References of "Fagnard, Jean-Francois"
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See detailNon-destructive investigation of hybrid of ferromagnet /(RE)BCO large grain bulks by flux extraction magnetometry and levitation force
Vanderbemden, Philippe ULiege; Philippe, Matthieu; Wera, Laurent ULiege et al

Conference (2017, July 10)

This work deals with bulk, large grain superconductors used as permanent magnet for rotating machines or levitation applications. It has recently been shown that the magnetic properties of bulk large ... [more ▼]

This work deals with bulk, large grain superconductors used as permanent magnet for rotating machines or levitation applications. It has recently been shown that the magnetic properties of bulk large grain superconductors can be improved easily by attaching a short section of a soft ferromagnetic material (F) to one of the faces of the bulk superconductor (S), thereby producing a hybrid F/S structure [1]. Here we investigate the contactless determination of the magnetic behavior of such structures using a recently constructed bespoke magnetometer based on the flux extraction technique [2]. This device allows magnetic moments as large as 1 Am² to be measured at 77 K and accommodates large bulk samples up to 20 mm diameter. This extends significantly the accessible measurement range of “off-the shelf” magnetometers. Unlike techniques based on recording the distribution of flux at the surface of the sample, the measured signal is representative of the superconducting currents flowing across the entire volume of the sample. In the present work we examine the properties of permanently magnetized superconductors and hybrid structures, and measure the irreversible demagnetization of these structures when they are subjected to magnetic field cycles that are not parallel to their magnetization. We also investigate the levitation behavior of hybrid structures subjected to the non-uniform field of a permanent magnet or a combination of permanent magnets used as guideway for levitation applications, and compare the results to those obtained with a bulk superconductor alone. This abstract is for the special session on bulk superconductivity. References: [1] Egan R. et al., Rev. Sci. Instrum. 86 (2015) 025107 [2] M. P. Philippe et al., Supercond. Sci. Technol. 28 (2015) 095008 Acknowledgments: We greatly acknowledge Nippon Steel & Sumitomo Metal Corporation for providing bulk large grain GdBa2Cu3O7 (GdBCO) samples. [less ▲]

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See detailMagnetic Shielding Above 1 T at 20 K With Bulk, Large Grain YBCO Tubes Made by Buffer-aided Top Seeded Melt Growth
Wera, Laurent ULiege; Fagnard, Jean-François ULiege; Kumar, Numbury Devendra et al

in IEEE Transactions on Applied Superconductivity (2017), 27(4),

YBCO tubes of ~ 10 mm diameter closed at one extremity were engineered by a Buffer-Aided Top Seeded Melt Growth fabrication process (BA-TSMG). These tubes can act as efficient “dc” magnetic shields and ... [more ▼]

YBCO tubes of ~ 10 mm diameter closed at one extremity were engineered by a Buffer-Aided Top Seeded Melt Growth fabrication process (BA-TSMG). These tubes can act as efficient “dc” magnetic shields and are observed to reduce axial flux densities of 1.5 T by a factor of 100 at 20 K. Such performances are comparable in magnitude to the record threshold inductions reported for bulk MgB2 and Bi-2212 materials at lower temperatures. Magnetic shielding measurements for open and closed tubes at 77 K also show that the presence of the cap improves substantially the shielding performance at the closed extremity since it reduces the penetration through the open end. This fabrication technique is extremely promising for shielding “dc” stray fields generated by HTS magnets operated in a temperature range obtained by cryocoolers, liquid hydrogen (20 K) or liquid neon (27 K). [less ▲]

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See detailBehaviour of bulk superconducting trapped field magnets subjected to magnetic fields with a transverse component
Vanderbemden, Philippe ULiege; Caps, Hervé ULiege; Vanderheyden, Benoît ULiege et al

Conference (2016, November 10)

Bulk superconductors can be used as trapped field magnets with unprecedented performances. Such trapped field magnets find a variety of engineering applications, in particular for brushless rotating ... [more ▼]

