Dynamic nuclear polarization system for the SANS-J-II spectrometer at JAEA

We have developed a dynamic nuclear polarization (DNP) system for the SANS-J-II spectrometer at the JRR-3 atomic research reactor of Japan Atomic Energy Agency (JAEA). The DNP system is composed of a split-type horizontal superconducting magnet (3.3 T), a Gunn oscillator as a microwave source (94 GHz), and a cryostat (1.2 K). In particular, a sample cell with 40 in inner diameter and the magnet with a field homogeneity of 5×10⁻⁵ in a volume of 25 mm×8 mm were employed to polarize samples with a diameter of 20 mm for the ultra small-angle scattering experiment using the magnetic lens installed at the SANS-J-II spectrometer [S. Koizumi, H. Iwase, J. Suzuki, T. Oku, R. Motokawa, H. Sasao, H. Tanaka, D. Yamaguchi, H.M. Shimizu, T. Hashimoto, J. Appl. Crystallogr. 40 (2007) s474]. We obtained the proton polarization |P|=32% in the polyethylene doped with 2,2,6,6-tetra-methyl-piperidine-1-oxyl (TEMPO).     

Evaluation of magnetic force distribution on a pair of superconducting bulk magnets

We study the construction of superconducting permanent magnets by RE123 bulk materials and the investigation of these industrial applications such as a magnetic separation. A bulk magnet can generate strong magnetic fields exceeding 2 T, which is the limit of ordinary iron-cored electromagnets, in a compact device with a low running cost. A magnetic field distribution of the bulk magnet is a cone shape, and it contributes to an increase of magnetic force which is proportional to the product of a magnetic field and its gradient. It is important to evaluate magnetic force when the application of the bulk magnet is discussed. In this paper, two Gd123 bulk materials of 65 mm in diameter were magnetized using a pair of superconducting bulk magnet system and three-axis components of magnetic flux density (B_x,⋅B_y, and B_z) in an open space between the magnetic poles were scanned with pitch of 2 mm in each direction. From these measured data, the axial and radial components of magnetic force factor, B_z⋅dB_z/dz and B_r⋅dB_r/dr, were calculated. At 10 mm gap, the B_z⋅dB_z/dz value reached 180.6 T²/m for a field of 2.33 T, which is comparable to B_z = 6.76 T for a common 10 T–100 mm∅ superconducting magnet.     

A simple, small and low cost permanent magnet design to produce homogeneous magnetic fields

A new portable, pocket-size NMR probe based on a novel permanent magnet arrangement is presented. It is based on a Halbach-type magnet design which mimics the field of a spherical dipole by using cylindrical bar and ring magnets. The magnet system is made up of only three individual magnets, and most field calculations and optimisations can be performed analytically. A prototype system has been built using a set of small, off the shelf commercially available permanent magnets. Proton linewidths of 50 ppm FWHM could be achieved at a field strength of 1T. Calculations show that with custom-sized permanent magnets, linewidths of less than 1 ppm can be achieved over sample volumes of up to 1 mm3, which would in theory enable chemical shift resolved proton spectroscopy on mass-limited samples. But even with the achieved linewidth of 50 ppm, this can be a useful portable sensor for small amounts of liquid samples with restricted molecular mobility, like gels, polymers or high viscosity liquids.     

Side-hole to anti-hole conversion in time-resolved spectral hole burning of ruby: Long-lived spectral holes due to ground state level population storage

We report time-resolved transient spectral hole burning of Verneuil-grown 20 ppm and ca. 0.6 ppm ruby (Al₂O₃:Cr³⁺) in zero field and low magnetic fields B∥c at 4 K. The hole-burning spectroscopy of the 20 ppm sample implies relatively rapid cross relaxation in the ⁴A₂ ground state on the ∼1 ms timescale both in zero field and in low magnetic fields, B∥c, up to 0.2 T. In the 0.6 ppm sample, side-hole to anti-hole conversion is observed both in zero field and in low magnetic fields. This conversion is caused by population storage in ⁴A₂ ground state levels. Spin-lattice relaxation, on the 200 ms timescale, is directly observed from the time dependence of the resonant hole and anti holes in B∥c, consistent with a very low cross-relaxation rate. However, in zero field cross relaxation in the ⁴A₂ ground state is still a significant relaxation mechanism for the 0.6 ppm sample resulting in hole decay in ∼50 ms.     

