Hall Probe Measurements Research Articles

Modeling and magnetic measurements of TNK synchrotron radiation source magnets

The TNK synchrotron radiation source is being built by Budker Institute of Nuclear Physics on the base of Lukin State Research Institute of Physical Problems. Magnetic system for the storage ring of TNK synchrotron radiation source was produced at the BINP. It consists of 6 superperiods and includes 24 dipole magnets, 72 quadrupole lenses, 36 sextupole lenses and 12 octupole lenses.The storage ring will operate in a wide range of energies—from 450 to 2200 MeV—which corresponds to 0.3–1.5 T magnetic field in dipole magnets. Dipole magnets have H-shape solid yokes from Armco iron with a curved form. Dipole gap is 42 mm and yoke straight length is 1447 mm. The results of 3D magnetic field modeling executed by means of Mermaid 3D are presented. All dipole magnets were magnetically measured by special Hall probe measurement system developed in BINP. The results of magnetic measurements and modeling are compared and analyzed.

Reliable plate–plate MRF magnetorheometry based on validated radial magnetic flux density profile simulations

The apparent shear stress from plate–plate magnetorheometry, using the commercial magnetocell MRD180/1T (Physica/Anton Paar) in standard configuration, is distinctly overestimated. The effect is due to a flux density maximum near the sample rim and radial migration of iron particles towards the rim. Radial magnetic flux density profiles were investigated by finite element simulations using the Maxwell®2D code and by direct Hall probe measurements. The reliability of the finite element method results, both for the empty magnetocell and with magnetorheological fluid (MRF) in the measuring gap, allows conclusions on the true flux density within the MRF, which cannot be accessed by Hall probe measurements. If the MRF sits on top of the bottom yoke (standard configuration), the flux density maximum reaches twice the plateau value (0.74 T for 3 A coil current and 0.3 mm gap height of the investigated MRF). This yields a higher effective flux density and causes radial iron particle migration, resulting in an additional magnetic flux increase near the rim due to augmented MRF magnetisation. As a result, the rotor torque at constant rotary speed increases with time. Reliable results are achieved by a modification of the magnetocell, such that the MRF sits on a non-magnetic Hall disc of 1.5 mm thickness, allowing an online flux density measurement by a FW Bell Hall probe. In this configuration, the radial flux density profile near the rim remains sufficiently flat and no iron particle migration is detected.

Atomic Layer Deposition of Indium Tin Oxide Thin Films Using Nonhalogenated Precursors

This article describes a new atomic layer deposition (ALD) method for preparing indium tin oxide (ITO) thin films using nonhalogenated precursors. The indium oxide (In2O3) was deposited using alternating exposures to cyclopentadienyl indium (InCp) and ozone, and the tin oxide (SnO2) used alternating exposures to tetrakis(dimethylamino) tin (TDMASn) and hydrogen peroxide. By adjusting the relative number of In2O3 and SnO2 ALD cycles, we deposited ITO films with well-controlled SnO2 content. The ITO films were examined using four-point probe and Hall probe measurements, spectrophotometry, ellipsometry, scanning electron microscopy, atomic force microscopy, X-ray fluorescence, and X-ray diffraction. The lowest resistivity (3 × 10-4 Ωcm) and highest optical transparency (92%) were obtained for films containing 5% SnO2. The ITO films were slightly thinner and contained more SnO2 than expected on the basis of rule-of-mixtures predictions. In situ measurements revealed that these discrepancies result from an inh...

