Showing papers in "Journal of Superconductivity and Novel Magnetism in 2011"

Improving the Physical Properties of (Bi, Pb)-2223 Phase by SnO2 Nano-particles Addition

Abstract: In this work, the effect of SnO2 nano-particles (40 nm) addition to the physical properties of Bi1.6Pb0.4Sr2Ca2Cu3O10+δ superconducting phase was studied. (Bi, Pb)-2223 superconductor phase added by SnO2 nano-particles was prepared by a conventional solid-state reaction technique. The SnO2 nano-particles concentrations x varied from 0.0 to 2.0 wt% of the sample’s total mass. The prepared samples were investigated by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and electron dispersive spectroscopy (EDS) for analyzing phase formation and microstructure. Also, the electrical resistivity and transport critical current density, for investigated samples, were measured by standard dc four-probe method. Phase examination by XRD indicated that SnO2 nano-particles enhanced the (Bi, Pb)-2223 phase formation up to x=0.4 wt%. On the other hand, the high concentrations of SnO2 nano-particles retarded the phase formation. Granular investigation, from scanning electron microscope, showed that both number and size of voids decreased as x increased from 0.0 to 0.4 wt%. The superconducting transition temperature and transport critical current density were found to have optimal values at x=0.4 wt%. The enhancement rates in Tc and Jc were 12 and 58%, respectively, which had a maximum enhancement in both Jc and Tc for all investigated nano-particles.

Mechanical Properties of (Cu0.5Tl0.5)-1223 Substituted by Pr

Abstract: Cu0.5Tl0.5Ba2Ca2−xPrxCu3O10−δ superconducting samples, with 0≤x≤0.15, were prepared by a single-step solid state reaction on a form of rectangular bar. The prepared samples were characterized using X-ray powder diffraction (XRD) and scanning electron microscope (SEM). The room temperature Vickers microhardness was measured at different loads (0.25–3 N). The experimental results were analyzed using Meyer’s law, Hays–Kendall approach, elastic/plastic deformation model, proportional specimen resistance model, and the indentation-induced cracking (IIC) model. Surprising results were obtained and showed that all samples in the form of rectangular bars exhibited reverse indentation size effect in contrary with those in the form of discs. Vickers microhardness values were decreased as Pr-content increased that consisting with the porosity results. Furthermore, the Young’s modulus was determined using the dynamic resonance technique. A relation between Young’s modulus (E) and Vickers microhardness (HV) was obtained.

Attractive vortex interaction and the intermediate-mixed state of superconductors

Abstract: The magnetic vortices in superconductors usually repel each other. Several cases are discussed when the vortex interaction has an attractive tail, and thus a minimum, leading to vortex clusters and chains. Decoration pictures then typically look like in the intermediate state of type-I superconductors, showing lamellae or islands of Meissner state or surrounded by Meissner state, but with the normal regions filled with Abrikosov vortices that are typical for type-II superconductors in the mixed state. Such intermediate-mixed state was observed and investigated in detail in pure Nb, TaN and other materials 40 years ago; last year it was possibly also observed in MgB2, where it was called “a totally new state” and ascribed to the existence of two superconducting electron bands, one of type-I and one of type-II. The complicated electronic structure of MgB2 and its consequences for superconductivity and vortices are discussed. It is shown that for the real superconductor MgB2 which possesses a single transition temperature, the assumption of two independent order parameters with separate penetration depths and separate coherence lengths is unphysical.