Bulk superconductors can be used as trapped field magnets with unprecedented performances. Such trapped field magnets find a variety of engineering applications, in particular for brushless rotating machines and levitation devices. Compared to conventional permanent magnets, however, the magnetic field generated by a superconducting trapped field magnet is much more sensitive to the presence of an external field that is not perfectly aligned with the superconductor magnetic moment, i.e. when the external field has a transverse component. A situation where the superconductor experiences a magnetic field with a transverse component is likely to happen several times during running operation of the device. As an example, in a “trapped flux” synchronous machine, any variation of the load of the shaft may lead to a slight misalignment of the magnetic moment of the superconductor (placed in the rotor) with respect to the rotating field (generated by the stator). In such a situation, the superconductor is subjected to a rotating field, and the situation is likely to be different from a situation where the superconductor is subjected to a pure transverse field. In the present work, we aim at examining different scenarios (i.e. transverse or rotating) where the trapped field magnet is subjected to a transverse component. These configurations are examined both experimentally and numerically. The numerical model is used to determine the distribution of currents in the cross-section of the superconductor. We examine the conditions where the rotating field leads to a partial remagnetization of the sample and is therefore less detrimental that the application of a pure transverse field. We also show that the magnetic moment of the sample may behave differently that the field at the surface of the superconductor. Finally, we investigate the behavior of structures where the superconductor is attached to a thin ferromagnetic disk, and show the beneficial effect of such hybrid structures compared to the superconductor alone. Acknowledgements : The research was funded through the University of Liège (ULg) and an ARC grant for Concerted Research Actions, financed by the French Community of Belgium (Wallonia-Brussels Federation), under reference ARC 11/16-03. We greatly acknowledge M. Morita, S. Nariki and H. Teshima from Nippon Steel & Sumitomo Metal Corporation, Futtsu, Chiba. [less ▲]

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See detailBulk high-temperature superconducting hollow cylinders used for magnetic shielding: effect of an initial trapped field on the shielding performances.
Wera, Laurent ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Poster (2016, November 09)

Bulk High-Temperature Superconductors (HTS) can be used as efficient passive magnetic shields with performances exceeding those of conventional ferromagnetic materials. The most common geometry for a ... [more ▼]

Bulk High-Temperature Superconductors (HTS) can be used as efficient passive magnetic shields with performances exceeding those of conventional ferromagnetic materials. The most common geometry for a superconducting shield is a bulk hollow cylinder. In this work, we investigate the magnetic shielding properties when the tube is not in the virgin state. Such a situation happens in practice when the tube has been initially subjected to a first magnetic field. The induced superconducting currents that are flowing in the tube generate a trapped magnetic moment which may deteriorate the shielding of a second field. Here we study experimentally how an initial trapped moment affects the shielding of a magnetic field, when they are both perpendicular to each other. In particular, we focus on the effect of an axial trapped moment on the transverse shielding performances and on the effect of a transverse trapped field on the axial or the transverse shielding performances. Finally, we show how the pristine state of the tube can be restored when applying an external field of adequate amplitude. The consequence for practical applications is that it is possible to recover shielding properties similar to those obtained in the virgin state without heating up the shield above its critical temperature TC. [less ▲]

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See detailMagnetic shielding above 1 T at 20 K with bulk, large grain YBCO tubes made by Buffer-aided Top Seeded Melt Growth.
Wera, Laurent ULiege; Fagnard, Jean-François ULiege; Kumar, Nambury Devendra et al

Poster (2016, September 07)

A superconducting tube can be used as an efficient magnetic shield. The best shielding performance levels are usually achieved when the tube is closed at one or both extremities. The purpose of the ... [more ▼]