Calculations of AC current losses and AC magnetic losses from the scanning Hall probe measurements for a coated conductor

We measured the field profiles near the surface of a coated conductor (CC) by using the scanning Hall probe method. The external currents, represented by I a, or the external fields, represented by H a, were applied during the measurements. The field was applied in the normal direction with respect to the tape surface. The measured field profiles could be expressed by H( x, X a), where the variable, X a, was either I a or H a. The x-axis was parallel to the in-plane transverse direction of the CC tape. The CC tape was fabricated by the IBAD-PLD method. The applied field (current), H a ( I a), was decreased stepwise from H peak to − H peak (from I peak to − I peak). From H( x, X a), we calculated the current profile, J( x, X a), by the iterative inversion method. From J( x, X a) and the corresponding flux densities, B( x, X a) we calculated the AC current loss, Q C( I peak), and the AC magnetic loss, Q M( H peak), for various values of I peak and H peak, respectively. The characteristic functions were defined by f C = π Q C / μ 0 I c 2 and f M = π Q M / μ 0 I c 2 , where I c is the critical current. We found that the calculated values of f C ( f M) were similar to the experimental values of f C ( f M) which were directly measured for the IBAD-PLD CCs. On the other hand, the values of f C ( f M) were smaller than (larger than) the values estimated from the experimentally observed Q C ( Q M) in RABiTS ™-MOD CCs.

Hysteresis in the de Haas–van Alphen effect

We present the experimental observation of hysteresis in high quality single crystals with different sample shapes of the simple metal beryllium. The hysteresis itself turns out to be very small. Here, Hall probe measurements at very low modulation frequency prove that the hysteresis is not due to eddy current effects. Furthermore, the hysteresis effect is measured using a standard AC method with various modulation levels and frequencies, and for samples with different demagnetization factors. We demonstrate that previously in rod-like samples observed effect is also present in a plate-like sample. The irreversible behavior of the magnetization occurs at the paramagnetic part of the de Haas–van Alphen (dHvA) period in case of Condon domain formation arising if the dHvA amplitude is strong enough. Moreover, we show that the resulting, extremely nonlinear response to a very small modulation field offers a simple way to find the Condon domain phase diagram.

Scanning Hall probe measurements of field distributions of a coated conductor underapplied fields

We measured the field profiles near the surface of a coated conductor (CC) under variousapplied fields by using the scanning Hall probe method. The field, applied inthe normal direction, was increased from zero to 171.5 Oe and then decreased to−58.8 Oe. We could not analyse our data completely by the direct use of Brandt’s calculation butby a modification with unusual field dependences of the introduced parameters.Since Brandt’s original calculation was based on homogeneous films, it was notsuitable for CCs with coarse granular structures. The modified calculations withappropriate parameters are related to the coarse granular structures. Those parameters,D,Jc,and R, represent the three characteristics of the flux penetration network: the averagedistance of flux penetrations, the density of critical sheet currents, and the range ofmeandering of the flux penetration front, respectively. The external field dependences ofthese parameters were different from those of the classical critical state model.

A straightforward specimen holder modification for remnant magnetic-field measurement in TEM

We report a simple means to modify an analytic sample holder to perform ambient Hall probe measurements of the sample area inside a transmission electron microscope (TEM). These measurements are important in the case of electron microscopy studies involving magnetic materials. We characterize the magnetic field of the JEOL 2100F-LM, a microscope dedicated in design to perform magnetic imaging, and also of the JEOL 3000F FEG-TEM operated in Lorentz mode. In the case of the 3000F, we measure vertical remnant field about 300 Oe due to the objective lens of the microscope. In the case of the 2100F, design of the objective lens reduces the remnant field to about 4 Oe. We characterize the field along two orthogonal directions, and spatial characteristics of the field profile are made for both microscopes during all stages of specimen entry into the column. In the case of the 2100F, we additionally measure the field conditions as a function of objective lens excitation, which is important for in situ magnetization experiments. Finally, we provide experimental results illustrating the importance of these measurements.

CHARACTERIZATION OF ZnO FILMS BY ION BEAM ANALYSIS

Ion Beam Analysis (IBA) techniques such as Rutherford Backscattering Spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA) were used to determine the composition, uniformity, impurity and elemental depth profiles of Zn , O and H in ZnO films deposited on silicon, quartz, glass and glassy carbon using radio frequency (RF) magnetron sputtering. For the films deposited under the same condition, it was observed that the variation of Zn/O ratios is independent of substrate material and depends on the film thickness. ERDA revealed that the hydrogen impurities were incorporated into the films. Higher hydrogen concentrations were found for the films deposited on glass and quartz compared to Si . The composition and thickness variation in relationship with three different substrates were explored with XRD, SEM, AFM, PL and Hall probe measurements. It was found that ZnO films (250-300 nm) deposited on Si have, the optical and electrical properties which make them suitable candidate for following p-type doping studies using ion implantation and annealing techniques.

