International Journal of Modern Physics B

We studied temperature and magnetic-field dependence of grain boundary pinning effect in MgB₂ films with columnar grains by measuring the angular dependence of the resistivity and the critical current density (J_c). MgB₂ films grown by using a hybrid physical chemical vapor deposition method under appropriate conditions exhibit a peculiar columnar growth with their columns oriented along the c axis. The pinning effect by grain boundaries was manifested in the comparable or even higher Jc for magnetic fields (H) parallel to the c axis (H‖c) than that for fields parallel to the ab plane (H‖ab) below certain temperature-dependent crossover fields. The crossover field, B_cr, was around 1.6 T at 5 K and decreased to around 0.9 T at 30 K depending weakly on the sample characters, and B_cr lay well below the upper critical fields in the phase diagram. Above B_cr, J_c for H‖c decreased very rapidly. The vortex spacing at B_cr was closely correlated with the temperature dependence of the penetration depth, indicating that the degree of the vortex-vortex overlap is an important parameter to determine the number of vortices allowed per unit length of grain boundaries. Thus, B_cr was thought to demarcate the accommodation capability of flux lines by the grain boundaries, and a rapid decrease of J_c above B_cr was ascribed to flux motion of the less strongly pinned vortices located away from the grain boundaries.

In this work we describe the effects of adding different amount of single-wall (sw) carbon nanotubes (CNT) on the superconducting properties of MgB₂. We observed that the amount of swCNT that partially dilute into the MgB₂ matrix is lower than for double-wall (dw) CNT samples for the same nominal content. This C incorporation leads to an increase of the upper critical field (H_c2), with a maximum H_c2(0) extrapolated value of 33.5 T, expected for a two-gap superconductor in the dirty limit. At the same time, the fraction of swCNT that has not been incorporated into MgB₂ lattice may retain their structural integrity acting as strong vortex pinning centers. In this case the optimum nominal swCNT content for increasing critical current density (J_c) is in the range 7.5-10%at.

YBa₂Cu₃O_7-δ (YBCO) films are prepared on the substrate of single crystal LaAlO₃ (LAO) by using metal trifluoroacetates deposition (TFA-MOD). The phase evolvement, magneto-transport and flux pinning characteristics in MOD-YBCO films are studied. It is revealed that Ba-Cu-O and Y-Cu-O precipitates are formed and can be controlled by optimum crystallization process. The field dependence of J_c (H//c) shows that the accommodation field H_acc decreases exponentially with increasing temperature and the power-law relationship holds in the medium fields with an exponent α of about 0.67, nearly independent of temperatures. Compared with the samples grown by pulsed laser deposition, MOD films have larger H_acc at higher temperatures (> 50 K) indicating that the value of the exponent α is affected by the precipitates entrapped in the YBCO matrix.

We studied the effect of sintering time on microstrain, critical current density (J_c), and resistivity (ρ) in MgB₂ bulks. All samples were fabricated by solid state reaction and sintered within a time range of 30 minutes – 60 hours. It has been found that microstrain increases as the sintering time increases, while the connectivity decreases with sintering time from 30 minutes to 20 hours, and then increases to 60 hours. Both the irreversibility field, H_irr, and the upper critical field, H_c2, were increased with microstrain. This indicates that the disorder significantly affects H_irr and H_c2. The sample with the sintering time of 30 minutes exhibited the highest J_c at all fields, due to its having the highest connectivity. When the sample is sintered for a very long time (60 hours), J_c at high fields can be improved. This is mainly attributed to increased H_c2.

The superconductivity of nano-SiC doped MgB₂ sintered in pulsed magnetic field (PMF) was investigated with Raman scattering measurements and Raman spectral fit analysis. The critical transition temperature, T_c, for the sample sintered in 5T PMF is improved compared with that of the sample sintered without PMF. The high T_c is attributed to the strengthening of the electron-phonon coupling (EPC) in MgB₂, as reflected by the broadened E_2g mode in the Raman spectra. The EPC constants are estimated as 0.876 and 0.874, with the electron-E_2g coupling contribution 2.30 and 2.25, respectively. Magnetic field processing technology has been proved to be a powerful tool to improve the superconducting properties of SiC-doped MgB₂ superconductor.