A superconducting tube can be used as an efficient magnetic shield. The best shielding performance levels are usually achieved when the tube is closed at one or both extremities. The purpose of the present work is to study experimentally the shielding performance of YBCO tubes obtained by Buffer-aided Top Seeded Melt Growth fabrication process (BA-TSMG). This fabrication process enables the tube to be closed at one extremity by a cap containing the seed and there is no air gap between the cap and the tube. The shielding effectiveness is characterized by two parameters: (i) the shielding factor, defined as the ratio between the applied magnetic induction and the local magnetic induction measured inside the shield, and (ii) the threshold induction, i.e. the applied magnetic induction above which a given value of the shielding factor cannot be achieved. The magnetic shielding performances of tubes with different geometry are characterized at 77 K in liquid nitrogen. Further magnetic shielding measurements are carried out on one tube at various temperatures ranging from 20 K to 77 K using a Physical Property Measurement System (PPMS). The tubes are subjected to a quasi-static (“DC”) uniform magnetic field. A Hall probe measures the axial component of the local magnetic induction along the axis of the tube as a function of the applied magnetic induction. In order to investigate how the cap prevents the magnetic flux from penetrating inside the tube, we also characterize open tubes where the cap is removed and compare their properties to those of closed tubes. Magnetic shielding measurements show that the threshold induction increases by a factor of 9 as temperature decreases from 77 K to 20 K. Measurements also show that the presence of the cap improves the shielding performance at the closed extremity of the order of 1000 as it reduces the penetration through the open end. Near the closed extremity, a threshold induction of 1.5 T was reached at 20 K. To our knowledge this threshold induction is the best value reported so far at 20 K, and is comparable in magnitude to the record threshold inductions reported for bulk MgB2 and Bi-2212 materials at lower temperatures. These results give evidence that efficient magnetic shields can be obtained with this fabrication technique. [less ▲]

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See detailNon-destructive measurements of the volume magnetic behavior of large bulk GdBCO single domains and ferromagnet/superconductor structures
Vanderbemden, Philippe ULiege; Egan, Raphael; Morita, Mitsuru et al

Conference (2016, September 05)

The present work deals with magnetic measurements on bulk large grain GdBa2Cu3O7 (GdBCO) single domains using a recently constructed bespoke magnetometer [Rev. Sci. Instrum. 86 025107 (2015)]. The device ... [more ▼]

The present work deals with magnetic measurements on bulk large grain GdBa2Cu3O7 (GdBCO) single domains using a recently constructed bespoke magnetometer [Rev. Sci. Instrum. 86 025107 (2015)]. The device enables the measurement of magnetic moments as large as 1 Am^2 (1000 emu) on large bulk samples up to 20 mm diameter at 77 K. This extends significantly the accessible measurement range of “off-the shelf” magnetometers (e.g. SQUID, VSM). Unlike Hall probe mapping, the measured signal is representative of the superconducting currents flowing across the entire height of the sample. Since the device is based on a flux extraction technique and does not include any Hall probe, another advantage is that no gap between a Hall probe and the sample needs to be taken into account to interpret the results. In the present work we first show how the dimensions of the sensing coils and integration bounds are chosen in such a way the magnetometer is sensitive to dipolar magnetic moment of the sample, while being insensitive to moments of higher order. Next we study the relaxation of the trapped magnetic moment under various magnetization processes. The sensitivity of the device allows small variations of m (e.g. ~ 2% for 1 hour) to be recorded. We show experimentally that the relaxation of the magnetic moment of the whole sample is reduced strongly if the superconductor is not fully magnetized, either in field cooled (FC) or zero-field cooled (ZFC) conditions. The practical consequence is that an abnormally slow magnetic relaxation (large E-J power law exponent n) under a conventional magnetization procedure indicates that the bulk superconductor could be magnetized further. In a second set of experiments, we study the increase of magnetic moment of a bulk superconductor when it is attached to a soft ferromagnetic disk. We also examine the irreversible demagnetization of these structures when subjected to crossed magnetic fields and compare to the magnetic moment of a classical (Nd-Fe-B) permanent magnets in the same conditions. Acknowledgements: This work is part of an "Action de Recherches Concertees" grant (ARC 11/16 -03) from the "Communaute Francaise de Belgique". [less ▲]

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See detailSuperconductors used as trapped field magnets : investigation of hybrid structures combining superconducting and ferromagnetic materials
Vanderbemden, Philippe ULiege; Fagnard, Jean-François ULiege; Caps, Hervé ULiege et al

Conference (2016, August 30)

In addition to their unique ability to carry electric currents without loss, superconductors are able to trap significant magnetic flux densities at low temperature. In so-called type-II irreversible ... [more ▼]

