External Magnetic Field Measurements Research Articles

Thermomagnetic instability in superconducting lead-porous glass nanocomposites

In this paper we examine the mechanisms leading to thermomagnetic instability in nanostructured materials and study their regularities. The appearance of thermomagnetic instabilities in the superconducting state of various materials remains one of the problems of applications of superconductors. We studied the temperature and the magnetic field dependences of the magnetization m(T, H) and heat capacity C(T, H) of a nanocomposite consisting of interconnected lead filaments embedded in nanoporous glass (Pb-PG) with filament diameter d = 7 nm. Porous glass contains an arbitrarily orientated multiply connected system of pores of approximately the same size; lead in the nanocomposite forms a replica of the pore system. Thermomagnetic instability was previously observed in this material in superconducting state at T ≤ 5 K on m(H) dependencies in magnetization studies. We established that high enough external heater power P used during the heat capacity measurements in external magnetic field C(H) at T ≤ 5 K can initiate thermomagnetic instability and penetration of magnetic flux into the nanocomposite. The thermomagnetic instability is generally preceded by a sequence of small heat release events with the same magnitude as P. Another sequence of small heat release events is observed in the region close to Hc2. These events are presumably linked to the thermomagnetic instability process and are caused by small redistribution of magnetic flux in the nanocomposite.

Vanishing RKKY interactions in Ce-based cage compounds

We report the results of thermodynamic measurements in external magnetic field of the cubic Ce-based cage compounds CeT 2Cd20 (T = Ni,Pd). Our analysis of the heat-capacity data shows that the Γ7 doublet is the ground state multiplet of the Ce3+ ions. Consequently, for the Γ7 doublet it can be theoretically shown that the Ruderman–Kittel–Kasuya–Yosida interaction between the localized Ce moments mediated by the conduction electrons, must vanish at temperatures much lower than the energy separating the ground state doublet from the first excited Γ8 quartet. Our findings provide an insight as to why no long range order has been observed in these compounds down to temperatures in the milliKelvin range.

An Innovative PEMFC Magnetic Field Emulator to Validate the Ability of a Magnetic Field Analyzer to Detect 3D Faults

An original non-invasive methodology of the fuel cell diagnosis is proposed to identify different positions of the faults in Proton Exchange Membrane Fuel Cell (PEMFC) stacks from external magnetic field measurements. The approach is based on computing the external magnetic field difference between normal and faulty PEMFC operating conditions. To evaluate the external magnetic field distribution, in this paper, we propose an improved design of the magnetic field analyzer. This analyzer amplifies the magnetic field around the cell to perform an accurate detection of the fault position. Moreover, the main contribution of this work is represented by conceiving and implementing a 3D multi-physical current distribution emulator of a proton exchange membrane fuel cell. The new concept of a proton exchange membrane fuel cell emulator has been specially designed to emulate the magnetic field of a real fuel cell stack. This emulator concept is also beneficial for a new model of the fuel cell, which implies a multi-physical coupling between electrochemical electric conduction and the generated magnetic field. Finally, finally, the numerical model and the emulator have been involved in the realization of numerical simulations and experimental analysis to prove the ability of the system to detect and localize 3D faults.

Design of the self-oscillating loop of the optically pumped cesium magnetometer

The self-oscillating loop is an important part of the optically pumped cesium magnetometer, and its working characteristics directly determine the accurate measurement of external magnetic field. The design of the self-oscillating loop has been discussed in this paper, including a signal conditioning circuit, a phase shifter and a frequency meter. It can be used to precisely improve the accuracy of resonance signal in a wide range of frequencies from 50 kHz to 350 kHz. The relative error of our system is less than 0.5×10−6 and it has a good prospect in the optically pumped cesium magnetometer.

External Magnetic Field Measurements in UME Kibble Balance

The new definition of kilogram in terms of the fixed value of Planck constant ensures the long-term stability of SI mass unit and enables traceability from more than one source. Kibble balance experiments offer an effective primary realization method for the new definition of kilogram. Kibble Balance apparatus operating at National Metrology Institute of Turkey is designed with a stationary coil and an oscillating magnet. In contradistinction to traditional moving coil Kibble balance experiments, external magnetic field brings an asymmetry between the Ampere’s law of force and the Faraday’s law of induction in moving magnet experiments. In this paper, we develop a method based on the external magnetic flux density difference measurements in vertical direction to take into account the effect of the external magnetic field on the realization of kilogram. The proposed model in this approach fits well with the data such that the kilogram realization requirement is met within the accuracy of the measuring instrument.