A Theory for the High-Tc Cuprates: Anomalous Normal-State and Spectroscopic Properties, Phase Diagram, and Pairing

Abstract: A theory of highly correlated layered superconducting materials is applied for the cuprates. Differently from an independent-electron approximation, their low-energy excitations are approached in terms of auxiliary particles representing combinations of atomic-like electron configurations, where the introduction of a Lagrange Bose field enables treating them as bosons or fermions. The energy spectrum of this field accounts for the tendency of hole-doped cuprates to form stripe-like inhomogeneities. Consequently, it induces a different analytical behavior for auxiliary particles corresponding to “antinodal” and “nodal” electrons, enabling the existence of different pairing temperatures at T ∗ and T c . This theory correctly describes the observed phase diagram of the cuprates, including the non-Fermi-liquid to FL crossover in the normal state, the existence of Fermi arcs below T ∗ and of a “marginal-FL” critical behavior above it. The qualitative anomalous behavior of numerous physical quantities is accounted for, including kink- and waterfall-like spectral features, the drop in the scattering rates below T ∗ and more radically below T c , and an effective increase in the density of carriers with T and ω, reflected in transport, optical and other properties. Also is explained the correspondence between T c , the resonance-mode energy, and the “nodal gap”.

Effect of Nano-Oxides Addition on the Mechanical Properties of (Cu0.5Tl0.5)-1223 Phase

Abstract: In this study, superconducting samples of type Cu0.5Tl0.5Ba2Ca2Cu3O10−δ,(Cu0.5Tl0.5)-1223, added by SnO2 and In2O3 in nano-scale were prepared by a solid-state reaction technique. The concentrations of both SnO2 and In2O3 were varied from 0.0 to 1.0 wt.% of the total sample’s mass. The prepared samples were characterized using X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) for phase analysis and microstructure examination, respectively. The electrical resistivity of the prepared samples was measured by the conventional four-probe technique from room temperature down to the zero superconducting transition temperature (Tc0). An increase in Tc is observed up to x=0.6 wt.% for (SnO2)x Cu0.5Tl0.5Ba2Ca2Cu3O10−δ, followed by a systematic decrease with increasing nano-SnO2 addition for x>0.6 wt.%. While, for (In2O3)x Cu0.5Tl0.5Ba2Ca2Cu3O10−δ the Tc is slightly changed with x. Room temperature Vickers microhardness measurements were carried out at different applied loads (0.49–2.94 N) for the study of the mechanical performance of the prepared samples and examination of the effect of different nano-oxides addition on the microhardness of (Cu0.5Tl0.5)-1223 phase. Furthermore, the true microhardness values (Ho), for both additions, were evaluated through different models and their results were compared with those estimated from the experimental results in the plateau region. Also, some important mechanical parameters, such as Young’s modulus (E), yield strength (Y), fracture toughness (K) and brittleness index (B), were calculated for both additions. The results clarified that these parameters are strongly dependent on both the applied loads and the nano-oxides addition.

Magnetic proximity effects in V/Fe superconductor/ferromagnet single bilayer revealed by waveguide-enhanced polarized neutron reflectometry

Abstract: Polarized neutron reflectometry is used to study the magnetic proximity effect in a superconductor/ferromagnet (SC/FM) system of composition Cu(32 nm)/ V(40 nm)/Fe(1 nm)/MgO. In contrast to previous studies, here a single SC/FM bilayer, is studied and multilayer artefacts are excluded. The necessary signal enhancement is achieved by waveguide resonance, i.e., preparing the V(40 nm)/Fe(1 nm) SC/FM bilayer sandwiched by the highly reflective MgO substrate and Cu top layer, respectively. A new magnetic state of the system was observed at temperatures below 0.7T C manifested in a systematic change in the height and width of the waveguide resonance peak. Upon increasing the temperature from 0.7T C to T C, a gradual decay of this state is observed, accompanied by a 5% growth of the diffuse scattering. This behavior can be explained in a natural way by the polarization of the superconducting electrons upon the SC transition, i.e., an appearance of additional induced magnetization within the SC, due to the proximity of the FM layer.