In addition to their unique ability to carry electric currents without loss, superconductors are able to trap significant magnetic flux densities at low temperature. In so-called type-II irreversible superconductors, these properties are due to the pinning of individual flux lines (vortices) by defects at the nanoscale level. In this talk the influence of pinning on the magnetic properties of bulk superconductors will be explained and illustrated, with the emphasis placed on how pinning can be exploited to produce “trapped field magnets” for practical applications. We will then show how the properties of these trapped field magnets are enhanced when superconductors (SC) are combined to soft ferromagnetic (FM) alloys [1,2]. In particular we investigate the properties of macroscopic FM/SC/FM structures where the superconductor is sandwiched between two ferromagnetic sections, one of each circular face. In particular we show how the properties of the whole structure is influenced by the saturation magnetization and the geometry of the ferromagnetic material. References: 1. M P Philippe, J-F Fagnard, S Kirsch, Z Xu, A R Dennis, Y H Shi, D A Cardwell, B Vanderheyden, and P Vanderbemden, Magnetic characterisation of large grain, bulk Y–Ba–Cu–O superconductor–soft ferromagnetic alloy hybrid structures. Physica C: 502 (2014) 20-30. 2. M P Philippe, M D Ainslie, L Wera, J-F Fagnard, A R Dennis, Y H Shi, D A Cardwell, B Vanderheyden, and P Vanderbemden, Influence of soft ferromagnetic sections on the magnetic flux density profile of a large grain, bulk Y–Ba–Cu–O superconductor. Superconductor Science and Technology: 28 (2015) 095008. [less ▲]

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See detailComparison of the magnetization decay due to crossed or rotated magnetic field measured on a Ferromagnet disk/GdBCO pellet structure
Fagnard, Jean-François ULiege; Morita, Mitsuru; Teshima, Hidekazu et al

Poster (2016, August)

Superconducting (SC) bulk materials are commonly used as permanent magnets in synchronous rotating machines. When the machine is in operation, the bulk materials that are placed in the machine rotor can ... [more ▼]

Superconducting (SC) bulk materials are commonly used as permanent magnets in synchronous rotating machines. When the machine is in operation, the bulk materials that are placed in the machine rotor can experience a certain misalignment with the magnetic field generated by the stator. This situation is likely to happen during transients or non-ideal behavior of the machine: the angle between the stator field and the SC magnetization can change because of a sudden change of mechanical load or vibrations of the device. The consequence is that the SC pellet can be subjected to a transverse component of magnetic field which is likely to cause a significant demagnetization and therefore a decrease of the machine performances. In previous works, we already have studied the problem of a “pure” transverse magnetic field (also called “crossed field”) as well as the beneficial influence of a ferromagnetic (FM) disk placed on the top of the SC pellet. To this aim, a bespoke insertion tool for a PPMS was designed and constructed. The tool can be used to characterize a SC sample whose orientation is changed from a parallel to a perpendicular direction with respect to the magnetic field direction. Here, this insertion tool was used to rotate the sample at various angles regarding to the field direction. A GdBCO pellet of 9 mm in diameter and 5 mm in height was characterized at 77 K in various experimental conditions by measuring the magnetic induction below the centre of the sample surface. We compared the results obtained with the GdBCO pellet alone to those with a FM disk of various thicknesses attached on the top of the sample. A first set of experiments consisted in applying pure transverse magnetic field cycles of 50 mT and 100 mT at low frequency, the polarity of the field being either always the same (unipolar) or reversed after half of the period (bipolar). A second set of experiments was carried out by rotating the applied magnetic field (applied first in the pellet magnetization direction) in several steps to a given angle (up to 90°) and back to 0°. All these experiments were carried out after having fully magnetized the sample and recorded the magnetization decay due to the flux creep during 30 min. The results show that (i) the FM disk reduces the magnetization decay in both pure transverse and rotated magnetic field experiments, (ii) rotated magnetic fields have a less detrimental effect than the “pure transverse” magnetic field. This phenomenon is likely to be caused by a partial remagnetization of the sample when the angle of the magnetic field is reduced back to 0°. Finite element modelling was carried out to sustain the measurements of the local magnetic induction and gives additional information on the supercurrent distribution inside the SC pellet during “pure transverse” and “rotated” applied fields. [less ▲]

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See detailMagnetic shielding with bulk high-temperature superconducting hollow cylinders: improvement of the shielded volume and investigation of sequential orthogonal applied fields
Vanderbemden, Philippe ULiege; Hogan, Kevin ULiege; Wera, Laurent ULiege et al