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.     

Oxygen defects relaxation in high-temperature YBa2Cu3O7 superconductors

The temperature dependences of normal state static magnet susceptibility χ(T) for high-temperature YBa₂Cu₃O₇ superconductors (T _c ≤ T ≤ 400 K) measured for the same sample before and after 22 years of storage are compared. It is shown that during longime storage of a maximally doped sample, its electronic system is more in equilibrium than it was in the initial state, while a similar underdoped sample decomposed completely. Comparing the χ(T) curves for YBa₂Cu₃O₇, we draw conclusions as to the nature of the Curie contribution and the fluctuation-induced contribution to the magnetic susceptibility of the investigated sample, and the experimental procedure for determining T _c.     

Feasibility of MR fingerprinting using a high-performance 0.55 T MRI system

BackgroundMR fingerprinting (MRF) is a versatile method for rapid multi-parametric quantification. The application of MRF for lower MRI field could enable multi-contrast imaging and improve exam efficiency on these systems. The purpose of this work is to demonstrate the feasibility of 3D whole-brain T1 and T2 mapping using MR fingerprinting on a contemporary 0.55 T MRI system.Materials and methodsA 3D whole brain stack-of-spirals FISP MRF sequence was implemented for 0.55 T. Quantification was validated using the NIST/ISMRM Quantitative MRI phantom, and T1 and T2 values of white matter, gray matter, and cerebrospinal fluid were measured in 19 healthy subjects. To assess MRF performance in the lower SNR regime of 0.55 T, measurement precision was calculated from 100 simulated pseudo-replicas of in vivo data and within-session measurement repeatability was evaluated.ResultsT1 and T2 values calculated by MRF were strongly correlated to standard measurements in the ISMRM/NIST MRI system phantom (R² > 0.99), with a small constant bias of approximately 5 ms in T2 values. 3D stack-of-spirals MRF was successfully applied for whole brain quantitative T1 and T2 at 0.55 T, with spatial resolution of 1.2 mm × 1.2 mm × 5 mm, and acquisition time of 8.5 min. Moreover, the T1 and T2 quantifications had precision <5%, despite the lower SNR of 0.55 T.ConclusionA 3D whole-brain stack-of-spirals FISP MRF sequence is feasible for T1 and T2 mapping at 0.55 T.     

Physical property characterization of single step synthesized NdFeAsO<Subscript>0.80</Subscript>F<Subscript>0.20</Subscript> bulk 50 K superconductor

We report an easy single step synthesis route of title compound NdFeAsO_0.80F_0.20 superconductor having bulk superconductivity below 50 K. The title compound is synthesized via solid-state reaction route by encapsulation in an evacuated (10^-3 Torr) quartz tube. Rietveld analysis of powder X-ray diffraction data shows that compound crystallized in tetragonal structure with space group P4/nmm. R(T)H measurements showed superconductivity with T _c (R = 0) at 48 K and a very high upper critical field (H _c2) of up to 345 T. Magnetic measurements exhibited bulk superconductivity in terms of diamagnetic onset below 50 K. The lower critical field (H _c1) is around 1000 Oe at 5 K. In normal state i.e., above 60 K, the compound exhibited purely paramagnetic behavior and thus ruling out the presence of any ordered FeO _x impurity in the matrix. In specific heat measurements a jump is observed in the vicinity of superconducting transition (T _c ) along with an upturn at below T = 4 K due to the AFM ordering of Nd⁺³ ions in the system. The Thermo-electric power (TEP) is negative down to T _c , thus indicating dominant carriers to be of n-type in NdFeAsO_0.80F_0.20 superconductor. The granularity of the bulk superconducting NdFeAsO_0.8F_0.2 sample is investigated and the intra and inter grain contributions have been individuated by looking at various amplitude and frequencies of the applied AC drive magnetic field.     

