Low External Magnetic Field Research Articles

Test Facility and Experimental Studies of Monocrystal HTS Pellets

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The paper describes a multipurpose facility and the results of experimental studies of magnetization, trapped flux mapping, flux creep and low amplitude external AC magnetic field effects for YBCO monocrystal pellets, as well as the technique and results of measuring the volt-ampere characteristics of YBCO thin-wall cylinder at 77, 73 and 66 K. The facility features with placing the pellets in separate demountable liquid nitrogen cryostats, and using the set of Hall probes for studies of the evolution of the trapped magnetic field. </para>

Magnetic and Electrical Control of Electron-Nuclear Spin Coupling in GaAs Double Quantum Dots

We magnetically and electrically control spin coupling between electrons and nuclei in a vertical GaAs double quantum dot in the Pauli spin blockade regime to characterize the two-electron exchange energy and the dynamic nuclear polarization (DNP). The Pauli blockade is lifted by a hyperfine-mediated transition from an electron triplet state to the singlet state. This transition progressively occurs when the triplet-singlet separation or exchange energy J is compensated by tuning either Zeeman energy with external magnetic field (i.e., magnetic control) or inter-dot energy detuning with bias voltage (i.e., electrical control). The blockade is then lifted with a current step for sweeping up an external magnetic field. The J value, which is evaluated from the step position for various inter-dot detuning values, is consistent with calculation. DNP takes place through the hyperfine coupling near the current step, leading to a shifted current step to lower external magnetic field for the down-sweep. For the electrical control, a similar current step appears for sweeping bias voltage. In this case we are able to control the two-electron state energies in a relevant manner based on real time before DNP is significantly influenced, because the voltage sweep is much faster than the magnetic field sweep. We use this technique to distinguish the current step with and without DNP contributions, and therefore quantify the nuclear Overhauser fields with external magnetic field as a parameter. We present a simple phenomenological model to reproduce the experiment and also discuss a possible approach to realize the full DNP.

A novel nanoscale catalyst system composed of nanosized Pd catalysts immobilized onFe3O4@SiO2–PAMAM

This study reports the syntheses ofFe3O4@SiO2–Gn–PAMAM–Pd(0) composites and their applications as magnetically recoverable catalysts for the hydrogenation of allylalcohol. The controlled growth of polyamidoamine (PAMAM) dendrimers with different generationson Fe3O4@SiO2 surfaces was monitored by FT-IR spectra. Subsequently, Pd nanoparticles withdiameters of about 2.5 nm were stabilized homogeneously on the surface ofFe3O4@SiO2–Gn–PAMAM (n = 1–4),investigated by thermogravimetry (TG) and transmission electron microscopy (TEM) measurements.The Fe3O4@SiO2–Gn–PAMAM–Pd(0) have high catalytic activity for the hydrogenation of allyl alcohol andthe rate of the reaction can be controlled by changing the generationof PAMAM. In particular, the composites made of superparamagneticFe3O4 nanocrystals with diameters of about 10 nm are very suitable as catalyst supports forcatalyst separation under a relatively low external magnetic field and catalyst re-dispersionafter removing the external magnetic field.

Magnetization Reversal Processes in the Nanocrystalline Fe-Co-Zr-Ti-Pr-B Magnets

Magnetization reversal processes in the magnets derived from the Fe60Co13Zr1Ti3Pr9B14 alloy were investigated. It was shown that the processing technique affects the magnetization reversal processes. For the nanocrystalline ribbon samples pinning of the domain walls arises at low external magnetic fields while nucleation of reversed domains occurs at higher fields. However, the nucleation fields are lower than the pinning fields for the nanocrystalline rod and tube samples produced by suction-casting technique.

How to determine critical current density in YBCO tapes from voltage–currentmeasurements at low magnetic fields

In this paper, an optimization method to determine themagnetic field dependence of the intrinsic critical current density,Jc(B),is introduced. First of all the critical current is measured in various external magnetic fields. Thenthe Jc(B)-dependence that fits optimally to the measurements is searched for. In thesecomputations the self-field of the sample is taken into account and thereby measurementsperformed in low external magnetic fields, below 0.1 T, can be exploited. Here theJc(B)-dependence of YBCO material is described by the Kim model, which is modified to include theanisotropy also. Thus, there are four parameters to search for: zero field critical current densityJc0, reference fieldB0, Kim model exponentα and anisotropyscaling factor γ. The search for these parameters was computationally challengingbut computation times were still satisfactory. As examples, theJc(B)-dependencesof two YBCO samples from different manufacturers were found. For both samples, all parameters exceptB0 were near each other.For example, Jc0 values forboth samples were about 0.9 × 1010 A m−2.