Hyperfine interactions in Fe/Co-B-Sn amorphous alloys by Mössbauer spectrometry

Hyperfine interactions in Fe78B15Sn7 metallic glass with Co/Fe substitution were investigated by Mössbauer spectrometry. As-quenched samples were prepared by planar flow casting and subsequently annealed at 673 K for 30 min which is in the region of phase transformations determined by differential scanning calorimetry. Magnetic thermogravimetry unveiled increase in Curie temperature after Co substitution. Structural analysis was performed by X-ray diffraction and transmission electron microscopy. The results show formation of primary bcc-Fe(Co) phase with proceeding nanocrystallization and indicate supersaturation of this phase with Sn. Hyperfine interactions between nuclei and electron shells were investigated by Mössbauer spectrometry at different temperatures and in external magnetic field. Modifications of the short-range order are demonstrated mainly by deviations in the positions of the net magnetic moment. They are driven by nanocrystallization as well as temperature of measurement and/or presence of external magnetic field.

Fault Identification on a Fuel Cell by 3-D Current Density Reconstruction From External Magnetic Field Measurements

An original approach of fuel cell diagnosis is presented. It is based on the solving of an inverse linear problem linking the magnetic field signature outside of the fuel cell to the current density distribution inside. The searched solution is a linear combination of conservative current distribution obtained by a set of electrokinetic problems solved by a finite face element method. As the problem is ill-posed, the solution is stabilized using a truncated singular value decomposition. The approach is validated to reconstruct a 3-D current density distribution in a stack simulator and in a fuel cell stack operating in the laboratory conditions.

2D and 3D fault basis for fuel cell diagnosis by external magnetic field measurements

An original approach used for the identification of faults in fuel cell stacks is presented. It is based on the 3D reconstruction of the current density from external magnetic field measurements which is an ill-posed magnetostatic linear inverse problem. A suitable and original current density and magnetic field basis are proposed in order to define both local and global faults on a fuel cell stack. The inverse problem is regularized by truncated singular value decomposition (SVD) to ensure the uniqueness of the solution.

Magnetic Properties and Exchange Bias Effect of Cu and Cr doped Sm2Co17 Alloy Film

Cu and Cr-doped Sm2Co17 films were synthesized using pulsed laser deposition method with the substrate of Si(100), and our results show that Cu and Cr incorporation can reduce the grain size and increase the coercivity of Cu and Cr-doped Sm2Co17 films. Furthermore, incorporations of Cu and Cr will introduce a positive effect for the exchange bias of Cu and Cr-doped Sm2Co17 films. The magnetic measurement of external magnetic field and zero-field-cooled (FC-ZFC) under the temperature ranging from 10K to 300 K prove that the critical value of FM-AFM transform is ~80 K in the Cu and Cr-doped Sm2Co17 films, which showed that the exchange bias origins from the coupling competition between ferromagnetic phase and antiferromagnetic phase.

Cu nuclear magnetic resonance study of charge and spin stripe order inLa1.875Ba0.125CuO4

We present a Cu nuclear magnetic/quadrupole resonance study of the charge stripe ordered phase of LBCO, with detection of previously unobserved ('wiped-out') signal. We show that spin-spin and spin-lattice relaxation rates are strongly enhanced in the charge ordered phase, explaining the apparent signal decrease in earlier investigations. The enhancement is caused by magnetic, rather than charge fluctuations, conclusively confirming the long-suspected assumption that spin fluctuations are responsible for the wipeout effect. Observation of the full Cu signal enables insight into the spin and charge dynamics of the stripe-ordered phase, and measurements in external magnetic fields provide information on the nature and suppression of spin fluctuations associated with charge order. We find glassy spin dynamics, in agreement with previous work, and incommensurate static charge order with charge modulation amplitude similar to other cuprate compounds, suggesting that the amplitude of charge stripes is universal in the cuprates.

Anisotropic properties of a single superconducting CaCuO2/SrTiO3 interface

The transport properties of CaCuO2/SrTiO3 single interfaces are studied by resistance versus temperature measurements in external magnetic fields. The superconducting anisotropy where and are the superconducting coherence lengths parallel and perpendicular to the interface, respectively, shows values higher than that previously obtained for CaCuO2/SrTiO3 superlattices deposited in the same conditions. The larger anisotropy, observed for the single interfaces, indicates that the charge carriers are confined inside a thin superconducting layer next to the interface rather than spread throughout the whole CaCuO2 block. The activation energy and the irreversibility line confirm this hypothesis, suggesting that quasi two-dimensional transport is dominant in this system. The interpretation of the experimental data in the framework of the Berezinskii–Kosterlitz–Thouless theory confirms that the thickness of the superconducting sheet layer is about 1 nm, corresponding roughly to two CaCuO2 unit cells.