Spin-vortex Superconductivity

Abstract: We present a theory of superconductivity based on the theoretical prediction that a macroscopic persistent current is generated by spin-vortices. It explains the origin of the phase variable θ that is canonical conjugate to the superfluid density as a Berry phase arising from the spin-vortex formation. This superconductivity does not require Cooper-pairs as charge carriers, thus, is not directly related to the standard theory based on the BCS one; however, it exhibits the flux quantization in the unit Φ0=hc/2|e|, where h is Planck’s constant, c the speed of light, and e the electron charge; and the AC Josephson frequency, fJ=2|e|V/h, where V is the voltage of the battery connected to the superconductor–insulator–superconductor junction. In due course, it is found that the standard derivation of the AC Josephson frequency misses a term arising from the flow of particles through the leads connected to the junction. If this contribution is included, the observed fJ indicates that the phase θ is a variable conjugate to the number density of charge e carriers instead of the currently accepted charge 2e carriers. We propose an experiment that discriminates whether it is e or 2e. If the above claim is verified, it means that the BCS theory cannot predict whether a particular compound is a superconductor or not since it does not explain the origin of θ. A connection between the present mechanism and the BCS mechanism is discussed; the fact that the BCS theory gives an excellent estimate of Tc is attributed to the fact that it predicts the temperature at which spin-vortices become long-lived due to the energy gap formation; since the stabilization by the electron-pair formation is compatible with the present mechanism, asymmetries observed in the even and odd number of electron systems are preserved. The most notable difference is that the persistent current generation is formulated in a strictly particle-number-conserving manner. Thus, it does not violate the superselection rule for the total charge.

Effect of Nano-Sized ZnO on the Physical Properties of (Cu0.5Tl0.25Pb0.25)Ba2Ca2Cu3O10−δ

Abstract: High-temperature superconductor phase of (Cu0.5Tl0.25Pb0.25)-1223 was synthesized by solid-state reaction technique and characterized using X-ray powder diffraction (XRD). XRD analysis revealed that the prepared sample was nearly monophase and exhibited tetragonal structure with space group P4/mmm. Nano-zinc-oxide, prepared by Co-precipitation method, was added to the sample. ZnO-concentrations y varied from 0.0 to 2.0 wt.% of the sample’s mass. The prepared samples were investigated through XRD, scanning electron microscope (SEM), energy dispersive X-ray (EDX), particle size analyzer (PSA), differential scanning calorimeter (DSC), electrical resistivity and transport critical current density measurement. X-ray data analysis showed that the nano-Zn addition does not affect the tetragonal structure of (Cu0.5Tl0.25Pb0.25)-1223 phase, whereas the lattice parameters showed an insignificant variation. The results of the superconducting transition temperature, the transport critical current density and melting point of the prepared samples were found to depend on nano-ZnO concentrations.

Precursor influence on the electrical properties of textured Bi-2212 superconductors

Abstract: Several synthetic methods, solid-state, sol-gel and polymer solution methods, have been used to prepare prereacted precursors, as well as a vitreous material obtained by melt quenching. The influence of the starting powder characteristics on the phase formation, microstructure, T c and J c of Bi-2212 textured rods prepared by directional crystallization from the melt has been analyzed. In all the cases, high transport critical current values (higher than 3000 A/cm2 at 77 K) have been obtained, independently of the precursor type. Samples obtained by the polymer route show improved T c values, associated to a lower oxygen content.

Mutual Inductance and Force Calculations Between Coaxial Bitter Coils and Superconducting Coils with Rectangular Cross Section

Abstract: Mutual inductance and force calculations between coaxial Bitter coils and superconducting coils with rectangular cross section in a hybrid magnet system using derived semi-analytical expressions based on two integrations were performed. The mutual inductance and force calculations are based on the assumption of the uniform current density distribution in superconducting coils. The current density distribution of a Bitter coil in radial direction, however, is inversely proportional to the radius of the Bitter coil. The influence of the current density redistribution caused by a cooling hole and an inhomogeneous temperature distribution of Bitter coil of a water-cooled magnet was not considered. The obtained expressions can be implemented by Simpson’s integration with FORTRAN programming. We confirm the validity of mutual inductance calculation by comparing it with a filament method, and give the accuracy of two methods. The mutual inductance values computed by two methods are in excellent agreement. The derived semi-analytical expressions of mutual inductance allow a low computational time compared with filament method to a specific accuracy. The force is derived by multiplying the currents of the two coils by their mutual inductance gradient.