Systematic investigation of structural, magnetic and transport properties of RuSr₂RE_2-xCe_xCu₂O₁₀ with RE being Sm and Eu has been performed. We present the experimental evidence that the synthesis of RuSr₂Eu_1.5Ce_0.5Cu₂O₁₀ under non-optimised heat treatment conditions may results in the formation of secondary RuSr₂EuCu₂O₈ phase. In contrast, similar preparation conditions for RuSr₂Sm_1.5Ce_0.5Cu₂O₁₀ samples lead to the pure phase. Consequently, RuSr₂Eu_1.5Ce_0.5Cu₂O₁₀ samples exhibit a small peak around 125 K in the temperature dependence of dc magnetization, pronounced irreversibility between zero-field cooled and field cooled curves at temperatures higher than the peak temperature, and the bell shaped behavior of the coercive field as the function of temperature above Curie's temperature. These anomalous features, which may arise due to the superposition effect of the main and secondary phases, which are both magnetic, are completely absent for RuSr₂Sm_1.5Ce_0.5Cu₂O₁₀ sample, which does not consist of the secondary phase.

The microstructure of the MgB₂ wires with hydrocarbon, pyrene (C₁₆H₁₀) addition has been investigated by transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDX). In our study, MgB₂ wires have been fabricated through powder-in-tube (PIT) process using pure Fe as sheath material and sintered at 600°C ~800°C for 30min to 4 hrs under high purity argon gas. TEM and EDS analyses were conducted on the specimens prepared by tripod polishing and ion milling method. The structural features such as second phases, interface and defects were investigated by electron microscopy.

We investigated the effects of different operating temperatures on the performance of transport critical current density, J_c, for MgB₂ + 10 wt%C₄H₆O₅MgB₂/Fe wires. It was shown that the J_c values of the malic acid doped wires sintered at 900°C reached 10⁴ Acm^-2 at 20 K and 5 T. The J_c value extrapolated to 2 T and 20 K exceeds the practical level of 10⁵ Acm^-2. According to the Kramer plots, the pinning force, F_K = J_c^1/2 x B^1/4, is expected to be a linear function of magnetic field B. The irreversibility field, B_irr, at which extrapolated F_K reaches zero, was 1.8 T at 32.8 K, 2.8 T at 30 K, 5.7 T at 25 K, 8.6 T at 20 K, and 12.5 T at 15 K, respectively.

The feasibility of the infiltration of molten magnesium into porous boron skeleton was examined. The particle size and crystalline state of starting boron powders as well as the confinement of Mg vapors play an important role in successful infiltration. Magnesium penetrates more readily into porous skeletons formed by large crystalline boron particles than those formed by fine amorphous powders. The infiltration of magnesium mainly occurs via capillary condensation rather than liquid flow. Although the infiltrated microstructures are seemingly very dense, nano-scale structures contain many tiny inter-particle voids due to random packing of MgB₂ grains. This is due to the absence of a liquid phase in final microstructures as a result of transient liquid-phase infiltration.

MgB₂ thin films have been synthesized by co-evaporation method by controlling the relative deposition rates of B and Mg on sapphire (Al₂O₃) substrate. Before the co-evaporation of Mg and B, the deposition rates of each element have been measured separately with various substrate temperatures. As the substrate temperature increased above the optimum growth condition, Mg started to evaporate from the already reacted MgB₂ and form MgB₁₂ as a second phase, which resulted in the tendency decreasing the critical temperature. Superconducting properties have been measured by PPMS and the microstructural features have been characterized with XRD, SEM and EDS.

