by X-ray powder diffraction (XRD), Mossbauer Effect (ME) spectroscopy, and vibrating sample magnetometer (VSM). Indexed XRD patterns confirm the formation of pure cubic spinel phase. The lattice parameters (a) of the rare earth (R) doped samples were smaller than that of the pure Ni-ferrite. Mossbauer effect spectroscopy was used to study the distribution of cations in tetrahedral (A) and octahedral [B] sites of the spinel. The hysteresis loops indicated that the saturation magnetization and coercivity increased with R-substitution and appeared to be greatly affected by the nature of R ions. The obtained results are interpreted based on the rearrangement of cations between the A-site and B-site.

Electrical Properties of Nanophase Ferrites Doped with Rare Earth Ions

Abstract: The electrical properties of nanocrystalline nickel ferrites doped with rare earth ions having a general chemical formula NixFe(3�x)O4 have been studied. The AC electrical conductance has been measured as a function of frequency in the range of 0.05-900 kHz at room temperature of 296 K. The conductance was found to be dependent on concentra- tion of nickel ions. Doping of these samples with rare earth ions has shown a change in the electrical conductance. The results are discussed in the light of a hopping model.

Synthesis and Study of Structural, Morphological and Magnetic Properties of ZnFe2O4 Nanoparticles

Abstract: Superparamagnetic zinc ferrite (ZnFe2O4) nanoparticles were prepared by a surfactant assisted hydrothermal method and subjected to the heat treatment. The structure, vibrational, morphology, and magnetic properties of synthesized product were characterized by XRD, FT-IR, HR-SEM, and VSM measurements. XRD result confirms the formation of regular spinel structured ZnFe2O4 with space group of Fd3m and an average crystalline size was calculated as 21 nm and 28 nm for the samples annealed in air atmosphere at 300 °C and 600 °C. The HR-SEM image shows that the particles are in spherical shape with small aggregation. A room temperature superparamagnetic behavior was observed for both samples. The saturation magnetization (M s) of 12.0 emu/g and 9.10 emu/g were observed for the samples annealed in air atmosphere at 300 °C and 600 °C, respectively.

Magnetocaloric Effect in Different Impurity Doped La0.67Ca0.33MnO3 Composite

Abstract: In this paper, we investigate the effect of nonmagnetic YSZ and magnetic Fe3O4 addition on the magnetocaloric properties of the La0.67Ca0.33MnO3 composite. The temperature dependences of the magnetization for pure LCMO and the impurity doped LCMO composite upon 0.1, 0.3, and 1 T magnetic field were simulated. By the help of the phenomenological model, magnetic entropy change and specific heat for magnetic field variation are predicted. The values of maximum magnetic entropy change, full-width at half-maximum, and relative cooling power were calculated at several magnetic fields. The ΔS M value is 2.45, 2.39, and 5.58 J kg−1 K−1 at 1 T for pristine LCMO, LCMO/YSZ, and LCMO/Fe3O4, respectively.

A Novel Approach for the Synthesis and Characterization Studies of Mn 2 + -Doped CdS Nanocrystals by a Facile Microwave-Assisted Combustion Method

Abstract: A simple and efficient microwave-assisted combustion method was developed to synthesize Mn 2+-doped CdS (Mn x Cd 1−xS: x= 0.0, 0.3, and 0.5) nanocrystallites. The study suggested that the application of microwave heating produced spherical shaped cluster of pure and Mn 2+-doped CdS nanocrystallites, which was achieved in few minutes. The effects of Mn 2+-doping on structural, morphological, optical and magnetic properties of CdS nanocrystallites were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), high-resolution scanning electron microscopy (HR-SEM) with energy dispersive X-ray analysis (EDX), high-resolution transmission electron microscopy (HR-TEM) with selected area electron diffraction (SAED), UV-Visible diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and vibrating sample magnetometer (VSM). The XRD results confirmed the formation of hexagonal CdS. The formation of pure and Mn 2+-doped CdS phase was also confirmed by FT-IR and EDX. The formation of spherical shaped cluster of nanocrystallites was confirmed by HR-SEM and HR-TEM. The as-synthesized nanocrystallites were found to have good optoelectronic properties that were determined by DRS and PL spectra. VSM results of the as-synthesized Mn 2+-doped CdS nanocrystallites showed ferromagnetic behavior.

