On some consequences of an external magnetic field applied to HTS coils

A computational model which enables to evaluate the distribution of the critical currents, electric fields and the voltage in the winding of a solenoidal high temperature superconducting (HTS) magnets subjected to an external magnetic field parallel with the magnet axis, was developed. The model comes out from the well-known power law between the electric field and the transport current of the HTS tape short sample. It allows to predict the voltage–current V(I) characteristics of both the pancake coils and the complete magnet. The model was applied to the magnet system consisting of 22 pancake coils made of multifilamentary Bi(2223)/Ag tape at 20 K, which is subjected to an external uniform magnetic field parallel with the coil axis. A rather unexpected behavior of the magnet at different operating conditions (operating current and external magnetic field strength) is predicted, analyzed and reported together with a theoretical explanation. On one hand, the external uniform magnetic field parallel with the coil axis increases the resulting magnetic field strength, however, on the other hand it simultaneously decreases the angle between the resulting magnetic field and the tape surface. Thus, the effect of higher magnetic loading caused by the presence of an external magnetic field strength which is acting on individual turns located close to the coil’s flanges is compensated by more favorable orientation of the tape with respect to the resulting magnetic field. As a result, increase in the critical currents of these turns is expected. Further, the results indicate, that in case of the high field HTS insert coils the anisotropy in the I_c(B) characteristic does not play a substantial role. As a consequence, the technology of the production of the tapes for high field insert HTS coils should concentrate rather on the tapes having the current carrying capacity as high as possible, than on the attempt how to decrease the anisotropy in the I_c(B) by changing the architecture of the filaments in the tape.     

The design of a body RF coil for low-field open MRI using pseudo electric dipole radiation and simulated annealing

The paper presents a new body RF coil design scheme for a low-field open MRI system. The RF coil is composed of four rectangular loops which are made of wide copper strips located near the surfaces of the bottom and top pole faces of the permanent magnet. The body RF coil has been designed by using the pseudo electric dipole radiation (PEDPR) method with the Metropolis algorithm. In the calculation of the RF fields via the finite difference time domain (FDTD) method, the computational time increases as the RF frequency becomes lower. Moreover, the computational process using the FDTD method takes a very long time when the RF coil is optimized. The optimization requires varying the configuration of the RF coil system and performing successive calculations of field strength and field homogeneity. When we perform these successive calculations, the computational time can be reduced by using the PEDPR method, where the segmented current elements of the RF coil are treated as pseudo electric dipole radiation sources. Because the RF coil is made of wide strips, the variation of the current density on the strip has been considered in the B₁-field calculation. For each configuration of the RF coil system, the current distribution is calculated via circuit analysis, where each copper strip is considered as a parallel combination of current element lines. The preliminary field calculation study by the FDTD method verifies both the circuit analysis method for the current distribution and the PEDPR method for the radiation field strength. The optimization of the RF coil configuration is performed by the Simulated Annealing (SA) process using the Metropolis algorithm. Simulations have been performed for a 10 MHz RF frequency. The optimized RF coil has four rectangular loops of 37 cm × 100 cm with 6.5 cm wide strips which are separated vertically 49 cm and horizontally center-to-center 63 cm. In the 25 cm diameter of spherical volume (DSV), the design results show a good field inhomogeneity of the B₁-field below 0.49 dB (5.8%).     

Effect of the substitution of Pr for Nd on microstructure and magnetic properties of nanocomposite Nd2Fe14B/α-Fe magnets

Microstructure and magnetic properties of melt-spun nanocomposite magnets with nominal compositions of (Nd_1−xPr_x)₉Fe₈₆B₅ (x=0–1) were investigated. Substitution of Nd by Pr could significantly improve the hard magnetic properties of the nanocomposite magnets; the intrinsic coercivity (_iH_c) and the maximum magnetic energy product ((BH)_max) increase from 414 kA/m and 124 kJ/m³ for x=0 to 493 kA/m and 152 kJ/m³ for x=0.6, respectively. Further substituting Nd by Pr (x>0.6) strongly weakens exchange-coupling interaction between magnetically hard and soft phases.     

Permanent magnet applications

Rare-earth permanent magnets are ideally suited to generate magnetic fields comparable to their spontaneous polarization J_S. Near-square hysteresis loops and large values of the coercivity and anisotropy fields greatly simplify magnet design, as each magnet block is effectively transparent to the magnetic fields produced elsewhere in the magnet assembly. The fields generated by compact and efficient magnet structures requiring no continuous expenditure of energy can be static or variable, uniform or nonuniform. Permanent magnets are fully competitive with electromagnets for fields up to 2 T, and fields as high as to 5 T can be produced in a small volume. When a field with a rapid spatial variation is required, permanent magnets may offer the only practicable solution. Both permanent magnet structures and the uses to which they are put are reviewed, classifying the magnet applications in terms of the nature of the field, the effect on the magnet and the physical effect exploited.     