Suitability of Bundle Approximation in AC Loss Analysis of NbTi Wires: Simulations and Experiment

Multifilamentary NbTi wires for ac applications are manufactured by embedding filament bundles into a metal matrix. In this stage of the manufacturing process, it is possible to affect the layout of the cross section and to choose whether to use few large or many small bundles in order to achieve a certain amount of filaments. All in all, up to 100 000 filaments are attainable for wire having the diameter of 1 mm. In this paper, ac loss measurements in external magnetic field on differently stacked NbTi samples are described. The measurements were performed in a LHe-cooled cryostat. The amplitude of the external field was varied between 250 mT and 3 T at frequencies of 0.02 and 0.12 Hz. We discuss possibilities to simulate the losses with finite element method. In particular, we concentrate on the filament bundle approximation and the possibilities to exploit it in the research and development process of new NbTi wires. In this approach, the filament bundles are considered as a homogenous mixture of matrix and superconducting filaments. According to the results, the bundle approximation greatly overestimates the losses. Furthermore, it should not be used for comparing, e.g., two wire structures where one has bundles of different size than the other. However, when considering how to situate the bundles on the cross section to achieve minimal ac loss, the bundle approximation can be a useful tool

PEMFC stack diagnosis based on external magnetic field measurements

A new diagnosis method based on the measurement of the magnetic field surrounding a PEMFC stack was developed. The method relies on the measurements of the magnetic field signature generated by the current distribution in the PEMFC. It is possible to estimate an averaged 2D current density identification by solving an inverse problem, based on the relationship between the currents and a magnetic field. In this work, 30 sensors fixed around the stack are used to perform the cartography within a second. This technique was implemented on a PEMFC stack consisting of 30 cells with a large active area of 220 cm 2 and was capable to capture the current density change from a healthy state to a faulty one. Our approach is robust and allows us to distinguish between either membrane drying or electrode flooding by varying the air stoichiometry.

Magnetic force microscopy study of domain structures in magnetic films

Magnetic force microscopy is used to investigate two different types of samples: thin metal films and ferrite garnet films. It is pointed out for garnet films that magnetic force microscopy allows us only to judge the domain structure of surface layers. Problems associated with conducting measurements in external magnetic fields, the effect of the magnetic field of the probe on the investigated domain structure, and using magneto-polarized optics in combination with magnetic force microscopy are considered.

Current Distribution Identification in Fuel Cell Stacks From External Magnetic Field Measurements

This paper presents a new non invasive technique for fuel cell diagnosis. The method relies on the measurements of the magnetic field signature generated by a working fuel cell. Knowing the relationship between currents and magnetic field, it is possible to estimate the current density by solving an inverse problem. This problem being ill-posed, original current and sensors basis are proposed to improve the reconstruction process. The experimental set-up remains simple, based on few fixed sensors and shows a good efficiency.

A Multiscale 3D Model of the Vacuum Arc Remelting Process

A three-dimensional, transient, multiscale model of the VAR process is presented, allowing novel simulations of the influence of fluctuations in arc behavior on the flow and heat transfer in the molten pool and the effect this has on the microstructure and defects. The transient behavior of the arc was characterized using the external magnetic field and surface current measurements, which were then used as transient boundary conditions in the model. The interactions of the magnetic field, turbulent metal flow, and heat transfer were modeled using CFD techniques and this “macro” model was linked to a microscale solidification model. This allowed the transient fluctuations in the dendritic microstructure to be predicted, allowing the first coupled three-dimensional correlations between macroscopic operational parameters and microstructural defects to be performed. It was found that convection driven by the motion of the arc caused local remelting of the mushy zone, resulting in variations in permeability and solute density. This causes variations in the local Rayleigh number, leading to conditions under which freckle solidification defects will initiate. A three-dimensional transient tracking of particle fall-in was also simulated, enabling predictions of “white spot” defects via quantification of the trajectory and dissolution of inclusions entering the melt.