Conference (2016, April 26)

Due to their ability to trap magnetic fields that exceed the saturation magnetization of conventional ferromagnets, bulk high-temperature superconductors have great potential to act as efficient passive ... [more ▼]

Due to their ability to trap magnetic fields that exceed the saturation magnetization of conventional ferromagnets, bulk high-temperature superconductors have great potential to act as efficient passive magnetic shields. Shielding efficiency is usually demonstrated in bulk hollow cylinders. One of the main drawbacks of this geometry is that the shielding is only effective in the central region of the tube and decreases towards the tube extremities because of the field penetration through the open ends. In order to improve the performances at tube extremities and hence increase the shielded volume, the tube should be closed using e.g. a superconducting cap. Gaps or holes, however, are needed for allowing connections between the inner part of the tube and the external world. In this communication we examine the shielding efficiency and volume when a superconducting tube is closed either at one extremity or at both extremities. The effect of the gap size between the cap and the tube is also studied. Next we examine more complex magnetic shielding configurations where an external magnetic field is applied first one direction and a second field is subsequently applied along another (perpendicular) direction. We discuss the influence of pre-existing shielding supercurrents on the magnetic shielding properties. [less ▲]

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See detailMagnetic moment and local magnetic induction of superconducting/ferromagnetic structures subjected to crossed fields: experiments on GdBCO and modelling
Fagnard, Jean-François ULiege; Morita, Mitsuru; Nariki, Shinya et al

in Superconductor Science and Technology (2016), 29(12), 125004

Recent studies have shown that ferromagnetic materials can be used together with bulk high temperature superconductors in order to improve their magnetic trapped field. Remarkably, it has also been ... [more ▼]

Recent studies have shown that ferromagnetic materials can be used together with bulk high temperature superconductors in order to improve their magnetic trapped field. Remarkably, it has also been pointed out that ferromagnets can help in reducing the crossed field effect, namely the magnetization decay that is observed under the application of AC transverse magnetic fields. In this work, we pursue a detailed study of the influence of the geometry of the ferromagnetic part on both trapped fields and crossed field effects. The magnetic properties of the hybrid superconducting/soft ferromagnetic structures are characterized by measuring the magnetic moment with a bespoke magnetometer and the local magnetic field density with Hall probes. The results are interpreted by means of 2D and 3D numerical models yielding the distribution of the superconducting currents as a function of the ferromagnet geometry. We examine in details the distortion of the shielding superconducting currents distribution in hybrid structures subjected to crossed magnetic fields. These results confirm the existence of an optimum thickness of the ferromagnet, which depends on the saturation magnetization of the ferromagnetic material and the current density of the superconductor. A hybrid structure providing an efficient protection against the crossed magnetic field while maintaining the magnetic induction along the axis of the structure is suggested. The limitations of the 2D modelling in this configuration are discussed. [less ▲]

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See detailMagnetic Shielding with Bulk High Temperature Superconductors: Improvement of the Shielded Volume in Hollow Cylinders
Wera, Laurent ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

in Muralidhar, Miryala (Ed.) Superconductivity: Applications Today and Tomorrow (2016)

The superconducting HTS hollow cylinder has been one of the first geometries to demonstrate its utility as an efficient passive magnetic shield, and particularly for DC magnetic shielding. The shielding ... [more ▼]

The superconducting HTS hollow cylinder has been one of the first geometries to demonstrate its utility as an efficient passive magnetic shield, and particularly for DC magnetic shielding. The shielding performances can be characterized by two parameters: (i) the shielding factor SF, defined as the ratio between the applied magnetic induction Bapp and the magnetic induction inside the shield Bin and (ii) the threshold induction Blim for which the shielding factor becomes lower than a given level. The main drawback of the hollow cylinder geometry is that the shielding is only effective in the central region of the tube and decreases towards the tube extremities because of the field penetration through the open ends. In order to improve the performances at tube extremities and hence increase the shielded volume, the tube should be closed. This can be achieved by using a superconducting vessel or by closing one or both extremities with a cap. This chapter describes the experimental and the numerical analysis of the DC shielding performances of a Bi-2223 tube closed by a superconducting Bi-2223 cap, for an axial configuration. The first part is dedicated to the experimental study of two tubes of different lengths when they are closed at one extremity or closed at both extremities. We also study the effect of the gap size between the cap and the tube. In the second part, we use numerical simulations to highlight the possible improvements and to optimize the design. In particular we study the influence of the critical current density and of the thickness of the cap. [less ▲]