Flux pinning in melt-processed nanocomposite single-grain superconductors

Insulating RE2BaCuO5 (RE-211) second-phase inclusions embedded within theREBa2Cu3O7−δ (RE-123) superconducting phase matrix are known to contribute significantly toflux pinning in melt-processed superconductors. The critical current density inthese materials has been observed to increase significantly in the presence of lowexternal magnetic fields, in particular due to the presence of the RE-211 phase.Jc at higher applied magnetic field, however, is unaffected by theseinclusions. Recently we have introduced successfully nanoscale(RE)2Ba4CuMOy (RE-2411,where M = W,Nb,Ag and Bi) phase inclusions into the RE-123 phase matrix using a top-seeded meltgrowth (TSMG) technique. Here we report the flux pinning behaviour of variousREBa2Cu3O7−δ/(RE)2Ba4CuMOy nanocomposites and compare their properties with those ofREBa2Cu3O7−δ/RE2BaCuO5 single-grain composites. A direct correlation between increasedJc and increased concentration of nanoscale inclusions is observed. Finally, thepresence of RE-2411 nanoscale inclusions is observed generally to improve bothJc and the irreversibility field of bulk RE–Ba–Cu–O superconductors processed by TSMG.

Bulk Superconducting Nano-Composites With High Critical Currents

Flux pinning sites are most effective if their size is comparable to the superconducting coherence length, which is on the nano-meter scale for RE-Ba-Cu-O superconductors [RE = rare earth element]. Introducing nano-phase inclusions directly into the bulk superconducting material has only been partially successful to date, however, due primarily to the absence of chemically stable phases that can co-exist with RE-Ba-Cu-O without suppressing its key superconducting properties. We have identified novel isostructural phases based on (RE) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> CuMO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> (where M = W, Zr, Nb, Ag and Bi) and have fabricated successfully superconducting bulk nano-composites with a high current carrying capability. The average size of the nano-inclusions is observed to vary from 20 nm to 300 nm depending on element M. An observed improvement in J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> under low and high external magnetic fields at 77 K correlates directly with an increased density of nano-inclusions in the superconducting matrix.

2D modeling and preliminary in vitro investigation of a prototype high gradient magnetic separator for biomedical applications

High gradient magnetic separation (HGMS) of magnetic materials from fluids or waste products has many established industrial applications. However, there is currently no technology employing HGMS for ex-vivo biomedical applications, such as for the removal of magnetic drug- or toxin-loaded spheres from the human blood stream. Importantly, human HGMS applications require special design modifications as, in contrast to conventional use where magnetic elements are permanently imbedded within the separation chambers, medical separators need to avoid direct contact between the magnetic materials and blood to reduce the risk of blood clotting and to facilitate convenient and safe treatment access for many individuals. We describe and investigate the performance of a magnetic separator prototype designed for biomedical applications. First, the capture efficiency of a prototype HGMS separator unit consisting of a short tubing segment and two opposing magnetizable fine wires along the outside of the tubing was investigated using 2D mathematical modeling. Second, the first-pass effectiveness to remove commercially available, magnetic polystyrene spheres from human blood using a single separator unit was experimentally verified. The theoretical and experimental data correlated well at low flow velocities (<5.0 cm/s) and high external magnetic fields (>0.05 T). This prototype separator unit removed >90% in a single pass of the magnetic spheres from water at mean flow velocity ≤8.0 cm/s and from blood mimic fluids (ethylene glycol–water solutions) at mean flow velocity ≤2.0 cm/s. In summary, we describe and prove the feasibility of a HGMS separator for biomedical applications.

Self-assembled magnetic nanowire arrays

Different self-assembled magnetic nanowire arrays were achieved with ∼100% trapping rate on templates with nanomagnet arrays under a low external magnetic field (∼10Oe). The principles of magnetic charge matching and dimension matching between the magnetic nanowires to be assembled and the gaps between two nanomagnets were proposed and demonstrated to be crucial for achieving low magnetostatic energy and high trapping rate. This templated self-assembly technique and the proposed template design principles have great potential for nanomanufacturing of regular arrays of magnetic nanostructures.