Modification of electrical conductivity in RF magnetron sputtered ZnO films by low-energy hydrogen ion implantation

Abstract The electrical conductivity of ZnO films deposited on Si by the RF sputtering technique were modified by low-energy hydrogen implantation. Hydrogen ions were implanted at 25 keV into various ZnO films with doses ranging from 1 × 1016 to 1.0 × 1017 ions cm−2. High resolution quantitative hydrogen depth profiles of the un-doped and doped samples were measured non-destructively with elastic recoil detection analysis using a finely focussed 2.5 MeV4He+ ion beam impinging onto the samples at 20°. Structural and electrical measurements were carried out on the films by SEM, XRD and Hall probe measurements to explore the effects of hydrogen implantation. It was observed that the electrical conductivity is decreased by as much as an order of magnitude after hydrogen implantation. The films remained polycrystalline after implantation. The results suggest that reducing the n-type conductivity using low-energy hydrogen implantation without annealing may provide a pathway for passivating the donors for a possible subsequent formation of p-type acceptors by ion implantation.

Grain Boundaries in Multi-Seeded Melt-Grown Superconductors

Future applications of high temperature superconductors require bulk materials of a complex shape. The multi-seeded-melt-growth process (MSMG) represents a promising technique for obtaining qualitatively well oriented bulk materials with different kinds of shape. In the MSMG process, several seeds are placed on a precursor pellet, from which the growth of the bulk starts. A certain problem of the MSMG process is that grain boundaries become inevitable when the growth fronts of two neighboring seeds collide. These grain boundaries are responsible for a reduction of the critical currents and pose a problem for high current applications. By polishing the sample step by step, the influence of the grain boundaries was investigated by scanning Hall probe measurements and by the magnetoscan technique. Additionally, optical microscopy and electron microscopy were employed to investigate the details of the microstructure.

SU-FF-T-329: Enhancing Magnetically Collimated Electron Beams for Breast Cancer Treatments

Purpose: To develop magnetically collimated electron beams and to study their feasibilities for breast tumor bed irradiations. Method and Materials: We constructed an enhanced magnetic collimator with a beam aperture of 5.5 cm from permanent magnets using high-density Neodymium-Fe compounds. The collimator is cylindrical in shape and can be mounted at the end of an electron cone. The magnetic field is symmetrically aligned with the beam axis. The central axis of the magnetic field was calculated via finite element analyses and validated with Hall probe measurements. To implement magnetically collimated electrons for breast tumor bed irradiations, we measured and modeled the central axis depth doses and dose profiles of magnetically collimated electron beams on a flat water phantom. An empirical pencil-beam model was developed to calculate three-dimension dose distributions of magnetically collimated electron beams in CT studies. We implemented the dose model for breast tumor bed treatment cases. Results: Based on the dose distributions of the fixed and the arc beams of magnetically collimated electrons on the flat water phantom, the entrance dose is significantly lower than the conventional electrons (< 30% of the maximum dose). When treating the breast tumor bed, conformal dose distributions to the tumor volume are achieved with magnetically collimated electrons in contrast to single enface beam dose distributions. Isodose surface such as 90% of the maximum dose can be used to fully cover a target volume at depth. Conclusion: It is feasible to produce magnetically collimated electron beams with appropriate beam apertures. Conformal dose distributions from magnetically collimated electrons potentially create new regimens for breast tumor bed treatments.