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See detailInfluence of crossed fields in structures combining large grain, bulk (RE)BCO superconductors and soft ferromagnetic discs
Philippe, Matthieu ULiege; Fagnard, Jean-François ULiege; Wera, Laurent ULiege et al

in Journal of Physics: Conference Series (2016), 695

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. When ... [more ▼]

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. When such superconducting (SC) “trapped field magnets” are combined to a ferromagnetic (FM) disc, the total magnetic moment is increased with respect to that of the superconductor alone. In the present work, we study experimentally the magnetic behaviour of such hybrid FM/SC structures when they are subjected to cycles of applied field that are orthogonal to their permanent magnetization, i.e. a “crossed-field” configuration. Experimental results show that the usual “crossed-field demagnetization” caused by the cycles of transverse field is strongly reduced in the presence of the ferromagnet. [less ▲]

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See detailOrigin of dc voltage in type II superconducting flux pumps: field, field rate of change, and current density dependence of resistivity
Geng, J.; Matsuda, K.; Fu, L. et al

in Journal of Physics : D Applied Physics (2016), 49(11), 1101

Superconducting flux pumps are the kind of devices which can generate direct current into superconducting circuit using external magnetic field. The key point is how to induce a dc voltage across the ... [more ▼]

Superconducting flux pumps are the kind of devices which can generate direct current into superconducting circuit using external magnetic field. The key point is how to induce a dc voltage across the superconducting load by ac fields. Giaever (1966 IEEE Spectr . 3 [http://dx.doi.org/10.1109/MSPEC.1966.5217702] 117 ) pointed out flux motion in superconductors will induce a dc voltage, and demonstrated a rectifier model which depended on breaking superconductivity. van de Klundert et al (1981 Cryogenics 21 [http://dx.doi.org/10.1016/0011-2275(81)90195-8] 195, 267 ) in their review(s) described various configurations for flux pumps all of which relied on inducing the normal state in at least part of the superconductor. In this letter, following their work, we reveal that a variation in the resistivity of type II superconductors is sufficient to induce a dc voltage in flux pumps and it is not necessary to break superconductivity. This variation in resistivity is due to the fact that flux flow is influenced by current density, field intensity, and field rate of change. We propose a general circuit analogy for travelling wave flux pumps, and provide a mathematical analysis to explain the dc voltage. Several existing superconducting flux pumps which rely on the use of a travelling magnetic wave can be explained using the analysis enclosed. This work can also throw light on the design and optimization of flux pumps. [less ▲]

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See detailCritical analysis of the paramagnetic to ferromagnetic phase transition in Pr0.55K0.05Sr0.4MnO3
Thaljaoui, R.; Pękała, M.; Fagnard, Jean-François ULiege et al

in Physica B: Condensed Matter (2016), 482

The critical properties of monovalent doped manganite Pr0.55K0.05Sr0.4MnO3 around the paramagnetic to ferromagnetic phase transition were investigated through various methods: the modified Arrott plots ... [more ▼]

The critical properties of monovalent doped manganite Pr0.55K0.05Sr0.4MnO3 around the paramagnetic to ferromagnetic phase transition were investigated through various methods: the modified Arrott plots (MAP), the Kouvel-Fisher method and the critical isotherm analysis. Data obtained near Tc were examined in the framework of the mean field theory, the 3D–Heisenberg model, the 3D–Ising model, and tricritical mean field. The deduced critical exponents values obtained using MAP method were found to be β = 0.44(4) with TC ~ 303 K and γ = 1.04(1) with TC ~ 302 K. Kouvel-Fisher method supplies the critical values to be β = 0.41(2) with TC ~ 302 K and γ = 1.09(1) with TC ~ 302 K. The obtained critical parameters show a tendency towards the mean-field behavior, suggesting the existence of long-range ferromagnetic order in the compound studied. The exponent δ deduced separately from isotherm analysis at T= 303 K was found to obey to the Widom scaling relation δ = 1+ γ/ β. The reliability of obtained exponents was confirmed by using the universal scaling hypothesis. The itinerant character of ferromagnetism in the present system was also tested by using Rhodes-Wohlfarth’s criterion. [less ▲]