Effects of ultrasonic agitation and surfactant additive on surface roughness of Si (1 1 1) crystal plane in alkaline KOH solution

In the silicon wet etching process, the “pseudo-mask” formed by the hydrogen bubbles generated during the etching process is the reason causing high surface roughness and poor surface quality. Based upon the ultrasonic mechanical effect and wettability enhanced by isopropyl alcohol (IPA), ultrasonic agitation and IPA were used to improve surface quality of Si (1 1 1) crystal plane during silicon wet etching process. The surface roughness R_q is smaller than 15 nm when using ultrasonic agitation and R_q is smaller than 7 nm when using IPA. When the range of IPA concentration (mass fraction, wt%) is 5–20%, the ultrasonic frequency is 100 kHz and the ultrasound intensity is 30–50 W/L, the surface roughness R_q is smaller than 2 nm when combining ultrasonic agitation and IPA. The surface roughness R_q is equal to 1 nm when the mass fraction of IPA, ultrasound intensity and the ultrasonic frequency is 20%, 50 W and 100 kHz respectively. The experimental results indicated that the combination of ultrasonic agitation and IPA could obtain a lower surface roughness of Si (1 1 1) crystal plane in silicon wet etching process.     

Oxygen on Fe(1 1 0): magnetic properties of the adsorbate system

Investigations concerning the electronic and magnetic properties of oxygen adsorbed on magnetized iron films were carried out by means of angle and spin resolving photoelectron spectroscopy. Iron(1 1 0), epitaxially grown on a W(1 1 0) crystal, served as the ferromagnetic substrate. Exchange splittings of the O 2p_x derived level were detected demonstrating a magnetic coupling between the chemisorbate and the iron layer. This observation indicates the presence of an induced magnetic moment within the adsorbate overlayer. Variations of the exchange splitting occurred as a function of the oxygen coverage, energy of the exciting radiation, and detection angle of the emitted photoelectrons pointing to a k₆-dependent exchange splitting. High oxygen exposures lead to a FeO overlayer at the surface, showing vanishing peak separations due to the antiferromagnetic behavior of iron oxide.     

Resistance oscillations in junctions of superconductor–magnetic system

Resistance oscillations as a function of magnetic field were observed in superconductor–magnetic tunnel junctions of Nb–Fe–FeO_x–SiO₂–Au–Nb. Junctions involving superconductor–magnetic layer superconductor system are exciting because for certain regime of ferromagnetic layer thickness, a Josephson coupling with an intrinsic phase difference of π might be stabilized. For fabrication of the tunnel junctions the thin films were deposited by RF/DC magnetron sputtering. Using photolithography and reactive ion etching, square junctions of size varying from 50 μm to 250 μm were defined. I–V characteristics and R vs. H characteristics were studied at 4.2 K. When the magnetic field is applied parallel to the junction plane, measurements of the junction resistance as a function of magnetic field at a fixed temperature show resistance peaks whenever the total magnetic flux through the junction equals an integral multiple of flux quantum. The penetration depth of the superconducting electrodes was estimated from the positions of the resistance peaks.     

Magneto-oscillations in a trapezoidal two-dimensional electron gas grown over GaAs wires

In this work we investigate a nonplanar two-dimensional electron gas (2DEG) that allows study of the electronic behaviour in random and sign-alternating magnetic fields. Shubnikov–de Haas oscillations were studied by measuring the magnetoresistance at different angles φ between the field and the substrate. We find that at low magnetic field the position of the oscillation peaks followsB_p ∼ Bsin (φ − θ), where θ is the angle between the field and the facets that effectively contribute to magnetoresistance. This is due to the fact that electrons follow different paths depending on the realization of a specific magnetic field.     