Tunable high magnetic field thermal hysteresis for exchange-coupled double layers

Two types of tunable thermal hysteresis are observed for the first time in exchange-coupleddouble layer (ECDL) structures. For low external magnetic fields, ECDLs show onecompensation temperature where the measurement of the magnetization as a function oftemperature displays a bow-tie shape. For high fields a new type of tunable thermalhysteresis is observed due to an interface wall created between the layers wherethe ECDL shows two different compensation temperatures. The widths of bothforms of thermal hysteresis are tunable with a change in external magnetic field.

Magnetic-Field Induced Efficient Alignment of Carbon Nanotubes in Aqueous Solutions

Efficient alignment of aqueous carboxyl-functionalized multiwalled carbon nanotubes having remanent iron catalyst particles are carried out in relatively low external magnetic fields (B ≤ 1017 mT). The nanotubes were grown by catalytic chemical vapor deposition and then functionalized in a multistep oxidation process using nitric acid and potassium permanganate. In the field-induced ordering, the ferromagnetic property of iron nanoparticles entrapped in the inner-tubular cavity of nanotubes is exploited. Considerable dichroism of nanotube solutions (up to 3.02) is measured and deposition of aligned CNT networks from the solutions on silicon substrates is demonstrated.

Synthesis of magnetically separable adsorbents through the incorporation of protected nickel nanoparticles in an activated carbon

We present a simple and versatile route to prepare magnetically separable porous carbons from commercial activated carbons. The magnetic properties of the composite are provided by nickel nanoparticles (around 8–15 nm), which are produced within the porous network by means of the thermal treatment at 600 °C of an activated carbon impregnated with a nickel salt and sucrose. This synthetic strategy allows the magnetic nanoparticles to be protected against acid attack. The magnetic composite thus obtained exhibits a large surface area and a high pore volume. An important advantage of this composite is that exhibits, at room temperature, low coercivity (∼0.1 kOe), allowing it to be easily manipulated by means of low external magnetic fields.

Response of super-paramagnetic beads in microfluidic devices with integrated magnetic micro-columns

Ferromagnetic micro-columns (FMCs) have been fabricated on glass substrates using standard photolithography and electroplating and then integrated in micro-fluidic channels in order to study the response of super-paramagnetic beads suspended in an aqueous solution. We observed that under low external magnetic fields, the integrated micro-columns could generate sufficiently strong magnetic field gradients to trap super-paramagnetic beads in a flowing stream of water. The trapping behavior of the integrated FMCs was studied by considering different micro-patterns and different magnetic field intensities. In addition, the formation of magnetic micro-beads chains has been observed due to the magnetic dipole interactions. We believe that the integration of ferromagnetic micro-columns in micro-fluidic channels can have a variety of advanced applications, considering different possibilities of surface functionalization and manipulation of super-paramagnetic beads in an aqueous solution.

Probing magnetism in human blood by muon spin relaxation

A marked difference in spin relaxation function was found between positive muon in oxy-hemoglobin containing human blood versus that in deoxy-hemoglobin containing human blood, at room temperature and under zero and low external longitudinal magnetic field. Future extension to the functional study of the human brain is suggested.

Magnetic and electrical properties of La0.7Sr0.3MnO3–Zn0.8Co0.2Al0.01O junctions on silicon substrates

p - n and n-p junctions based on the p-type La0.7Sr0.3MnO3 (LSMO) and the n-type Zn0.8Co0.2Al0.01O (ZCAO) have been fabricated on silicon substrates via pulsed laser deposition. Both LSMO and ZCAO thin layers deposited on Si had good remanent magnetic characteristics at room temperature. The metal-semiconductor transition temperature (Tp) of the ZCAO∕LSMO n-p junction is lower than that of the pure LSMO layer, which is due to the influence of the top ZCAO layer. The low-field (±3kOe) magnetoresistance (LFMR) value of the ZCAO∕LSMO n-p junction is about −15% at 80 K, which is lower than that of the pure LSMO layer on Si (−21%). As deposited on top of ZCAO∕Si, the LSMO layer has no LFMR property. The LSMO∕ZCAO p-n junction has good current flow versus voltage (I–V) property with a threshold voltage of about 1.5 V at 80–300 K, which is very similar to normal Si-based p-n diode. Its I–V characteristics cannot be modulated by low external magnetic fields. However, the ZCAO∕LSMO n-p junction demonstrated a nonlinear I–V behavior, and its current flow can be modulated by low external magnetic fields. The resistance variation of the ZCAO∕LSMO n-p junction was −15% under forward bias voltages as the external magnetic field varied from 0 to 3 kOe.