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See detailThermal properties of Ti-doped Cu–Zn soft ferrites used as thermally actuated material for magnetizing superconductors
Stachowiak, Piotr; Mucha, Jan; Szewczyk, Daria et al

in Journal of Physics : D Applied Physics (2016), 49

A great majority of widely used ferrite ceramics exhibit a relatively high temperature of order–disorder phase transition in their magnetic subsystem. For applications related to the magnetization process ... [more ▼]

A great majority of widely used ferrite ceramics exhibit a relatively high temperature of order–disorder phase transition in their magnetic subsystem. For applications related to the magnetization process of superconductors, however, a low value of Tc is required. Here we report and analyze in detail the thermal properties of bulk Ti-doped Cu–Zn ferrite ceramics Cu0.3Zn0.7Ti0.04Fe1.96O4 and Mg0.15Cu0.15Zn0.7Ti0.04Fe1.96O4. They are characterized by a Curie temperature in the range 120–170 K and a maximum DC magnetic susceptibility exceeding 20 for the Cu0.3Zn0.7Ti0.04Fe1.96O4 material. The temperature dependence of both the specific heat Cp and of the thermal conductivity κ, determined between 2 and 300 K, are found not to exhibit any peculiar feature at the magnetic transition temperature. The low-temperature dependence of both κ and the mean free path of phonons suggests a mesoscopic fractal structure of the grains. From the measured data, the characteristics of thermally actuated waves are estimated. The low magnetic phase transition temperature and suitable thermal parameters make the investigated ferrite ceramics applicable as magnetic wave producers in devices designed for magnetization of high-temperature superconductors. [less ▲]

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See detailDesign of the superconducting end caps of a tubular magnetic shield.
Wera, Laurent ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Poster (2015, September 09)

Shielding a sensitive device against DC or AC stray magnetic fields can be realized very efficiently with high-Tc superconductors. A simple magnetic shield can be obtained with a tube enclosing the device ... [more ▼]

Shielding a sensitive device against DC or AC stray magnetic fields can be realized very efficiently with high-Tc superconductors. A simple magnetic shield can be obtained with a tube enclosing the device, which is protected against external fields until the magnetic flux diffuses either across the tube wall or through the end openings. A better shielding vessel is obtained by using superconducting end caps. In the simplest design, the end caps, consisting of superconducting disks placed at the extremities of the tube, are making a 90 degree angle with the tube wall. Because of demagnetization effects, this arrangement results in a 'weak spot' for the penetrating magnetic flux. In this work, we investigate numerically different shapes for the end caps so as to avoid sharp angles and obtain a smoother magnetic flux penetration. The external magnetic induction is applied parallel to the shield axis. The magnetic flux distribution and the shielding currents are calculated with a finite element approach. The shielded volume is defined as the region where the local magnetic flux density is attenuated below a given fraction of the external field. It is studied as a function of the shape of the cap, its superconducting properties, the aspect ratio of the assembly, and the presence of a cabling hole. Curved and elongated caps, which could in principle be obtained by melt cast processing or different coating techniques, are shown to substantially increase the shielded volume. [less ▲]

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See detailMagnetic shielding properties of a bulk Bi-2223 superconducting hollow cylinder subjected to the magnetic stray field of a nearby magnetic source
Hogan, Kevin ULiege; Wera, Laurent ULiege; Fagnard, Jean-François ULiege et al

Poster (2015, September 09)

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. At low frequencies, their efficiency even surpasses that of conventional ferromagnetic materials ... [more ▼]