Colossal magnetoresistance effect in epitaxially grown La2/3Ca1/3MnO3 perovskite-like manganite thin films

We report in this work, study on colossal magnetoresistance (CMR) effect in epitaxial La_2/3Ca_1/3 MnO₃ thin films grown on SrTiO₃ (0 0 1) substrates by pulsed laser deposition (PLD) technique. The films were grown on as-received SrTiO₃ substrates and on SrTiO₃ substrates prepared by HF etching (Koster et al., Appl. Phys. Lett. 73 (1998) 2920; V. Leca et al., Wet etching methods for perovskite substrates, University of Twente, MESA+ Research Institute, Low Temperature Division). Two of the samples were annealed in different conditions to investigate the films heat treatment effect on electric and magnetic properties. Electrical resistance was done using the four-probe method at temperatures in the range of 2–375 K without a magnetic field and in an external field of 5 T applied in the film plane. Resistance-magnetic field (R vs. H) at 77 K for the two annealed samples was done in a 5 T sweep magnetic field. The surface morphology and structural information of the films were obtained using atomic force microscopy (AFM) and X-ray diffraction (XRD), respectively. Secondary ion mass spectroscopy (SIMS) analysis was performed on the annealed samples to investigate any possible chemical reaction between La_2/3Ca_1/3MnO₃ thin films and SrTiO₃ substrate.     

I–V characteristics and magnetic field profile studies in high Tc BSCCO based Helmholtz coil

A double pancake wound high T_c Helmholtz coil has been fabricated using commercial grade BSCCO tape. The current voltage (I–V) characteristics of this HTS coil has been carried out using standard four probe technique. From the I–V characteristics, the critical current based on self field dependent of HTS coil and quality index “n” value have been calculated. Magnetic field profiles have been studied along the axis of this coil for various amplitudes of applied current ranging from 10 A to 50 A at 77 K. The measured field profiles have a very good agreement with that of theoretical values predicted in the literature.     

Design and experiment of spectrometer based on scanning micro-grating integrating with angle sensor

A compact, low cost, high speed, non-destructive testing NIR (near infrared) spectrometer optical system based on MOEMS grating device is developed. The MOEMS grating works as the prismatic element and wavelength scanning element in our optical system. The MOEMS grating enables the design of compact grating spectrometers capable of acquiring full spectra using a single detector element. This MOEMS grating is driven by electromagnetic force and integrated with angle sensor which used to monitored deflection angle while the grating working. Comparing with the traditional spectral system, there is a new structure with a single detector and worked at high frequency. With the characteristics of MOEMS grating, the structure of the spectrometer system is proposed. After calculating the parameters of the optical path, ZEMAX optical software is used to simulate the system. According the ZEMAX output file of the 3D model, the prototype is designed by SolidWorks rapidly, fabricated. Designed for a wavelength range between 800 nm and 1500 nm, the spectrometer optical system features a spectral resolution of 16 nm with the volume of 97 mm × 81.7 mm × 81 mm. For the purpose of reduce modulated effect of sinusoidal rotation, spectral intensity of the different wavelength should be compensated by software method in the further. The system satisfies the demand of NIR micro-spectrometer with a single detector.     

Thin films of molecule-based charge transfer complex cobalt tetracyanoethylene: In situ X-ray photoemission study

Thin films of molecule-based charge transfer magnet, cobalt tetracyanoethylene [Co(TCNE)_x, x ~ 2] consisting of the transition metal Co, and an organic molecule viz. tetracyanoethylene (TCNE) have been deposited by using physical vapor deposition method under ultra-high vacuum conditions at room temperature. X-ray photoelectron spectroscopy (XPS) technique has been used extensively to investigate the electronic properties of the Co(TCNE)_x thin films. The XPS measurements show that the prepared Co(TCNE)_x films are clean, and oxygen free. The stoichiometries of the films, based on atomic sensitive factors, are obtained, and yields a ~ 1:2 ratio between metal Co and TCNE for all films. Interestingly, the positive shift of binding energy position for Co(2p), and negative shifts for C(1s) and N(1s) peaks suggest a charge-transfer from Co to TCNE, and cobalt is assigned to its Co(II) valence state. In the valence band investigation, the highest occupied molecular orbital (HOMO) of Co(TCNE)_x is found to be at ~ 2.4 eV with respect to the Fermi level, and it is derived either from the TCNE^? singly occupied molecular orbital (SOMO) or Co(3d) states. The peaks located at ~ 6.8 eV and ~ 8.8 eV are due to TCNE derived electronic states. The obtained core level and valence band results of Co(TCNE)_x, films are compared with those of V(TCNE)_x thin film magnet: a well known system of M(TCNE)_x type of organic magnet, and important points regarding their electronic properties have been brought out.     