Simple mobile single-sided NMR apparatus with a relatively homogeneous B0 distribution

This work presents a simple design for a mobile single-sided nuclear magnetic resonance (NMR) apparatus with a relatively homogeneous static magnetic field (B₀) distribution. In the proposed design, the B₀ magnetic field of the apparatus is synthesized using only two permanent magnet blocks, i.e., a cube (main) magnet and a small shim magnet placed above the main magnet. The magnetic flux of the shim magnet partially cancels out that of the main magnet, subsequently creating a smooth B₀ profile above the shim magnet where low-resolution NMR experiments are performed. Compared with many previously published designs, this straightforward design simplifies the construction of the apparatus and simultaneously generates a B₀ field parallel to the apparatus surface, allowing the use of a simple loop-type radiofrequency (RF) coil. Additionally, an apparatus prototype is constructed according to the proposed design. Weighing only 1.8 kg, the constructed apparatus has a compact structure and can be held in the palm of a hand. The apparatus generates a B₀ strength of about 0.0746 T. Within a B₀ field deviation of 3 mT, the region with a relatively homogeneous B₀ distribution extends to about 11 mm above the shim magnet. The proposed apparatus can detect a clear Hahn echo or Carr-Purcell-Meiboom-Gill (CPMG) echoes of a pencil eraser block or a bottle of oil placed on the apparatus in 5 s with signal averaging using an RF transmitter power of only 19 W; the detection range of the apparatus exceeds 6 mm. The strength of the residual static magnetic field gradient of the apparatus is roughly estimated at 0.58 T/m. Applying different CPMG echo spacings in this residual static gradient leads to various transverse relaxation time (T₂) contrasts for liquids with distinct viscosities such as water and oil. Two nondestructive inspection applications of the apparatus, including correlating the concentrations of magnetic nanoparticle solutions with their measured transverse relaxation rates (R₂) and monitoring the outgassing from an opened bottle of oxygen-supersaturated water by measuring its longitudinal relaxation rate (R₁), are also demonstrated.     

Gas sensing property of lithium tetraborate clad modified fiber optic sensor

The micro structured plate-like lithium tetraborate, Li₂B₄O₇ (1 μm in diameter) has been prepared by sol–gel method and characterized structurally by X-ray diffraction and morphologically by scanning electron microscopy. UV–Vis spectrum shows about 60% transparency in the visible region and the optical energy band gap is found to be 3.5 eV which is also confirmed by strong near band edge emission from luminescence spectrum. The spectral characteristics of the cladding modified fiber optic sensor coated with microcrystalline Li₂B₄O₇ are studied for various concentrations of ethanol, methanol and ammonia (50–500 ppm). At 298 K, the sensitivity for ethanol is ?10 counts/ppm which is relatively higher than ammonia (?4 counts/ppm) and methanol (?3 counts/ppm). The time response of the sensor is presented for pure Li₂B₄O₇ with ethanol gas.     