Evolution of Magnetocaloric Behavior in Oxygen Deficient La 2/3 Ba 1/3 MnO 3−δ Manganites

Abstract: Effects of oxygen deficiency on the thermomagnetic properties of La2/3Ba1/3MnO3−δ polycrystalline perovskites have been predicted. By the help of the phenomenological model, the temperature dependences of the magnetization for La2/3Ba1/3MnO3−δ with δ=0.0, 0.02, 0.05, 0.08, and 0.1 upon 1 T magnetic field were simulated. The behavior of the temperature dependent magnetocaloric effect, in the vicinity of magnetic phase transitions, was investigated. The magnetic entropy change, specific heat, and adiabatic temperature change for several δ were obtained. The values of maximum magnetic entropy change, full-width at half-maximum, and relative cooling power, in 1 T magnetic field variation, were calculated. As the oxygen content increases, the magnetocaloric effect of La2/3Ba1/3MnO3−δ , decreases and shifts to room temperature. The results obtained show a strong dependence on the oxygen deficiency of the materials. The magnetocaloric effect of these materials is large and tunable, suggesting a possible technical application of the materials at moderate magnetic fields near room temperature. It is shown that for La2/3Ba1/3MnO3−δ , the magnetic entropy change and adiabatic temperature change follows a master curve behavior.

Simulation of Magnetocaloric Effect in La0.7Ca0.3MnO3 Ceramics Fabricated by Fast Sintering Process

Abstract: The enhanced low-field magnetocaloric effect (MCE) is simulated for La0.7Ca0.3MnO3 (LCMO) ceramics that were fabricated by fast sintering process with different Al2O3 contents. It is shown that LCMO exhibits magnetic entropy change (ΔS M ) much more uniform than that of gadolinium. The results show that the peak in the MCE at the ferromagnetic to paramagnetic phase transition is improved as the sintering temperature decreases. Furthermore, the samples open up a new way in which to tune the intrinsic properties of mixed-valence manganites. Through these results, LCMO has some potential applications for magnetic refrigerants in an extended high-temperature range. It is suggested that the fast sintering process with different Al2O3 contents for LCMO is an efficient way to obtain a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas.

Magnetocaloric Effect in Nanopowders of Pr 0.67 Ca 0.33 Fe x Mn 1−x O 3

Abstract: Magnetocaloric effect in nanopowders of Pr0.67Ca0.33Fe x Mn1−x O3 (x=0,0.1,0.3,0.4), in the vicinity of magnetic phase transitions, was investigated. It is shown that Pr0.67Ca0.33MnO3 exhibits the largest magnetic entropy change (ΔS M ) of 0.61 J/kg/K at 208 K upon 0.5 T magnetic field variation. Furthermore, the ΔS M distribution of the Pr0.67Ca0.33Fe x Mn1−x O3 is much more uniform than that of gadolinium. Because of these results, nanopowders of Pr0.67Ca0.33Fe x Mn1−x O3 have some potential applications for magnetic refrigerants in an extended high-temperature range. Moreover, it can be used as a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas.

First-Principles Investigation of Half-metallic Ferromagnetism in V-doped BeS, BeSe, and BeTe

Abstract: We have investigated the electronic structure and half-metallic ferromagnetism in zinc blende phase of Be1−x V x M (M=S, Se, Te) at concentration x=0.125 by employing a first-principles calculations within the framework of density functional theory (DFT) based on the linearized augmented plane wave method (FP-LAPW), as implanted in the WIEN2k code with generalized gradient approximation functional proposed by Wu and Cohen (WC-GGA). The electronic properties exhibit half-metallic behavior. So the density of states shows the hybridization between the p (S, Se, Te) and 3d (V) states that creates the antibonding states in the gap, which stabilizes the ferromagnetic ground state associated with the double-exchange mechanism, whereas the spin polarized band structures depict half-metallic gap that increases from Be0.875V0.125S to Be0.875V0.125Se to Be0.875V0.125Te. These compounds are robust half-metallic ferromagnets with spin polarization of 100 % and predicted to be potential candidates for spin injection applications in spintronic devices. Therefore, our predictions require an experimental confirmation in the future.