A Study of the Magnetic Properties of Bi1.64−xPb0.36CdxSr2Ca2Cu3Oy Superconductor

Abstract: The flux pinning energy and magnetic properties of Bi1.64−x Pb0.36Cd x Sr2Ca2Cu3O y (BPCSCCO) with x=0.0, 0.02, 0.04 and 0.06 were studied. A series of Bi-2223 superconductor samples with a nominal composition of BPCSCCO was synthesized and the effect of Cd substitution for Bi was investigated. As a result, Cd addition has been found to improve the superconducting properties of the Bi-Pb-Sr-Ca-Cu-O system. The effects of the annealing time and the amount of Cd doping on the structure, AC magnetic susceptibility, ρ–T curves and flux pinning energy were investigated. Also, for all samples the relation between the current and voltage in the mixed state was found to follow the model relationship V=α I β . The maximum value of β is 22.30, which is obtained for the sample with an annealing time of 270 h and a Cd content of 0.04.

Fractal Structure Favoring Superconductivity at High Temperatures in a Stack of Membranes Near a Strain Quantum Critical Point

Abstract: The statistical physics of the 3D ordered oxygen interstitials has been measured in La2CuO4+y using an advanced tool, scanning x-ray diffraction with focused synchrotron radiation. The observed fractal scale invariant distribution is found in a cuprate in the proximity to a stripes critical point in the 3D Aeppli-Bianconi phase diagram of cuprates, where T c is function of both hole doping and superlattice misfit strain. Therefore high-temperature superconductivity is favored by complex fractal systems while on the contrary standard low temperature superconductivity is favored in simple periodic crystals. This work shows that the fractal structural distribution in a stack of membranes favors the macroscopic quantum coherent condensate at high temperature. This result opens new perspectives for the understanding the relationship between emergent scale-free distribution in living matter and possible quantum coherent phenomena able to resist to the attacks of temperature decoherence effects.

An Introduction to Numerical Methods in Superconductors

Abstract: A summary is given of some of the various numerical methods which have been used to solve the field equations in superconductors. Some comparisons are made between the commonest methods and constitutive equations used. The difficulties in extending methods to three-dimensional situations are discussed. These include not only the numerical problems but also the lack of suitable parameters for describing flux cutting. Other problems which need to be addressed are losses in complete solenoids of pancake coils and the results of many hysteretic cycles.

Spin-vortices and Spin-vortex-induced Loop Currents in the Pseudogap Phase of Cuprates

Abstract: We explain several anomalous phenomena observed in the pseudogap phase of hole-doped cuprates based on the recently proposed spin-vortex superconductivity theory. In this theory, doped-holes become almost immobile small polarons, and spin-vortices are formed with those small polarons as their centers. A Hartree–Fock field for conduction electrons that is optimized for the interaction energy of local moments is derived; it contains a fictitious magnetic field arising from spin-vortices, and yields current carrying states. The obtained currents are loop currents around spin-vortices, i.e., the spin-vortex-induced loop currents (SVILCs), and a collection of them produces a macroscopic current. The SVILC explains (1) nonzero Kerr rotation in zero-magnetic field after exposed in a strong magnetic field; (2) the change of the sign of the Hall coefficient with temperature change; (3) the suppression of superconductivity in the x=1/8 static-stripe ordered sample; and (4) a large anomalous Nernst signal, including its sign-change with temperature change. We show that the hourglass-shaped magnetic excitation spectrum is the evidence for the existence of spin-vortices. We further argue that the “Fermi-arc” in the ARPES is a support for the presence of localized moments in the bulk; a disconnected arc-shaped Fermi surface is obtained by assuming an antiferromagnetic interaction between the localized moments in the bulk and itinerant electrons in the surface region.