The MgB₂ films have been prepared on the c-cut sapphire substrates by ultrasonic spray pyrolysis (USP) technique using Boric acid and Magnesium acetate tetrahydrate dissolved into distillated water and ethanol as starting materials. The overall concentrations were fixed at 0.1mol/L. During the deposition, 96% Ar - 4% H₂ was used as carrier gas. The reaction temperatures were 850 °C, 900 °C and 950 °C . The deposited MgB₂ films showed strongly (1 0 1) plane orientation. In the sample deposited at 950 °C for 5min, MgB₂ film showed a homogeneous surface, with good grain connectivity. The critical temperature (T_c) was observed to be 31.5 K. The crystal structural and microstructural properties of the MgB₂ films prepared by USP were observed to be strongly dependent on the reacting temperature.

Perspective for qubits Josephson Mo-Re alloy-oxide–Pb, Mo-Re alloy-normal metal–oxide–MoRe alloy, and NbN -normal metal–oxide–Pb junctions have been fabricated and investigated.

The quality superconducting YBa₂Cu₃O₇ films and YBa₂Cu₃O₇/NdBa₂Cu₃O₇ multilayered film structures have been grown by pulsed laser deposition on metallic templates with magnesium oxide (MgO) buffer layer grown by incline substrate deposition. Different supplementary buffer layers have been deposited between the superconductor and MgO buffer layer. This additional buffer layer reduces the crystal structure misalignment of the superconducting layers. The critical current density has been observed to be higher in the multilayered structures in comparison with the same thickness of monolayer YBa₂Cu₃O₇ films. The origin of this enhancement is the improved microstructure of the multilayered systems obtained.

YBa₂Cu₃O_7-δ (YBCO) films with SrZrO₃ (SZO) doping have been prepared successfully by a non-vacuum, low cost, and easily scalable metal–organic deposition method, and their microstructures and physical properties were investigated. From the analysis of x-ray diffraction and φ-scan results, the doped films had sharp biaxial textures. Although the doped YBCO film had a wider critical temperature transition width than that of the un-doped YBCO film, the critical current density, J_c, was significantly enhanced under applied fields as compared to the un-doped film. Furthermore, the ratio of J_{c, doped} to J_{c, un-doped} became gradually larger with increasing temperature and magnetic field, indicating that an effective pinning force was created by SZO doping. These results clearly demonstrate that there are appealing prospects for applications of the doped film in high magnetic fields and temperatures.

The effect of processing atmosphere on the microstructure and superconducting properties of MgB₂ was studied for bulk samples made using the in situ reaction technique under argon gas with three different levels of purity: ultra-high, high, and welding grade. The critical temperature, T_c, decreased by 0.5 K, while the full width at half maximum (FWHM) of the MgB₂ peaks, and the resistivity showed an increase in the sample processed in welding Ar. The critical current density, J_c, and the upper critical field, H_c2, for the welding Ar sample were improved in comparison with the samples treated in ultra-high and high purity argon. Optimal J_c in MgB₂ can be achieved by using readily available and economical welding grade Ar as the protective atmosphere instead of using ultra-high or high purity Ar.

Influence of substrate-temperature on the interfacial structure of Fe₃Si/FeSi₂ layered films deposited on a Si(111) substrate were studied. Fe₃Si/FeSi₂ films with sharp interfaces were grown at room substrate-temperature. At a substrate-temperature of 300 °C, interfaces between the Fe₃Si and FeSi₂ layers were obviously unsharpened, while the crystallinity of Fe₃Si was enhanced. The compositional periodic structure was barely unsharpened and it was nearly the same as that of the films deposited at room substrate-temperature. Epitaxial growth of Fe₃Si layers across FeSi₂ layers was carried out. This substrate-temperature is the upper limit at which the heterostructure formation takes place. At 400 °C, ε-FeSi was formed due to activated interdiffusion, and the structure of Fe₃Si changed partially from B2-type to DO₃-type.