Magnetic Properties of Zinc Ferrite Nanoparticles Synthesized by Coprecipitation Method

Abstract: The influence of the precipitant and ferric concentration on the magnetic properties of coprecipitated zinc ferrite nanoparticles has been investigated. The nanoparticles were characterized using X-ray diffraction, scanning and transmission electron microscope, and vibrating sample magnetometer techniques. The results showed that the single-phase zinc ferrite with partially inverse spinel structures can be formed at high concentrations. The inversion coefficient calculated by the Rietveld method decreases with increasing of the concentrations, may be due to the crystal growth. The magnetic measurements exhibited that the coprecipitated zinc ferrite nanoparticles were superparamagnet and magnetization decreases with increasing of the concentrations through decreasing of inversion coefficient.

Magnetocaloric Effect of Perovskite Eu0.5Sr0.5CoO3

Abstract: Magnetocaloric properties of the Eu0.5Sr0.5CoO3 system near a phase transition from a ferromagnetic to a paramagnetic state are investigated. It is shown that the magnetic entropy change (ΔSM) peak spanning over a broad range of temperature leads to a remarkably wide working temperature region, yielding a significant performance in terms of refrigerant efficiency. Moreover, ΔSM distribution is very uniform, which is desirable for Ericsson-cycle magnetic refrigerator. Eu0.5Sr0.5CoO3 can be used as a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas.

Magnetocaloric Effect in (001)-Oriented MnAs Thin Film

Abstract: In this paper, a phenomenological model is used to calculate magnetocaloric properties of (001)-oriented MnAs films Based on this model, it has been predicted the values of the magnetocaloric properties from calculation of magnetization as a function of temperature under very low external magnetic field change of 003 T Moreover, a control of the change of a broad range of temperature could be achieved by tuning several substrate temperatures This advantage should result in more efficient and global cooling devices It is suggested that (001)-oriented MnAs films are suitable candidates as refrigerants near a room-temperature region with a large temperature span

Low Temperature and High Magnetic Field Dependence and Magnetic Properties of Nanocrystalline Cobalt Ferrite

Abstract: The DC magnetic hysteresis loop measurements were carried out for temperatures varying from 5 to 300 K over a field range of ±10 T on nanocrystalline (∼35 nm) cobalt ferrite samples (crystallized to $Fd\bar {3} m$ space group with cubic symmetry) to validate the law of approach at low temperature for the nanocrystalline cobalt ferrite. A magnetocrystalline anisotropy constant and saturation magnetization have been obtained by analyzing the magnetization curve in saturation using the “law of approach (LA) to saturation.” The magnetocrystalline anisotropy constant is found to be almost constant in the temperature range of 5 to 150 K due to the freezing of spin at low temperature. Also, spin freezing leads to a decrease of coercivity with the increase in the temperature.

Non-magnetic Impurity Induced Magnetism in Li-Doped SnO2 Nanoparticles

Abstract: We address the possibility of non-magnetic doping induced magnetism, in Li-doped SnO2 nanoparticles. The compounds have been prepared by solid-state route at equilibrium and were found to be crystallized in single rutile phase. The magnetization measurements have shown that Li doping induces magnetism in SnO2 for a particular range of Li concentration. However, for other Li concentrations, including pure SnO2, the samples exhibit diamagnetism. To investigate the possible origin of the induced magnetism, we have studied the variation of the magnetization as a function of the average nanoparticle radius. Possible scenarios for the appearance of magnetism in these compounds are discussed.

Superparamagnetic Behavior of Zinc-Substituted Nickel Ferrite Nanoparticles and its Effect on Mossbauer and Magnetic Parameters

Abstract: Zinc-substituted nickel ferrite (Ni 1−x Zn x Fe 2 O 4 with x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) nanoparticles were synthesized by solgel auto-combustion technique at low temperature and characterized by using X-ray diffraction, scanning electron microscopy, pulse field hysteresis loop technique, and Mossbauer spectroscopy. X-ray diffraction studies confirmed the formation of single-phase spinel structure of the prepared ferrite samples with average crystallite size of 30 nm, very close to that of the critical size for nanoparticles exhibiting superparamagnetism. Scanning electron micrographs of the ferrite samples showed uniform spherical morphology of nanograins with homogenous microstructure. Further investigations on magnetic properties by pulse field hysteresis loop technique and Mossbauer spectroscopy indicated the presence of superparamagnetic phases in the ferrite samples attributed to occupation of octahedral [B] sites by zinc ions in these Ni–Zn samples and also to the nanometer sizes of the ferrite particles. Magnetic behavior of the Ni–Zn ferrite system is in agreement, initially, with Neel’s two-sublattice collinear model and then with the Yafet–Kittel model for samples with higher zinc content (x ≥ 0.4). Value of hyperfine splitting is found to decrease with increase in zinc content and is attributed to the reduction in particle size giving rise to superparamagnetism. Other Mossbauer parameters like quadrupole splitting and the isomer shift are within the reported range for those of ferrites with spinel structure.