Magnetic Alignment of Nanowires

Spatial manipulation and ability to assemble and position nanostructures in a controlled manner so they are registered to lithographically defined contacts is a critical step toward scalable integration in high-density nanodevices. By integrating ferromagnetic ends on nanostructures and using the magnetic interaction between ferromagnetic ends and electrodes, we demonstrated assembling, positioning, and spatial manipulating of nanostructures on ferromagnetic contacts. Segmented nickel/gold/nickel (Ni/Au/Ni) and nickel/bismuth/nickel (Ni/Bi/Ni) nanowires with controlled dimensions were fabricated by template-directed electrodeposition. One hundred percent magnetic alignment of nanostructures to the imposed magnetic fields was achieved by applying a low external magnetic field of 200 Oe. In addition, directional controllability of the magnetic assembling technique was demonstrated by assembling nanostructures with angles from 45° to 135° with respect to the electrodes. This magnetic assembly technique was shown to have potential for high-density interconnects without registration and individually addressable nanostructures with the use of different substrate architectures for two-dimensional control of nanostructures placement.

Shear flow behavior of confined magnetorheological fluids at low magnetic field strengths

The non-linear properties of iron based magneto-rheological (MR) fluids are investigated at low magnetic field strengths (0–1.7 kA/m) and different gap thickness (0–500 μm) in a plate-plate configuration. Single-width chain models qualitatively predict the low-shear flow behavior when plotting the field-specific viscosity, ηF, as a function of the Mason number, Mn: a slope close to −1 is observed in log-log representations. Wall depletion effects are observed when the suspensions are sheared under the presence of low external magnetic fields applied and/or large gap distances. These results are correlated to frictional yield stress measurements and chain length distribution calculations in the presence of the external magnetic field. Finally, an equivalent slip layer thickness is calculated using the method of Yoshimura and Prud’homme.

Magneto-modified catalyst on the base of nanocrystalline CuO

Abstract The weakly oxidized nanocrystalline copper powders of different sizes (average particle size of 30 and 100 nm) were used as the reactants at copper phthalocyanine (PcCu) formation. The nanocrystalline cupric oxide located at the particles surface serves as a heterogeneous magneto-controlled catalyst. A new effect of acceleration of chemical reaction rate controlled by low external steady magnetic field (∼2 kOe) at room temperature was revealed and investigated on an example of formation of a coordination compound of PcCu. The rate of PcCu formation accelerated by 7–8 times after applying of a magnetic field.

Temperature dependent self-field effect in measured V(I,B)-characteristics of Bi-2223/Ag tapes

Abstract When the voltage–current characteristics of a Bi-2223/Ag tape are measured at low external magnetic field the self-field created by the current in the tape tends to reduce the current carrying capacity. The self-field effect in the voltage–current measurement is strongest in high quality tapes at low temperatures. In this paper, the voltage–current characteristics of a Bi-2223/Ag tape have been measured at temperatures of 4.2–77 K. These characteristics are studied computationally in order to distinguish the temperature dependence of material properties from effects related to the self-field. Special attention is paid to the determination of the critical current density at zero magnetic field and the influence of self-field on n -value at different temperatures.

Magnetic phase transitions in Pr1−xDyxMn2Ge2and Ce1−xDyxMn2Ge2

The magnetic properties of the compounds Pr1−xDyxMn2Ge2 (0 ≤ x ≤ 1) andCe1−xDyxMn2Ge2 (0 ≤ x ≤ 1)were investigated by means of temperature-dependent DC magnetizationmeasurements in low external magnetic fields. The substitution of Dy for Pr or Celeads to a linear decrease in the lattice parameters. Below about 330 K, theinterlayer magnetic coupling in the Mn sublattice is ferromagnetic for Pr-rich andCe-rich compounds and antiferromagnetic for Dy-rich compounds. At lowtemperatures, the Dy sublattice also orders and reconfigures the ordering in theMn sublattice. This leads to ferrimagnetic ordering for Dy-rich compounds.