We present structural and magnetic properties of ZnO films doped with rare-earth Gd ions at various concentrations, achieved by ion implantation technique and pulsed laser deposition (PLD) on ZnO (0001) single crystals and sapphire. X-ray diffraction shows Gd doping in ZnO and crystal quality degrades with increasing Gd concentration. Magnetization as a function of temperature revealed a positive magnetization at 305 K and a concave trend was observed for all samples for both implanted and PLD grown samples. The highest saturation magnetization was achieved for Gd concentration of x = 7.9 % (0.7 µ_B/Gd) and at dosage of 9.0 × 10¹⁵cm^-2 (2.6 µ_B/Gd) for the PLD grown and implanted samples, respectively. We believe that the Gd ion solubility limit in the implanted and PLD grown samples, of around 3% and 7.9%, respectively, are reached.

Magnetic properties of multiferroic materials consisting of ferroelectric phase and ferrite phase have been investigated. Typical magnetic hysteresis loops of prepared multiferroic materials have been observed. The coercivity increases with the increase of ferroelectric phase. However, the saturation magnetization of multiferroic materials linearly decreases with the increase of ferroelectric phase. On increasing the content of ferroelectric phase, the initial permeability of multiferroic materials decreases and the peak of the quality factor tends to shift toward higher frequency. The Curie temperature of prepared multiferroic materials shifts toward higher temperature with the increase of ferroelectric phase. The microstructures of prepared multiferroic materials also have been studied.

The structure of Ru doped FePt nanoparticles using polyol process was studied. The particle size grown is around 5 nm, and a shell structure might be formed. By selecting the time and temperature of adding the Ru precursors into solution, three different processes to synthesize the FePtRu particles were studied resulting in different growing mechanics. The possible models during the reaction process are also discussed. The phase transition temperature for the as-grown FCC FePt nanoparticle to transform into L1₀FePt nanoparticle is about 823 K which is about the same as the one without doping Ru atoms. From the XAS study of each element, the possible scenario is that: although Ru atoms with the size close to the Pt, they do not totally replace the Pt sites in the FePt alloy. Instead, most of Ru formed a shell outside the FePt nanoparticles and Fe atoms are replaced.

Electrostatic Discharge (ESD) effects have been identified as one of the most dangerous causes of giant magnetoresistive (GMR) recording head damage. These phenomena have been studied at all levels of hard-disk drive manufacturing ¹. The head gimbal assembly (HGA) is mainly studied because of its exposure to the environment. The standard models are typically based on the human body model (HBM), the machine model (MM) and the charged device model (CDM) where research and practical tests are incompatible. In production, one or more ESD models are normally effective while the other is undergone under a separate model. In addition, tests in the time domain are more accurate than those in the frequency domain. However, picosecond measurements are taken with difficulty where the frequency domain measurement provides non-real time results. Therefore, this is the first report of serial ESD detection using the new technique of wavelet transform. It has been found that the glitch occurs when the ESD level of HBM – MM and HBM – CDM serial ESD on GMR heads are in the ranges of 1.2-2.6 V and 12-15 V respectively.

FePt-PtTe₂ two phase nanorods have been produced by a polyol process. The shape and magnetic properties of two phase nanorods with different phase ratio are investigated. L1₀ phase transformation of FePt in the nanorods has been accomplished at annealing temperature as low as 400 °C with H_c above 500mT. High temperature annealing causes the disintegration of the nanorods due the melting/evaporation of Te element.

We report the study of a promising Ge-based magnetic heterojunction diode composed of a Mn-doped p-Ge film grown on lightly doped n-Ge film. Magnetic field dependent current rectification of the diode occurs at a low temperature. The measurements of resistivity and magnetoresistance in plane at low temperatures reveal the presence of impurity band in Ge due to Mn doping.