Simulation of Magnetocaloric Properties of Antiperovskite Structural Ga1−XAlXCMn3

Abstract: In this work, low-enhanced magnetic field magnetocaloric effect for Ga1−X Al X CMn3 (X = 0, 005, 007, 012, and 015) system near a second-order phase transition from a ferromagnetic to a paramagnetic state is investigated theoretically It is found that magnetic entropy change distribution is much more uniform than that of gadolinium, which is desirable for an Ericson-cycle magnetic refrigerator Moreover, the results show that the magnetocaloric effect in this system is tunable by Al doping, which is beneficial for manipulating magnetocaloric refrigeration that occurs in various temperature ranges including room temperature This makes the Ga1−X Al X CMn3 samples potential candidates for practical applications

Theoretical Investigation of Structural, Electronic, and Magnetic Properties of V-Doped MgSe and MgTe Semiconductors

Abstract: In this study, we have explored the structural, electronic, and magnetic properties of V-doped zincblende MgSe and MgTe compounds using density functional calculations. The Wu-Cohen generalized gradient approximation is used for optimizing the structural properties, while the modified Becke and Johnson local (spin) density approximation functional has been employed to compute the electronic and magnetic properties. The spin dependent band structures, electronic density of state, and magnetic moments calculated for V-doped MgSe and MgTe semiconductors exhibit occurrence of 100 % spin polarization at the Fermi level which confirms stable half-metallic ferromagnetism in these materials. The spin-down gaps and the half-metallic gaps are analyzed in terms of V-3d and Se-4p (Te-5 p) hybridization, where it is observed that the V-3dstates play a key role in generating spin polarization and the magnetic moment in these compounds. The exchange constants N 0 αand N 0 β have been calculated to demonstrate the effects resulting from exchange splitting process. Furthermore, spin-polarized charge density calculation is presented for elucidating the bonding nature, while pressure dependence of total magnetic moment for three concentrations of V-doped MgSe and MgTe are also discussed.

Improvement of Superconducting Parameters of Bi1.8Pb0.4Sr2Ca2Cu3O10+δ Added with Nano-Ag

Abstract: Bi1.8Pb0.4Sr2Ca2Cu3O10+δ superconductor samples were synthesized by the conventional solid-state reaction technique. Nano-Ag was introduced by small weight percentages (0.2, 0.4, 0.6, 1, and 1.5 weight %) in the final step of the synthesis process. Phase formation and microstructure were investigated using x-ray powder diffraction, differential scanning calorimetry, and scanning electron microscopy. The real elemental-content and oxygen-content were examined using particle induced X-ray emission (PIXE) and Rutherford backscattering (RBS) techniques, respectively. Electrical resistivity as function of the temperature was carried to evaluate the relative performance of samples. Moreover, Electric field-Current density (E–J) characteristic curves were measured at 77 K. The electrical and granular properties were greatly enhanced, indicating more efficient pinning mechanisms. An improvement of the critical current density of 229 % was obtained with x=0.6 wt.%, while the superconducting transition temperature is improved by 2.5 %.

Magnetocaloric Effect in Sr 0.4 Ba 1.6−x La x FeMoO 6

Abstract: Magnetocaloric effect in half-metallic double perovskite Sr0.4Ba1.6−x La x FeMoO6 (x = 0, 0.1, 0.2, 0.3) were investigated. It is shown that Sr0.4Ba1.6−x La x FeMoO6 exhibits the largest magnetic entropy change (ΔS M ) of 0.086 J/kg K upon 0.2 T magnetic field variation. Furthermore, ΔS M distribution of the Sr0.4Ba1.6−x La x FeMoO6 is much more uniform than that of gadolinium. Through these results, polycrystalline samples of Sr0.4Ba1.6−x La x FeMoO6 have some potential applications for magnetic refrigerants in a wide temperature range, including room temperature.