Physical property characterization of bulk MgB<Subscript>2</Subscript> superconductor

We report synthesis, structure/micro-structure, resistivity under magnetic field [ρ(T)H], Raman spectra, thermoelectric power S(T), thermal conductivity κ(T), and magnetization of ambient pressure argon annealed polycrystalline bulk samples of MgB₂, processed under identical conditions. The compound crystallizes in hexagonal structure with space group P6/mmm. Transmission electron microscopy (TEM) reveals electron micrographs showing various types of defect features along with the presence of 3–4 nm thick amorphous layers forming the grain boundaries of otherwise crystalline MgB₂. Raman spectra of the compound at room temperature exhibited characteristic phonon peak at 600 cm^-1. Superconductivity is observed at 37.2 K by magnetic susceptibility χ(T), resistivity ρ(T), thermoelectric power S(T), and thermal conductivity κ(T) measurements. The power law fitting of ρ(T) give rise to Debye temperature (Θ_D) at 1400 K which is found consistent with the theoretical fitting of S(T), exhibiting Θ _D of 1410 K and carrier density of 3.81 × 10²⁸/m³. Thermal conductivity κ(T) shows a jump at 38 K, i.e., at T_c, which was missing in some earlier reports. Critical current density (J_c) of up to 10⁵ A/cm² in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K is seen from magnetization measurements. The irreversibility field, defined as the field related to merging of M(H) loops is found to be 78, 68 and 42 kOe at 4, 10 and 20 K respectively. The superconducting performance parameters viz. irreversibility field (H_irr) and critical current density J_c(H) of the studied MgB₂ are improved profoundly with addition of nano-SiC and nano-diamond. The physical property parameters measured for polycrystalline MgB₂ are compared with earlier reports and a consolidated insight of various physical properties is presented.     

High performance batch production of LREBa2Cu3Oy using novel thin film Nd-123 seed

A batch production for fabrication of LREBa₂Cu₃O_y (LRE: Sm, Gd, NEG) “LRE-123” pellets are developed in air and Ar-1% O₂ using a novel thin film Nd-123 seeds grown on MgO crystals. The SEM and XRD results conformed that the quality and orientation of the seed crystals are excellent. On the other hand, new seeds can withstand temperatures >1100 °C, as a result, the cold seeding process was applied even to grow Sm-123 material in Air. The trapped field observed in the best 45 mm single-grain puck of Gd-123 was in the range of 1.35 T and 0.35 T at 77.3 K and 87.3 K, respectively. The average trapped field at 77.3 K in the 24 mm diameter NEG-123 samples batch lies between 0.9 and 1 T. The maximum trapped field of 1.2 T was recorded at the sample surface. Further, the maximum trapped field of 0.23 T at 77 K was recorded in a sample with 16 mm diameter of Sm-123 with 3 mol% BaO₂ addition. As a result we made more then 130 single grain pucks within a couple of months. Taking advantage of the single grain batch processed material, we constructed self-made chilled levitation disk, which was used on the open day of railway technical research Institute. More then 150 children stood on the levitation disk and revel the experience of levitation. The present results prove that a high-performance good-quality class of LREBa₂Cu₃O_y material can be made by using a novel thin film Nd-123 seeds.     

NMR study and electrical properties investigation of Zn<Subscript>2</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>

The α-Zn₂P₂O₇ compound was obtained by conventional solid-state reaction. The sample was characterized by X-ray powder diffraction, solid state ³¹P NMR MAS, and electrical impedance spectroscopy. The solid state ³¹P MAS NMR, performed at 121.49 MHz, shows three isotropic resonances at −21.1, −18.8, and −15.8 ppm, confirming the non-equivalency of the three PO₄ groups in the α-Zn₂P₂O₇ form. They are characterized by different chemical shift tensor parameters with the local geometrical features of the tetrahedra. Electrical impedance measurements of β-Zn₂P₂O₇, form stable for temperature greater than 403 K, were performed as a function of both temperature and frequency. The electrical conduction and dielectric relaxation have been studied. The AC conductivity obeys the universal power law. The approximation type correlated barrier hopping model explains the universal behavior of the n exponent. The impedance plane plot shows semicircle arcs at different temperatures, and an electrical equivalent circuit has been proposed to explain the impedance results. The circuits consist of the parallel combination of bulk resistance R _p and constant phase elements CPE. The simulated spectra show a good correlation with the experimental data.     