Comparison and analysis of Hall probe scanning, magneto-optical imaging and magneticknife measurements of Bi-2223/Ag tape

Magneto-optical (MO) imaging, Hall probe measurements and the magnetic knife methodwere used for sample quality studies of multi-core Bi-2223/Ag tape with a columnar60-filament structure made by the TIRT (tape in rectangular tube) process. While themagneto-optical imaging was made at 25 K and without transport current, Hall probe andmagnetic knife measurements were performed for the sample carrying a direct current at77 K.The MO image recorded at 25 K and external field 30 mT has shownnon-uniform flux distribution in the three filament columns. 2D field profileswere analysed by the finite element method assuming ideal filament homogeneity.Bz(x) was computed analytically with an assumption that current density flows straight throughthe cross-section of the filament columns. The best agreement of computed profiles withmeasured ones is achieved for equal currents flowing in all filament zones. According to themagnetic knife measurements, current density is lowest in the centre column, but in thismethod the magnetic self-field is hardly influencing the current distribution. A betterunderstanding of such experimental differences is obtained with 3D modelling. Threedifferent cases of meandering transport current were studied computationally in order toexplain the measured results. Finally, we discuss the advantages and drawbacks of themethods used and present how they can together produce the most complex information.

Magnetic flux penetration into melt-textured superconductors

Scanning Hall probe measurements were made on melt-textured Y-123 monoliths undersequentially increasing activation fields. The remnant field profiles are compared tomeasurements of the local response of the superconductor to a small magnetic field inducedby a permanent magnet (‘magnetoscan’). Optical microscopy is employed to relate the fluxpatterns with the microstructure of the sample.

Measuring the Magnetic Field in the CMS Steel Yoke Elements

Flux loops and Hall probes are being installed on selected segments of the steel flux return of the 4 T solenoid of the compact muon solenoid (CMS) detector under construction at CERN (European Center for Nuclear Research). This steel also serves as part of the muon detection system of CMS and accurate characterization of the magnetic flux density in the steel as elsewhere in the detector is required. Voltages induced in the flux loops during fast discharge of the solenoid will be sampled and integrated to measure the change in average flux density in the steel during the discharge. Hall probes mounted on the surface of the steel segments will provide information about the fields internal and external to the steel. In the laboratory work reported herein small iron discs with flux loops on their peripheries and Hall probes on their flat surfaces are magnetized between the pole tips of a laboratory standard magnet and controlled power supply. The voltages induced in the flux loops during charging and discharging of the magnet are integrated and compared with the Hall probes which sample the fields immediately external to the discs. The experimental work reported here will provide interpretation of the flux coil and Hall probe measurements from the CMS magnet when it is commissioned in 2005.

Analysis of the vortex dynamics in high- Tc superconducting thin films with regular arrays of artificial defects

The dynamics of vortices in circular YBa 2Cu 3O 7− x thin films with and without arrays of antidots was experimentally examined via local Hall probe measurements. To detect changes in local field values and time evolution of magnetic field profile inside the samples, four Hall probes were placed at different radial positions and simultaneously read out. Effective pinning energy of thermally activated flux creep was determined in a range of temperatures from relaxation measurements on samples without antidots and with antidots of different sizes (1.4–2.4 μm). The results demonstrate that magnetic flux penetration is facilitated by presence of antidots.

Current distribution in Y/sub 1/Ba/sub 2/Cu/sub 3/O//sub 7-γ/ disk-shaped thin film by scanning hall probe measurements

We have measured the shielding and trapped (remanent) magnetic field profiles both perpendicular (H/sub z/(r)) and parallel (H/sub r/(r)) to the planes of a zero-field-cooled disk-shaped YBCO thin film. H/sub z/(r) and H/sub r/(r) were obtained using transverse and radial scanning Hall probes at various temperatures ranging from 30 K to 70 K and at small magnetic fields up to 750 G. The current distributions in the samples were inferred from a direct comparison of our experimental profiles to the theoretical ones obtained using the critical state models by Mikheenko et al. (1993) and Zhu et al. (1993). The models are in good agreement with the experimental trapped field profiles; however field dependence of the current should be taken into account for the shielding profiles.