Study of Structural and Magnetic Properties of Superparamagnetic Fe 3 O 4 –ZnO Core–Shell Nanoparticles

Abstract: In this work, Fe3O4–ZnO core–shell nanoparticles have been successfully synthesized using a simple two-step co-precipitation method. In this regard, Fe3O4 (magnetite) and ZnO (zincite) nanoparticles (NPs) were synthesized separately. Then, the surface of the Fe3O4 NPs was modified with trisodium citrate in order to improve the attachment of ZnO NPs to the surface of Fe3O4 NPs. Afterwards, the modified magnetite NPs were coated with ZnO NPs. Moreover, the influence of the core to shell molar ratio on the structural and magnetic properties of the core–shell NPs has been investigated. The prepared nanoparticles have been characterized utilizing transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and vibrating sample magnetometer (VSM). The results of XRD indicate that Fe3O4 NPs with inverse spinel phase were formed. The results of VSM imply that the Fe3O4–ZnO core–shell NPs are superparamagnetic. The saturation magnetization of prepared Fe3O4 NPs is 54.24 emu/g and it decreases intensively down to 29.88, 10.51 and 5.75 emu/g, after ZnO coating with various ratios of core to shell as 1:1, 1:10 and 1:20, respectively. This reduction is attributed to core–shell interface effects and shielding. TEM images and XRD results imply that ZnO-coated magnetite NPs are formed. According to the TEM images, the estimated average size for most of core–shell NPs is about 12 nm.

The Unique Properties of Superconductivity in Cuprates

Abstract: Copper oxides are the only materials that have transition temperatures, T c, well above the boiling point of liquid nitrogen, with a maximum $T_{\mathrm {c}}^{\mathrm {m}}$ of 162 K under pressure. Their structure is layered, with one to several CuO2 planes, and upon hole doping, their transition temperature follows a dome-shaped curve with a maximum of $T_{\mathrm {c}}^{\mathrm {m}}$ . In the underdoped regime, i.e., below $T_{\mathrm {c}}^{\mathrm {m}}$ , a pseudogap Δ* ∝ T* is found, with T* always being larger than T c, a property unique to the copper oxides. In the superconducting state, Cooper pairs (two holes with antiparallel spins) are formed that exhibit coherence lengths on the order of a lattice distance in the CuO2 plane and one order of magnitude less perpendicular to it. Their macroscopic wave function is parallel to the CuO2 plane near 100 % d at their surface, but only 75 % d and 25 % s in the bulk, and near 100 % s perpendicular to the plane in yttrium barium copper oxide (YBCO) [1]. There are two gaps with the same T c [2]. As function of doping, the oxygen isotope effect is novel and can be quantitatively accounted for by a vibronic theory or by the presence of bipolarons [2, 3]. These cuprates are intrinsically heterogeneous in a dynamic way. In terms of quasiparticles, bipolarons are present at low doping and aggregate upon cooling [2] so that probably ramified clusters and/or stripes are formed, leading over to a more Fermi liquid-type behavior at large carrier concentrations.

Field Dependence of Magnetocaloric Properties in La 0.6 Pr 0.4 Fe 10.7 Co 0.8 Si 1.5

Abstract: In this paper, the field dependence of magnetocaloric properties of La 0.6Pr 0.4Fe 10.7Co 0.8Si 1.5 with second-order phase transition material is studied using a phenomenological model. The model parameters were determined from the magnetization data adjustment and were used to give better fits to magnetic transition and to calculate the magnetocaloric thermodynamic quantities. The entropy curves have been observed to behave as an asymmetrical broadening of ΔS M peak with increasing magnetic field. For larger fields, the peak shifts to higher temperatures, while the overall shape of the curve broadens over a wide temperature range. The values of maximum magnetic entropy change, full width at half maximum, and relative cooling power, at several magnetic field variations, were calculated. The maximum magnetic entropy changes of 3.957(5) and 14.197(4) J kg −1 K −1 and the relative cooling power (RCP) values of 95.420(3) and 392.729(2) J kg −1 are obtained for 1 and 5 T, respectively. The theoretical calculations are compared with the available experimental data. The critical exponents associated with ferromagnetic transition have been determined from magnetocaloric effect (MCE) methods. By using the field dependence of ΔS M max≈a(μ 0 H) n and the distance (T peak−T c)≈b(μ 0 H)1/Δ, we have investigated the critical behavior of La 0.6Pr 0.4Fe 10.7Co 0.8Si 1.5. From the analysis of the relationship between the local exponent n and the gap exponent Δ, we have calculated other exponents: β, γ, and δ. The large MCE, relatively high RCP, high magnetization, and low cost jointly make the present compound a promising candidate for magnetic refrigerant near room temperature.