Synthesis and magnetic properties of single-crystalline BaFe12O19 nanoparticles

Rod-like and platelet-like nanoparticles of simple-crystalline barium hexaferrite (BaFe₁₂O₁₉) have been synthesized by the molten salt method. Both particle size and morphology change with the reaction temperature and time. The easy magnetization direction (0 0 l) of the BaFe₁₂O₁₉ nanoparticles has been observed directly by performing X-ray diffraction on powders aligned at 0.5 T magnetic field. The magnetic properties of the BaFe₁₂O₁₉ magnet were investigated with various sintering temperatures. The maximum values of saturation magnetization (σ_s=65.8 emu/g), remanent magnetization (σ_r=56 emu/g) and coercivity field (H_ic=5251 Oe) of the aligned samples occurred at the sintering temperatures of 1100 °C. These results indicate that BaFe₁₂O₁₉ nanoparticles synthesized by the molten salt method should enable detailed investigation of the size-dependent evolution of magnetism, microwave absorption, and realization of a nanodevice of magnetic media.     

A new simple tubular flow cell for use with variable angle spectroscopic ellipsometry: A high throughput in situ protein adsorption study

A simple and novel flow cell design is presented here for use with variable angle spectroscopic ellipsometry (VASE) to study the adsorption of liquid-borne species on reflective surfaces. The flow cell allows a sample as large as 6 mm × 75 mm to be probed point by point and at any common ellipsometric angle of incidence, unlike other designs. Using our flow cell system with VASE, combinatorial films of Al_1−xNb_x, Al_1−xTa_x, and Al_1−xTi_x (0  x  1) were tested in situ for fibrinogen affinity along their 75 mm long compositional gradients. Fibrinogen adsorption on the films was found to be closely correlated to the various surface oxide fractions, with high alumina content at the surface leading to low amounts of adsorbed fibrinogen for each binary library. Adsorbed amounts measured in situ were in agreement with previously obtained values found using ex situ techniques.     

Indigenous design and fabrication of a 6.5 T superconducting magnet and a magnetotransport measurement set-up

A low-cost apparatus for measuring Hall effect and magnetoresistance is designed and built indigenously. This includes a 6.5 T superconducting magnet and a variable temperature sample holder assembly. A superinsulated liquid helium dewar with a low liquid helium boil-off rate is chosen as the low-temperature bath for doing magnetotransport measurements. A pair of high-T _c superconducting leads for energizing the magnet reduces the liquid helium consumption further and makes it economically beneficial, especially for laboratories with limited budget. The performance of the apparatus is tested over a wide range of temperatures (4.2 to 300 K) and fields up to 6.5 T. Reproducible magnetotransport data are obtained with excellent temperature and field stability.      

Magnetic properties of Nd1−xKxMnO3 compounds

Polycrystalline Nd_1−xK_xMnO₃ (x=0.10–0.20) compounds have been prepared in single phase form with Pbnm space group. The magnetic properties were studied by measuring dc magnetization and ac susceptibility. They exhibit paramagnetic to ferromagnetic transition with transition temperature ranging from 116 to 128 K. The magnetization data have been analyzed by using Brillouin function model and by taking into account the ferromagnetic interaction. The effective spin contribution towards ferromagnetic interaction and spin canting angle have been estimated. The spin canting angle is found to decrease with increase in doping. Magneto-caloric effect (MCE) has been studied and the maximum change in entropy was found to be 1.76 J/kg K for 1 T field. Metal–insulator transition and colossal magnetoresistance of the order of 60% for 1 T field have been observed for x=0.20 sample.     