Electrostatic discharge (ESD) effects on GMR recording heads have been reported as the major cause of head failure. Since the information density in hard-disk drives has dramatically increased, the GMR head will be no longer in use. The tunneling magnetoresistive (TMR) read heads are initially introduced for a 100 Gbit/in² density or more. Though the failure mechanism of ESD in GMR recording heads has not been explicitly understood in detail, a study to protect from this effect has to be done. As the TMR head has been commercially started, the ESD effect must be considered. This is the first time that the TMR equivalent circuit has been reported in order to evaluate the ESD effect. A standard human body model (HBM) is discharged across R+ and R- where the capacitances of flex on suspension (FOS) are varied. It is intriguingly found that the electrical characteristics of the TMR head during the discharge period depend on the discharge position. This may be explained in terms of the asymmetry impedance of TMR by using adapted Thevenin's theory. The effect of FOS components on TMR recording heads is also discussed.

We investigate the magneto-transport behaviour of Ge_1-xMn_xTe (x = 0.98) grown by solid-source molecular-beam epitaxy. The magnetization curves measured in different field orientations show that part of the magnetic moments lies out-of-plane, and becomes enhanced as temperature increases. A transition from negative to positive magnetoresistance (MR) was observed at T ≥ 95 K in the MR measurements with field applied perpendicular to sample plane. This transition temperature corresponds to our measured Curie temperature T_C ~ 95 K.

We have investigated the thermodynamic, transport and magnetotransport properties of free charge carriers in a diluted magnetic semiconductor with a quantum well In_0.17Ga_0.83As in GaAs with δ-doped by C and Mn. In order to determine the density of the holes in a quantum well, we carried out thermodynamic calculations of the system of free holes, atoms Mn⁰ and ions Mn^-. We calculated the temperature dependence of resistance and magnetoresistance of holes in the quantum well. The contributions of various scattering mechanisms of holes to the resistance were analyzed. The negative magnetoresistance are explained as the reduction of spin-flip scattering by aligning spins of the magnetic field.

Nanocrystalline CO₃O₄ particles were successfully synthesized by hydrothermal method under pulsed magnetic field. The effect of magnetic field and aging time on the morphology and microstructure were examined. Different morphologies were observed from SEM images for the samples fabricated with or without pulsed magnetic field. The pulsed magnetic field made CO₃O₄ sphere compact and more smooth surface. The hollow sphere morphology and refined grain of CO₃O₄ were formed after aging process.

Nanocrystal MnO₂ was successful synthesized by hydrothermal method under pulsed magnetic field. The effect of pulsed magnetic field on the nucleation and growth of MnO₂ was studied by XRD and SEM analysis. It was found that the morphology of MnO₂ has been changed comparing without magnetic field. However, there were no different phases presented when pulsed magnetic field was applied.

Magnetorheological (MR) suspensions, composed of colloidal particles dispersed in a carrier liquid, possess tunable rheological characteristics by applying an external magnetic field, showing dramatic changes of yield stress and shear viscosity caused by transformation between solid-like to liquid-like state. As a new MR material, we synthesized hollow polystyrene/magnetite (PS/Fe₃O₄) microspherical composite. Morphology of the PS obtained and the loaded magnetite were examined via SEM, and TGA spectra confirmed the composition. Magnetic property was tested by VSM data. MR characteristics of MR suspension based on PS/Fe₃O₄ composites were studied via both steady shear and oscillatory tests using a rotational rheometer equipped with a magnetic field generator.

This article presents the effect of Radio Frequency Identification (RFID) at 125 kHz on GMR recording heads. The study is mainly concerned with the RFID effect on recording heads in terms of the distance (r) between the antenna reader of the RFID and the recording head, the coil radius (a) of the antenna reader, and the angle (θ) between antenna reader and recording head. The maximum error of the writing current is found in this experiment of about 2.5% (where r < a). An increase of angle affects the writing current error when the radius increases. It is found that the writing current error decreases when r > a. SEM result shows that radiated fields of RFID do not visibly degrade the writer part but only disturbance to recording heads in testing is found.