The Structural, Superconducting and Transport Properties of the Compounds Y 3 Ba 5 Cu 8 O 18 and Y 3 Ba 5 Ca 2 Cu 8 O 18

Abstract: This work is related to the structural, superconducting, and transport properties of the compounds Y3Ba5Cu8O18 and Y3Ba5Ca2Cu8O18 prepared by the sol-gel method. The influence of the doping of Ca atoms into the compound Y3Ba5Cu8O18 was studied by employing XRD, SEM, AFM, EDX, DTA, TGA, and the electrical resistivity (ρ), Hall coefficient (R H), Hall mobility (μ H), and magnetoresistance measurements. The XRD spectra showed that Y3Ba5Ca2Cu8O18 almost has the same crystal structure as that of Y3Ba5Cu8O18, except with some impurity peaks. The resistivity measurements have pointed out that the compounds Y3Ba5Cu8O18 and Y3Ba5Ca2Cu8O18 have their T c-onset temperatures at approximately 92.7 and 86.6 K, respectively. The Hall coefficients R H and Hall mobilities μ H have been measured at the 10–300 K temperature interval in a magnetic field of 0.55 T. The signs of R H and μ H are found to be positive for both samples, which indicate that the conduction is p-type in our samples. As expected, the magnetoresistance results clearly demonstrate a considerable decrease of the offset temperatures with increasing magnetic field.

Magnetic Phase Transitions in Cobalt Chromite Nanoparticles

Abstract: Cobalt chromite (CoCr2O4), an insulating normal spinel compound, is a potential multiferroic material. We report that pure, nanosize CoCr2O4 particles are synthesized through a conventional coprecipiation technique by controlling the pH of the precipitation. Both single-crystal and polycrystalline samples develop long-range ferrimagnetic order below the Curie temperature, T c (97 K), and a sharp phase transition at T s∼31 K, attributed to the onset of long-range spiral magnetic order. However, we observed a transition from paramagnetic to superparamagnetic phase at T c. Further lowering the temperature below T c (97 K), the superparamagnetic phase transforms to ferrimagnetic phase at blocking temperature, T b, which is found to be between 50 and 60 K. This intermediate superparamagnetic phase in between paramagnetic and long-range ferrimagnetic phases is attributed to a nanosize effect.

Cation Distribution in Zn Doped Cobalt Nanoferrites Determined by X-ray Absorption Spectroscopy

Abstract: Information on local crystal and electronic structure with elemental specificity is of paramount importance to understand many scientific problems. X-ray Absorption Spectroscopy (XAS) is particularly suited for this. Spinel structured ferrites exhibit a range of electrical and magnetic properties that make them particularly appealing for many technological applications such as permanent magnets, microwave absorbers, catalysts, and chemical sensors. Since the peculiar properties of ferrites are strictly related to the distribution of cations between octahedral and tetrahedral sites in the spinel structure, the control of cation distribution provides a means to tailor their properties. An EXAFS study of CoxZn1−xFe2O4 nanoparticles is presented here. Using this technique, the information about the site distribution for Fe and Co/Zn is determined. The information obtained on the cation distribution is important to understand the microstructure of spinel ferrites which is useful to study their effects on structural, electrical, and magnetic properties.

The Effect of Silver Substitution in Bi1.7Pb0.3Sr2Ca2−xAgxCu3Oy

Abstract: The effect of Ag substitution on the properties of high-temperature superconductor Bi1.7Pb0.3Sr2Ca2−x Ag x Cu3O y system have been investigated. The electrical and structural properties of the samples, prepared by the conventional solid-state reaction method, have been characterized by X-ray diffraction (XRD), electrical resistance and scanning electron microscopy (SEM) studies. XRD analysis reveals a multiphase structure of the samples, whereas SEM micrographs indicate some morphological changes induced by silver addition. It was found that an increase of the amount of Ag2O addition leads to an enhancement of the critical temperature and the percentage of Bi-2223 phase in the phase mixture.