Dilution Effect on Nanographene Magnetic Properties

Abstract: The magnetic properties of a diluted ferromagnetic–antiferromagnetic Ising model on a nanographene lattice, constituted of pure ferromagnetic layer with spin S=5/2 and diluted antiferromagnetic layer with a spin σ=3/2, have been studied using Monte Carlo simulations. The considered Hamiltonian takes into account nearest-neighbor interactions, crystal field, and external magnetic field. The calculated magnetizations versus the dilution of the bonds in block A depend on the temperature, crystal field Δ, and external magnetic field h. The saturation magnetization and the magnetic remanent are also obtained for the two blocks.

Solvothermal Synthesis of Pure SrFe12O19 Hexaferrite Nanoplatelets

Abstract: Strontium hexagonal ferrite, SrFe12O19, with platelet-like structure was synthesized directly by hydrothermal method without any calcination process. The product was characterized by Scanning electron (SEM), X-ray powder diffraction (XRD), Fouier transform infrared spectroscopy (FT-IR), and vibrating sample magnetometry (VSM) techniques. The optimum Fe/Sr moler atio is identified as 8:1, and the optimum weight of NaOH was 4.0 g for the synthesis of the Flake-like SrFe12O19 nanoparticles. The average crystallite size of the product was calculated as 28±5 nm. The low coercivities of the synthesized platelet crystals indicate soft magnetic behavior, which is a consequence of the large shape anisotropy (the platelet crystals are highly anisotropic). The reduced Ms can be explained by increasing surface area of the platelets.

Magnetocaloric Effect in LaFe 10.7 Co 0.8 Si 1.5 Compound Near Room Temperature

Abstract: Magnetocaloric properties of LaFe10.7Co0.8Si1.5 with the cubic NaZn13-type structure were investigated around their Curie temperature T C . By the help of the phenomenological model, the magnetization curves for LaFe10.7Co0.8Si1.5 at several magnetic fields were simulated. The magnetic entropy change and specific heat are obtained. The values of maximum magnetic entropy change, full-width at half-maximum, and relative cooling power, at several magnetic field variation, were calculated. It is shown that for LaFe10.7Co0.8Si1.5 the magnetic entropy change follows an asymmetrical broadening of ΔS M peak with increasing field. The maximum magnetic entropy change of this compound is 7.10 JK−1 kg−1 and relative cooling power is 201.37 J/K under a magnetic field of 2 T. The magnetocaloric effect of this material is large and tunable, suggesting a possible technical application of the material at moderate magnetic fields near room temperature.

Effect of Fe2O3 Nano-Oxide Addition on the Superconducting Properties of the (Bi,Pb)-2223 Phase

Abstract: The effect of different weight percentages of nano-Fe2O3 on the properties of Bi1.8Pb0⋅4Sr2Ca2Cu3O10+δ superconducting phase was studied. Phase formation, elemental real contents and granular microstructure of the investigated samples were carried out using X-Ray powder Diffraction (XRD), Energy Dispersive X-ray emission (EDX), Proton Induced X-ray Emission (PIXE), Rutherford Backscattering Spectrometry (RBS) and Scanning Electron Microscopy (SEM). XRD data indicated that the volume fraction of (Bi,Pb)-2223 decreased as nano-Fe2O3 wt.% increased. PIXE and EDX analyses showed that the PIXE technique is more accurate for detecting the nano-Fe2O3 than the EDX technique. The oxygen content, determined from RBS, decreased as nano-Fe2O3 wt.% increased. The superconducting properties of the prepared samples were investigated using electrical resistivity and I–V curves, at 77 K, measurements. It was found that the granular structure and the critical current density were improved up to 0.2 wt.% of nano-Fe2O3 addition. The superconducting transition temperature T c decreased as nano-Fe2O3 wt.% increased, attributing this to the decrease of the volume fraction or trapping of mobile holes. The enhancement rate of J c for (Bi,Pb)-2223 phase added with nano-Fe2O3 is 9 %, which is lower than that of (Bi,Pb)-2223 phase added with SnO2, Ag2O, Al2O3 and MgO. This means that the nano-magnetic addition has the lowest enhancement rate in both J c and T c.