This works reports, for the first time, the thorough study and optimisation of Phase Change Memory (PCM) structure with thin metal inserted chalcogenide via electrical resistivity (ρ) using finite element modeling. PCM is one of the best candidates for next generation non-volatile memory. It has received much attention recently due to its fast write speed, non-destructive readout, superb scalability, and great compatibility with current silicon-based mass fabrication. The setback of PCM is a high reset current typically higher than 1mA based on 180nm lithography. To reduce the reset current and to solve the over-programming failure, PCM with thin metal inserted chalcogenide (bottom chalcogenide/metal inserted/top chalcogenide) structure has been proposed. Nevertheless, reports on optimisation of the electrical resistivity using the finite element method for this new PCM structure have never been published. This work aims to minimize the reset current of this PCM structure by optimizing the level of the electrical resistivity of the PCM profile using the finite element approach. This work clearly shows that PCM characteristics are strongly affected by the electrical resistivity. The 2-D simulation results reveal clearly that the best thermal transfer of and self-joule-heating at the bottom chalcogenide layer can be achieved under conditions; ρ_bottom chalcogenide > ρ_metal inserted > ρ_top chalcogenide More specifically, the optimized electrical resistivity of PCMTMI is attained with ρ_top chalcogenide: ρ_metal inserted: ρ_bottom chalcogenide ratio of 1:6:16 when ρ_top chalcogenide is 10^-3 Ωm. In conclusion, high energy efficiency can be obtained with the reset current as low as 0.3mA and with high speed operation of less than 30ns.

Potassium-sodium-rubidium niobate single crystals are grown using an original pulling down method, to improve their composition change during a crystal growth, by means of co-doping of small ionic size sodium and large ionic size rubidium into potassium niobate. Even by the co-doping, single crystals can be grown with orthorhombic single-phase at room temperature, as well as pure potassium niobate. Their electric properties, such as the dielectric constant and the impedance, are changed depending on the doping ions.

The aim of present research is to study the influence of sintering temperature on the preparation of nanocrystalline Yttrium Iron garnet (YIG) with improved magnetic properties. The nanocrystalline YIG powders were synthesized using Microwave-Hydrothermal (M-H) method. The synthesized powders were characterized using X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). The powders were sintered at various temperatures using microwave sintering method. The sintered samples were characterized using XRD. The complex permeability, dielectric constant and loss tangent of sintered YIG ceramic were also measured and discussed in this paper

Despite the intense interest in peculiar electronic structure of graphene layers for potential application, especially in the microelectronics, the growth mechanism of graphene on substrates is still unclear. Understanding the quantitative correlation between layer number n and the critical radius R_c of the graphene cylinder will provide new insight in the fundamental science for the potential applications of graphene. In this work, the R_c(n) function of graphene layers deposited on SiO₂ substrate is established with nanothermodynamics. The finding in which R_c increases linearly as n increases agrees with the experimental results. This reveals the actual mechanism of the graphene growth behavior. In addition, the atomic structure of the graphene/SiO₂ interface was optimized by density functional theory.

The microwave properties of some of the low cost materials which can be used in high frequency applications with low transmission losses are investigated in this paper. One of the most accurate microwave characterization techniques, Split Post Dielectric Resonator technique (SPDR) is used for the experimental investigation. The dielectric constants of the 3 materials scrutinized at room temperature and at 10K are 3.65, 2.42, 3.61 and 3.58, 2.48, 3.59 respectively. The corresponding loss tangent values are 0.00370, 0.0015, 0.0042 and 0.0025, 0.0009, 0.0025. The high frequency transmission losses are comparable with many of the conventional materials used in low temperature electronics and hence these materials could be implemented in such applications.

Crystalline zinc oxide (ZnO) particles with different morphologies have been synthesized by the hydrothermal method under the magnetic field. It is found that the magnetic field influences the nucleation and growth of ZnO crystals. ZnO crystals obtained under high pulsed magnetic field were more uniform and well integrated.