Magnetic measurements and tuning of the LCLS prototype undulator

A full-length 30-mm-period prototype undulator segment has been constructed for the Linac Coherent Light Source (LCLS) project. It is a 3.42-m-long planar permanent magnet hybrid device with a fixed gap near 6.35mm. The magnetic structure was assembled from a set of 500 magnets that were purchased from Shin-Etsu. A subset of the magnets was measured in a Helmholtz coil system to check the accuracy of the vendor's measurements of the magnetic moments. A specially designed half-period fixture was used for Hall probe measurements of the magnetic field of all the magnet blocks. The magnets were sorted based on the vendor's data of the total magnetic moment and our measurements of the field integrals. The trajectory calculated from Hall probe measurements of the assembled device was within the specified tolerance (deviation <2μm in both planes) but showed a trajectory offset and kick at the entrance. A small number of shims was applied to minimize the trajectory offset and kick and to increase the amplitude of the spontaneous radiation. The measured maximum gap taper at the ends of the device and the temperature dependence of the effective magnetic field are also reported. The tuned device met all specified tolerances.

Current distribution maps in large YBCO melt-textured blocks

We report on the progress on computing the current maps from the magnetic induction in bulk samples of YBCO melt-textured compounds. Typical sample size is in the cm range and induction maps B z ( x, y, z) are obtained by Hall probe measurements. Currents are obtained by solving the inverse Biot–Savart problem. No assumption is required on the number of domains or geometry of the sample; the only constraint in the model is that the current circulation should be confined in the basal plane and uniform in the sample thickness. A current map with a resolution of 0.25–0.5 mm is typically obtained. Critical tests of the stability and accuracy of the current flow solutions have been performed. We have applied the algorithms to remanent flux maps obtained after both FC and ZFC magnetization processes. The algorithms can be readily used and thus can constitute a helpful tool for superconductivity engineers. The use of our software implementation is available free of charge at http://jaumetor.upc.es:8080/caragol.

Measurements of magnetic field fluctuations using an array of Hall detectors on the TEXTOR tokamak

Hall detectors have been used to measure the magnetic field together with its’ fluctuations in the boundary of a tokamak. The results show, that the measurements which have been performed so far, mainly by use of coils together with subsequent integration, either on-line or later by computer, can be substituted by Hall probe measurements giving the desired value of B directly. Because the integration of the coil signal becomes more and more difficult with long pulses, Hall detectors may give advantages in future fusion devices. We implemented a stack of nine Hall detectors mounted on three planes on a rod in such a way, that the three components of the magnetic field can be measured. To avoid capacitive and charge pickup from the plasma, the probes are electrically shielded. The damping due to skin effect within this shield has been taken into account. The probes have been calibrated using a known magnetic field of a straight wire driven with a LC bank. This field has been precisely measured with a Rogowski coil. The dependence of the Hall coefficient on the frequency has been measured and the pickup in the feeds due to Ḃ has been withdrawn from the results. We demonstrate the method with measurements on the TEXTOR tokamak, where we could clearly detect the small stray fields associated with magnetohydrodynamic (MHD) fluctuations. On TEXTOR we have been able to detect the MHD activity preceding discharge disruptions as well as the precursors of the so called sawteeth. The results are compared to those of other diagnostics on TEXTOR as, e.g., magnetic loops and electron cyclotron emission, and they do well compare.

Phase transitions in isolated vortex chains

In very anisotropic layered superconductors (e.g. Bi$_2$Sr$_2$CaCu$_2$O$_x$) a tilted magnetic field can penetrate as two co-existing lattices of vortices parallel and perpendicular to the layers. At low out-of-plane fields the perpendicular vortices form a set of isolated vortex chains, which have recently been observed in detail with scanning Hall-probe measurements. We present calculations that show a very delicate stability of this isolated-chain state. As the vortex density increases along the chain there is a first-order transition to a buckled chain, and then the chain will expel vortices in a continuous transition to a composite-chain state. At low densities there is an instability towards clustering, due to a long-range attraction between the vortices on the chain, and at very low densities it becomes energetically favorable to form a tilted chain, which may explain the sudden disappearance of vortices along the chains seen in recent experiments.