The Fabrication of THz Emitting Mesas by Reactive Ion-Beam Etching of Superconducting Bi2212 with Multilayer Masks

Abstract: Generation of powerful THz radiation from intrinsic Josephson Junctions (IJJs) of Bi2Sr2CaCu2O8+δ (Bi2212) may require mesas with large lateral dimensions However, there are difficulties in fabrication of perfect rectangular mesas The lateral angles of mesas should be close to 90 degrees to obtain IJJs with same planar dimensions for synchronization of IJJs We patterned Ta/photoresist and photoresist/Ta/photoresist masks on Bi2212 and used selective ion etching to overcome the thick photoresist layer shading on the lateral dimension of mesa during the ion-beam etching The reactive ion-beam etchings have been done with ion beams of Ar, N2 and O2, and we have obtained mesas about 1 μm with lateral angle of approximately 50 to 75°, which is better than the mesas fabricated with photoresist mask

Magnetoelectric Ordering of BiFeO3 from the Perspective of Crystal Chemistry

Abstract: In this paper we examine the role of crystal chemistry factors in creating conditions for formation of magnetoelectric ordering in BiFeO3. It is generally accepted that the main reason of the ferroelectric distortion in BiFeO3 is concerned with a stereochemical activity of the Bi lone pair. However, the lone pair is stereochemically active in the paraelectric orthorhombic s-phase as well. We demonstrate that a crucial role in emerging of phase transitions of the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order types belongs to the change of the degree of the lone pair stereochemical activity—its consecutive increase with the temperature decrease. Using the structural data, we calculated the sign and strength of magnetic couplings in BiFeO3 in the range from 945 °C down to 25 °C and found the couplings, which undergo the antiferromagnetic → ferromagnetic transition with the temperature decrease and give rise to the antiferromagnetic ordering and its delay in regard to temperature, as compared to the ferroelectric ordering. We discuss the reasons of emerging of the spatially modulated spin structure and its suppression by doping with La3+.

Investigation of Gd Addition Added on Magnetic and Structural Properties of Bi1.8Pb0.35Sr1.9Ca2.1Cu3GdxOy Superconductors by ac Susceptibility

Abstract: This study reports the effect of Gd addition on magnetic and structural properties of Bi1.8Pb0.35Sr1.9Ca2.1Cu3GdxOy superconductor with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5 by means of ac susceptibility measurements at various ac fields (ranging from 270 to 1352 A/m) and scanning electron microscopy (SEM) images. Critical onset (\(T_{\mathrm{c}}^{\mathrm{on}})\) and loss peak temperatures (Tp) were qualitatively estimated from the ac susceptibility curves. The peak temperature at zero ac-magnetic field (Tp0) and intergrain critical current densities (Jc) were theoretically calculated from the ac susceptibility plots via the critical state models. The results show that peak temperatures and critical current densities were found to decrease with increasing Gd addition. Moreover, using a self-field approximation together with Jc dependence on temperature, the characteristic length (Lc) associated with the pinning force is estimated to be approximately the same as the average grain size (Rg) of the pinning center because of the linear decrease in Jc with increasing temperature. Surface morphology and grain connectivity of the samples were also obtained to degrade with increase in the Gd addition from SEM investigations.

Electrical and Structural Properties of RE3Ba5Cu8O18 (RE=Y, Sm and Nd) Superconductors

Abstract: This paper presents the synthesis and characterization of the new superconducting compounds with nominal composition, RE3Ba5Cu8O18, RE=Y, Sm and Nd. The onset critical temperatures of the present samples are 97.5, 97 and 95 K for the RE=Y, Sm and Nd, respectively. The first two values are the highest for the RE–Ba–Cu–O superconductors recorded to date. The RE3Ba5Cu8O18 samples crystallize in the orthorhombic system, with increased unit cell volume as Sm and Nd replace the element Y.

Oxygen Isotope Effect in Cuprates Results from Polaron-induced Superconductivity

Abstract: The planar oxygen isotope effect coefficient measured as a function of hole doping in the Pr- and La-doped YBa2Cu3O7 (YBCO) and the Ni-doped La1.85Sr0.15CuO4 (LSCO) superconductors quantitatively and qualitatively follows the form originally proposed by Kresin and Wolf [Phys. Rev. B 49, 3652 (1994)], which was derived for polarons perpendicular to the superconducting planes. Interestingly, the inverse oxygen isotope effect coefficient at the pseudogap temperature also obeys the same formula. These findings allow the conclusion that the superconductivity in YBCO and LSCO results from polarons or rather bipolarons in the CuO2 plane. The original formula, proposed for the perpendicular direction only, is obviously more generally valid and accounts for the superconductivity in the CuO2 planes.