Experimental Evidence for a Transient Tayler Instability in a Cylindrical Liquid-Metal Column

In the current-driven, kink-type Tayler instability (TI) a sufficiently strong azimuthal magnetic field becomes unstable against nonaxisymmetric perturbations. The TI has been discussed as a possible ingredient of the solar dynamo mechanism and a source of the helical structures in cosmic jets. It is also considered as a size-limiting factor for liquid metal batteries. We report on a liquid metal TI experiment using a cylindrical column of the eutectic alloy GaInSn to which electrical currents of up to 8kA are applied. We present results of external magnetic field measurements that indicate the transient occurrence of the TI in good agreement with numerical predictions. The interference of TI with the competing large-scale convection, resulting from Joule heating, is also discussed.

Magnetic force microscopy measurements in external magnetic fields—comparison between coated probes and an iron filled carbon nanotube probe

We performed magnetic force microscopy (MFM) measurements in external magnetic fields parallel to the sample plane to qualitatively study their effect on the magnetization of different kinds of MFM probes. As a test structure we used an array of rectangular ferromagnetic thin film elements aligned with the external magnetic field direction. MFM images were taken while the field was increased stepwise to monitor the onset of a tilt in the MFM probe magnetization. Three different probes were investigated: a conventional pyramidal probe coated with 40 nm of CoCrTa, a coated high aspect ratio MFM probe, and a MFM probe based on an iron filled carbon nanotube (FeCNT). The results show that the magnetization of the pyramidal probe is only stable in in-plane fields of up to ∼60 mT, in larger fields a considerable in-plane component of the tip magnetization is observed. This makes it difficult to distinguish the effect of the external field on the sample from those on the MFM probe. The coated high aspect ratio probe is at first stable up to ∼200 mT and then shows a history dependent behavior, in the second sweep the onset of the magnetization tilt is already visible at ∼100 mT. With the FeCNT probe, no tilt of the probe magnetization could be observed in in-plane fields up to the experimental limit of 230 mT. Due to the large shape anisotropy of the enclosed iron nanowire its direction of magnetization stays mainly oriented along the long axis even in moderate fields that deviate from this easy axis. Consequently, this probe is best suited for MFM measurements in external fields.

Magnetic properties of a linear trinuclear manganese compound [Mn3(PhCO2)6(Bipy)2] · H2O

The reaction of the trinuclear oxo-centered mixed-valence complex [Mn3O(O2CPh)6(Py)2(H2O)] with 2,2′-bipyridyl (Bipy) and another potential tripodal ligand affords the title compound [Mn3(PhCO2)6(Bipy)2] · H2O in good yield. The X-ray crystallographic diffraction study reveals that three mangenese ions are arranged in a linear mode with Mncenter-Mnterminal and Mnterminal-Mnterminal diatances of 3.588 and 7.176 A, respectively. Molar magnetic susceptibility of the compound gradually decreases from 12.23 (300 K) to 4.45 cm3 K mol−1 (2 K). Taking into account the structure of this compound, the data in the 2.0–300 K range were fit to the appropriate theoretical expression to give J = −2.73 cm−1, ρ = 2.07%, Na = −0.0004 cm3 mol−1, g = 1.992, and R2 = 0.99996. The magnetization versus external magnetic field measurements at 2 K shows that the ground state is ST = 5/2.

Superconductivity and enhancement of critical current density of Ta-substituted TlSr1212 with addition of nanosize MgO

In this paper, the authors report the effect of nanosize MgO particles addition on superconducting and transport critical current density Jc of TlSr1212 superconductors prepared from Tl0·9Bi0·1Sr1·95Ta0·05Ca0·9Y0·1Cu2O7 starting composition using conventional solid state synthesis. X-ray diffraction analyses of Tl0·9Bi0·1Sr1·95Ta0·05Ca0·9Y0·1Cu2O7+xMgO (x=0 to 0·8 wt-%) showed formation of dominant 1212 phase (>90 vol.-%) together with minor 1201 phases. Undoped TlSr1212(x=0) was observed to superconduct with a zero-resistance critical temperature Tc zero of 65 K. The x=0·2 sample recorded the highest Tc zero (80 K) but for higher x, Tc zero was gradually suppressed. The effect of the MgO addition also caused Jc enhancement in zero field with highest value at x=0·2. Using the self-field approximation model, the variation of Jc with temperature for x=0 to 0·4 samples indicate that MgO addition reduces the characteristics length Lc associated with the pinning force. Jc measurements in external magnetic field of 0–0·8 T showed smallest drop in Jc at x=0·2. The Jc enhancements in self-field and in the external fields are suggested to be due to increased flux pinning of the samples as a result of the MgO addition.