Large spontaneous Hall angle in [Co,CoFe/Pt] multilayer

We have quantitatively investigated the Hall effect in [Co, CoFe/Pt] multilayer films. The [Co, CoFe/Pt] multilayers exhibit large spontaneous Hall resistivity (ρ_H) and Hall angle (ρ_H/ρ). Even though the Hall resistivity in [Co, CoFe/Pt] multilayer films (2.7–4 × 10⁻⁷ Ω cm) is smaller than that of amorphous RE–TM alloy films which show large spontaneous Hall resistivity (<2 × 10⁻⁶ Ω cm), the Hall angle of multilayer (6–8%) is almost twice than that in amorphous rare earth–transition metal alloy films (∼3%). The Hall angle provides evidence of the effects of the exchange interaction of the Hall scattering. The exchange is between conduction electron spins and the localized spins of the transition metal. The large Hall angle of [Co, CoFe/Pt] multilayer can be considered due to the high spin polarization and high Curie temperature of Co and CoFe transition metal layers. Even though the role of interfaces and surfaces in the magnetic properties of multilayer films may dominate that of the bulk, the Hall effects in [Co, CoFe/Pt] multilayer may be mainly dominated by the bulk effect.     

Electromagnetic wave absorption properties of ε-Fe3N/Y2O3 nanocomposites derived from Y2Fe17 intermetallic compound

The electromagnetic wave absorption properties of ε-Fe₃N/Y₂O₃ nanocomposites were characterized in a frequency range of 0.05–20.05 GHz. The imaginary part of relative permeability μ_r″ exhibited “twin peak” dispersion and μ_r″ value retained high over a 0.5–10 GHz range. The real part (ε_r′) and imaginary part (ε_r″) of relative permittivity almost kept a low constant in a region of 0.5–10 GHz, respectively. As a result, the resin composites with 51 vol% ε-Fe₃N/Y₂O₃ powders exhibited excellent electromagnetic wave absorption properties (RL<−20 dB) in a frequency range of 0.6–4.4 GHz, with a thickness of 3.3–19.3 mm. A minimum reflection loss of −55 dB was observed at 1.8 GHz with an absorber thickness of 7.05 mm.     

Microstructure refinement and significant improvements of magnetic properties in Pr2Fe14B/α-Fe nanocomposites

Exchange coupled (Pr,Tb)₂(Fe,Nb,Zr)₁₄B/α-Fe nanocomposites have been produced by melt spinning. A trend for perpendicular and planar c-axis orientation of the 2:14:1 phase was observed in the free surface of ribbons spun at speeds below 10 m/s and at optimal speeds, respectively. Higher wheel speeds led to the formation of an amorphous phase that transformed to 2:14:1 phase around 680°C. Optimum magnetic properties were found in samples spun at 14–17 m/s and annealed at 700°C for 20 min. The loop squareness was also found to depend mainly on the microstructure that is very sensitive to the sample composition. A few percentage of Nb and Zr suppressed the grain growth, resulting in a drastic improvement of magnetic properties, with approximate 50% enhancement in the intrinsic coercivity and an increase in maximum energy product from 5.6 kOe and 14.7 MGOe for the (Nb,Zr)-free sample to 8.2 kOe and 20.3 MGOe for the (Nb,Zr)-substituted samples, respectively. The significant improvement in magnetic properties originated from a much finer and homogeneous nanocomposite microstructure with an average grain size of 20 nm, leading to a high remanence of 0.73 M_s. Henkel plots indicate the enhancement of exchange coupling between hard and soft magnetic phases.     

Gap-related trapped magnetic flux dependence between single and combined bulk superconductors

Aiming at examining the trapped-flux dependence between single and combined bulk superconductors for field-pole applications, three rectangular Y_1.65Ba₂Cu₃O_7?x (YBCO) bulks with a possibly compact combination were employed to investigate the trapped-flux characteristics of single and combined bulks with a field-cooling magnetization (FCM) method. A gap-related dependence was found between them. At lower gaps of 1 mm and 5 mm, the peak trapped fields and total magnetic flux of combined bulks are both smaller than the additive values of each single bulk, which can be ascribed to the demagnetization influences of the field around the bulk generated by the adjacent ones. While, at larger gaps like 10 mm, the situation becomes reversed. The combined bulks can attain bigger peak trapped fields as well as total magnetic flux, which indicates that the magnetic field by the bulk combination can reach higher gaps, thanks to the bigger magnetic energy compared with the single bulk. The presented results show that, on one hand, it is possible to estimate the total trapped magnetic flux of combined bulks by an approximate additive method of each single bulk while considering a demagnetization factor; on the other hand, it also means that the performance of combined bulks will be superior to the addition of each single bulk at larger gaps, thus preferable for large-scaled magnet applications.     