Tailoring of Magnetic Properties of Magnetostatically-Coupled Glass-Covered Magnetic Microwires

Abstract: We report on tailoring of magnetic properties of Fe- and Co-rich microwires through magnetostatic coupling among them. We studied hysteresis loops of the arrays containing different number of the Co67Fe3.9Ni1.5B11.5Si14.5M0.6 and Fe74B13Si11C2 amorphous microwires. Fe74B13Si11C2 microwires have rectangular hysteresis loop, while Co67Fe3.9Ni1.5B11.5Si14.5M0.6 with vanishing magnetostriction constant posses inclined hysteresis loop with low coercivity. The presence of neighboring microwire (Fe either Co-based) significantly modifies hysteresis loop of whole microwire array. In a microwire array containing Fe-based microwires, we observed splitting of the initially rectangular hysteresis loop with a number of Barkhausen jumps correlated with number of Fe-rich microwires. In Co–Co arrays, we observed a change of inclination of overall hysteresis loop, and consequently magnetic anisotropy field under influence of the additional of Co-based microwire. In the case of mixed arrays containing Fe and Co-rich microwires, we were able to obtain irregular hysteresis loops with unusual shape. In this case, considerable increase of harmonics has been observed. Magnetic field amplitude and frequency affect the behavior of all studied arrays. Increasing the amplitude the shape of hysteresis loop of microwire array containing Fe-based microwires transforms from multi-step to single above certain magnetic field amplitude. In the array with Co-based microwires, we observe a change of coercivity. Observed dependences have been attributed by us to the magnetostatic interaction between the microwires with different magnetic domain structure. Together with the conventional method, such as thermal treatment, designing of arrays containing different types of microwires can serve for tailoring of their magnetic properties.

A Comparative Study of Fe1+δTe1−xSex Single Crystals Grown by Bridgman and Self-flux Techniques

Abstract: Single crystals of Fe1+δ Te1−x Se x (0≤x≤0.5) were grown via both Bridgman and self-flux techniques. Large crystals of size ∅10×50 mm could be obtained with the Bridgman method. The excess of iron, δ≥0.07, at interstitial sites was observed to deteriorate the superconductivity of the samples. Study of semiconducting and Curie–Weiss-like behavior indicates that an appearance of a hump for Fe1+δ Te0.60Se0.40 (δ≤0.04) is more pronounced for the self-flux growth than for the Bridgman method. This was observed via measurement of the normal state of resistivity and magnetic susceptibility, which decrease with lower temperature. Furthermore, our results give evidence that the phase with x∼0.40 is readily formed in the self-flux method despite the use of various ratios of initial mixtures.

Excess Conductivity Studies in Zn0.95Mn0.05O and ZnO Added YBa2Cu3Oy Superconductors

Abstract: We report electrical conductivity fluctuation analyses on YBa2Cu3O y (denoted as YBCO) granular samples added with nanosize ZnMnO (ZnMnO for brevity) and ZnO (30 nm) particles. Nanoparticles are added to the precursor powders during the final sintering cycle of a two-step preparation process. Phase analysis by X-ray diffraction and granular structure examination by transmission electron microscopy (TEM) were carried out. When ZnMnO and ZnO are added to the YBCO, the orthorhombic structure is maintained. TEM and energy dispersive X-ray spectroscopy analysis show the presence of inhomogeneities embedded in the superconducting matrix. The temperature dependence of electrical resistivity in zero magnetic field has been measured on free, 1 wt.% ZnMnO and 1 wt.% ZnO added samples and the effect of microscopic inhomogeneities in the paraconductivity region has been reported. Data about the dimensionality of the thermodynamic fluctuation are obtained by analyzing the excess of conductivity Δσ as a function of the reduced temperature $\varepsilon =\ln(\frac{T}{T_{c}^{mf}}-1)$ on the basis of the Aslamazov–Larkin theory. In the mean-field region a crossover from 3D to 2D was observed for each sample. 1D behavior of fluctuation conductivity was found at high temperatures (above the 2D regime) for nanoparticle added samples. The zero-temperature coherence length, the effective layer thickness of the two-dimensional system, the wire cross-sectional area for one-dimensional system, critical magnetic fields and critical current density are estimated. Superconducting parameters are affected by the nanoparticle additions.