High-field QCPMG NMR of large quadrupolar patterns using resistive magnets

Spectroscopy in a high magnetic field reduces second-order quadrupolar shift while increasing chemical shift. It changes the scale between quadrupolar and chemical shift of half-integer quadrupolar spins. The application of QCPMG multiple echo for acquiring large quadrupolar pattern under the high magnetic field of a 25 T resistive magnet is presented for acquiring large quadrupolar patterns. It shows that temporal field fluctuations and spatial homogeneity of the Keck magnet at the NHMFL contribute about ±20 ppm in line broadening. NMR patterns which have breadths of hundreds to thousands of kilohertz can be efficiently recorded using a combination of QCPMG and magnetic field stepping with negligible hindrance from the inhomogeneity and field fluctuations of powered magnets.     

Susceptibility artifacts from metallic markers and cardiac catheterization devices on a high-performance 0.55 T MRI system

IntroductionVisualization of passive devices during MRI-guided catheterizations often relies on a susceptibility artifact from the device itself or added susceptibility markers that impart a unique imaging signature. High-performance low field MRI systems offer reduced RF-induced heating of metallic devices during MRI-guided invasive procedures, but susceptibility artifacts are expected to diminish with field strength, reducing device visualization. In this study, field strength and orientation dependence of artifacts from susceptibility markers and metallic guidewires were evaluated using a prototype high-performance 0.55 T MRI system.Materials and methodsArtifact volume from nitinol and stainless steel passive susceptibility markers was quantified using histogram analysis of pixel intensities from three-dimensional gradient echo images at 0.55 T, 1.5 T and 3 T. In addition, visibility of commercially available clinical catheterization devices was compared between 0.55 T and 1.5 T using real-time bSSFP in phantoms and in vivo.ResultsA low-tensile strength stainless-steel marker produced field strength- and orientation-dependent artifact size (1.7 cm³, 1.95 cm³, 2.21 cm³ at 0.55 T, 1.5 T, 3 T, respectively). Whereas, a high-tensile strength steel marker, of the same alloy, produced field strength- and orientation-independent artifact size (3.35 cm³, 3.41 cm³, 3.42 cm³ at 0.55 T, 1.5 T, 3 T, respectively). Visibility of commercially available nitinol guidewires was reduced at 0.55 T, but imaging signature could be maintained using high-susceptibility stainless steel markers.Discussion and conclusionHigh-susceptibility stainless-steel markers generate field-independent artifacts between 0.55 T, 1.5 T and 3 T, indicating magnetic saturation at fields <0.55 T. Thus, artifact size can be tailored such that interventional devices produce identical imaging signatures across field strengths.     

Magnetic-Field-Controlled Quantum Critical Points in the Triangular Antiferromagnetic Cs2CuCl4-xBrx Mixed System

The triangular antiferromagnetic Cs₂CuCl_4-xBr_x mixed system is studied by neutron single-crystal diffraction in magnetic field. It shows a rich magnetic phase diagram consisting of four regimes depending on the Br concentration and is characterized by different exchange coupling mechanisms. For the investigated compositions from regime I (0 < x ≤ 1.5), a critical magnetic field B_c is found for a Br concentration x = 0.8 at B_c = 8.10(1) T and for x = 1.1 at B_c = 7.73(1) T and from regime IV (3.2 < x < 4) for x = 3.3 at B_c = 0.99(3) T. For magnetic fields larger than the respective B_c, magnetic superlattice reflections of these compounds are not found. The incommensurate magnetic wave vector q = (0, 0.470, 0) appears below the ordering temperature T_N = 0.51(1) K for Cs₂CuCl_3.2Br_0.8, and q = (0, 0.418, 0) below T_N = 1.00(6) K for Cs₂CuCl_0.3Br_3.7. Neutron diffraction experiments at around 60 mK for x = 3.7 in a magnetic field show the critical magnetic field at B_c = 7.94(16) T and the formation of the second magnetic phase at around 8.5 T depending on the temperature. Inelastic neutron scattering experiments for the compound from regime III (2 < x ≤ 3.2) with x = 2.2 show dynamical correlations at a temperature around 50 mK giving evidence for a spin liquid phase.