Anisotropic vortex pinning in the β-pyrochlore oxide superconductor KOs2O6

Vortex pinning in the β-pyrochlore oxide superconductor KOs₂O₆ with T_c = 9.6 K is investigated by measuring magnetic torque. A large anisotropy of magnetic torque is observed in the superconducting state below T_p = 7.6 K, where a first-order structural transition takes place, in spite of the inherent isotropic nature of the structural and electronic properties. Magnetic torque is enhanced at external magnetic fields parallel to the [1 1 1] and [0 0 1] directions. Moreover, a pronounced peak effect is also observed in the magnetic field dependence of the torque in these two directions. We consider that the observed anisotropy is related to a microstructure associated with the structural transition.     

Improvement of microstructure and mechanical properties of large single domain GdBaCuO superconductor with low void density

We fabricated large single domain of GdBaCuO (Gd-123) densified superconductor with a seeding and temperature gradient method in oxygen at the melt stage then in air at the growth stage for future applications such as a superconducting flywheel for energy storage system, magnetic separation, motor, current lead, and a bulk magnet. The innovative melt-textured single domain of Gd-123 system superconductor with low void density was about 46 mm in diameter and 15 mm in thickness. Gd-123 densified superconductors are expected to have high J_c and trapped magnetic field properties with higher mechanical strength. In this paper, we discuss processing, superconducting and mechanical properties of the densified Gd-123 bulk for improving the microstructure and strength of the RE-123 bulk. The relation between the microstructure and the flexural strength of the Gd-123 bulk was clearly shown.     

Large improvement of RF transmission efficiency and reception sensitivity for human in vivo 31P MRS imaging using ultrahigh dielectric constant materials at 7 T

In vivo ³¹P MRS provides a unique and important imaging tool for studying high-energy phosphate metabolism and bioenergetics noninvasively. However, compared to ¹H MRS, ³¹P MRS with a relatively low gyromagnetic ratio (γ) has a lower and limited sensitivity even at ultrahigh field. The proof of concept has been recently demonstrated that the use of high dielectric constant (HDC) materials between RF coil and object sample could increase MRI signal and reduce required RF transmission power for reaching the same RF pulse flip angle in the region of interest. For low-γ MRS applications operated at relatively lower frequency, however, it demands the dielectric materials with a much higher permittivity for achieving optimal performance. We conducted a ³¹P MRS imaging study using ultra-HDC (uHDC; with a relative permittivity of ~ 1200) material blocks incorporated with an RF volume coil at ultrahigh field of 7.0 T. The experimental results from phantom and human calf muscle demonstrate that the uHDC technique significantly enhanced RF magnetic transmit field (B₁⁺) and reception field (B₁⁻) and the gain could reach up to two folds in the tissue near the uHDC blocks. The overall results indicate that the incorporation of the uHDC materials having an appropriate permittivity value with a RF coil can significantly increase detection sensitivity and reduces RF transmission power for X-nuclei MRS applications at ultrahigh field. The uHDC technology could provide an efficient, cost-effective engineering solution for achieving high detection sensitivity and concurrently minimizing tissue heating concern for human MRS and MRI applications.     

A 32-channel coil system for MR vessel wall imaging of intracranial and extracranial arteries at 3T

PurposeTo develop a RF coil system for joint imaging of intracranial and extracranial arterial vessel wall at 3T.Materials and methodThe coil system consists of a 24-channel head coil combined with an 8-channel carotid coil. It is compared with a standard coil configuration (12-channel head coil + 4-channel neck coil + 8-channel carotid coil) for SNR and g-factors in phantoms and healthy volunteers. The clinical relevance of the proposed coil system is also evaluated in patients.ResultsIn phantom experiments, the SNR of the proposed coil system is 53% higher than the maximum SNR of the standard coil configuration at the center of the phantom which usually corresponds to the intracranial region of the head. The g-factors of the proposed coil system in the sagittal plane are lower than the standard coil configuration (by 10.8% and 26.6% for R = 2 and 4 respectively) in the same experiment. In healthy volunteer experiments, 55% of the pixels have SNR above 100 for the proposed coil system, which is 33% more than that of the standard coil configuration. The maximum g-factors in the standard configuration are higher than those from the new coil design by 12% at R = 2 and up to 36% at R = 4 in the sagittal plane. In patients, in-vivo intracranial and extracranial arterial wall images at an isotropic spatial resolution of 0.6 mm can be acquired using the proposed coil system. Plaques are well depicted from the images.ConclusionsThe performance of the proposed coil set is superior to the standard coil configuration, providing high SNR, low g-factor and good spatial coverage needed for simultaneous high resolution imaging of intracranial and extracranial arterial walls. Images acquired in 7.6 min using the proposed coil system can achieve an isotropic spatial resolution of 0.6 mm and can be used to depict plaques on the intracranial and extracranial arterial walls in patients.