Effect of Barium on the Properties of Lead Hexaferrite

Abstract: An analysis of the properties of a family based on lead-barium substitution in the M-type hexaferrite is presented. The samples were prepared by the ceramic method according to the general formula Pb x Ba1−x Fe12O19. The barium content was varied with x=0.1, 0.3, 0.5, 0.7, and 0.9; no secondary phases were detected in any composition. Rietveld refinement analysis was done in order to determinate crystallographic parameters, content of phases and degree of substitution. The effects of the barium on the morphological and magnetic properties were studied. Iron Mossbauer spectroscopy was used for determining the hyperfine parameters of the iron nucleus and their environment; also, the cationic occupancy was evaluated and the results were checked with X-ray refinement results.

Inorganic Magnetite Precipitation at 25 °C: A Low-Cost Inorganic Coprecipitation Method

Abstract: An easy, low-cost coprecipitation method to inorganically produce magnetite nanoparticles from solutions, in free-drift experiments, under anoxic conditions, at 25 °C and 1 atm pressure is here presented. By using this method, pure magnetite is obtained as the final solid, which shows the typical magnetic properties and thermal stability behavior of magnetite produced by other methods. The size of the magnetite crystals produced by the present method varies from relatively big sizes (200–300 nm), to sizes within the single magnetic domain range, just depending on the incubation time. The solution from which magnetite precipitates may be representative of certain natural environments where bacteria that produce magnetite may live and, thus, our magnetite may be used as an inorganic reference to compare to biologically produced magnetites.

Characterization of Pb-doped Sr-Ferrites at Room Temperature

Abstract: We report the magnetic characteristics of Pb-doped Sr-ferrites at room temperature. The polycrystalline samples of the series \(\mathrm{Sr}_{0.5}\mathrm{Pb}_{0.5}^{2 +} \mathrm{Fe}_{12 - x}\mathrm{Pb}_{x}^{3 +} \mathrm{O}_{19}\) (x=0, 0.2, 0.4, 0.6, 0.8, 1.0) have been prepared by the standard ceramic technique with the aim to study the magnetic properties including coercivity, remanence, and energy at room temperature. The measurements show decreasing trends in coercivity and remanence from 4682 Oe to 1783 Oe and from 1833 G to 1511 G for the sample with x-content of 0.0 to 1.0, respectively. A minute addition of Pb affects the behavior of the materials by decreasing its energy product.

Magnetoelastic anisotropy induced effects on field and temperature dependent magnetization reversal of Ni nanowires and nanotubes

Abstract: Vertically aligned Ni nanowires and nanotubes have been electrodeposited in alumina templates at room temperature. The detailed study of angular dependent coercivity and squareness demonstrates that the magnetic easy axis of Ni nanowires is perpendicular to that of Ni nanotubes axis. The mechanisms of magnetization reversal in Ni nanowires and Ni nanotubes are found to occur through the nucleation mode with the propagation of transverse domain wall and curling mode, respectively. Field dependant magnetization results at different temperatures have depicted that the magnetocrystalline anisotropy might cause a crossover of easy axis at room temperature to that of low temperature in both Ni nanowires and nanotubes. Furthermore, the variation in temperature dependent coercivity illustrates that the magnetoelastic anisotropy induced by the alumina matrix plays a dominant role in the magnetization reversal of the nanowires and nanotubes at low temperature.