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. At low frequencies, their efficiency even surpasses that of conventional ferromagnetic materials. Therefore, they can be very useful for many applications which require low or very low magnetic field in intense quasi-static magnetic environments. The vast majority of studies on superconducting screens has been conducted in homogeneous magnetic field configurations. Few works have been done concerning inhomogeneous magnetic field configurations while those are representative of practical situations. Moreover, those configurations are non-trivial given the magnetic hysteretic behaviour of type-II superconductors. In the present work, we aim at investigating and understanding the shielding properties of a superconducting hollow cylinder made of Bi-2223 subjected to the magnetic stray field of a coil placed in the vicinity. All experiments are carried out at 77K. [less ▲]

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See detailCharacterization of the magnetic properties of a continuously coated YBCO coated conductor cylinder in persistent current mode
Wera, Laurent ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Poster (2015, September 09)

YBCO coated conductors are good candidates for magnetic shielding applications because of (i) their high current density and (ii) their lower weight compared to magnetic shields made of bulk HTS materials ... [more ▼]

YBCO coated conductors are good candidates for magnetic shielding applications because of (i) their high current density and (ii) their lower weight compared to magnetic shields made of bulk HTS materials. In particular, an efficient superconducting magnetic shield can be built as an assembly of milled YBCO 2G coated conductor tape sections, or, as in this case, a continuously coated cylinder. The purpose of the present work is to characterize experimentally the DC magnetic properties of a continuous band of YBCO film (about 1 cm width) deposited around a hastelloy cylinder of 6.4 cm in diameter. In particular, we characterize the distribution and the time dependence of the trapped field inside the cylinder due to persistent current loops. We also investigate the magnetic shielding properties at several sweep rates when the applied magnetic field is parallel to the main axis of the cylinder. All measurements are carried out at 77 K. The measurement of the trapped field of the cylinder gives evidence that a persistent current of about 80 A can be induced in the YBCO band. Magnetic shielding measurements show that, thanks to the persistent current flowing in the band, the cylinder is able to attenuate by a factor of 2 the applied magnetic field. These results open interesting perspectives for a further development of magnetic shields made of coated conductor cylinders. We investigate how the shielding performances could be further improved by increasing the number of layers and by increasing the height of the cylinder. The results are compared to a multilayer coated conductor cylinder with joints and to persistent current loops obtained with milled coated conductor tape sections. [less ▲]

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See detailMagnetic shielding properties of tubular bulk superconductors subjected to stray magnetic fields
Hogan, Kevin ULiege; Fagnard, Jean-François ULiege; Wera, Laurent ULiege et al

Conference (2015, September 04)

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. The vast majority of studies on superconducting shields has been conducted in homogeneous ... [more ▼]

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. The vast majority of studies on superconducting shields has been conducted in homogeneous magnetic field configurations. Only few works have been done concerning inhomogeneous magnetic field configurations while those are representative of practical situations. Moreover, those configurations are non-trivial given the magnetic hysteretic behaviour of type-II superconductors. In the present work, we aim at investigating and understanding the magnetic response of a superconducting hollow cylinder made of Bi-2223 subjected to the magnetic stray field of various magnetic sources. Thanks to a bespoke 3D mapping system and a three-axis Hall probe, we measure the magnetic flux density distribution in the vicinity of the cylinder when the latter is subjected to those stray fields. All experiments are carried out in liquid nitrogen. Our experimental data allow us to apprehend the magnetic field penetration inside the superconductor in inhomogeneous field configurations and to compare those to results obtained with homogeneous fields. Then we compare our results to a finite element model of an infinite tube subjected to an inhomogeneous magnetic field in order to gather information on the magnetic field penetration and the current density distribution inside the superconductor. [less ▲]

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See detailCrossed field effect measured on a GdBCO pellet at various temperatures below 77 K
Fagnard, Jean-François ULiege; Debois, Simon; Morita, Mitsuru et al

Conference (2015, September)

In engineering applications, superconducting bulk materials might be subjected to cycles of variable magnetic fields transverse to their trapped field which can cause a significant demagnetization. Up to ... [more ▼]

In engineering applications, superconducting bulk materials might be subjected to cycles of variable magnetic fields transverse to their trapped field which can cause a significant demagnetization. Up to now, several studies have been carried out on bulk superconducting pellets in order to characterize this so-called “crossed fields” effect at 77 K only. When attempting to carry out experiments at lower temperature, the issue is that the commercial systems with rotating sample holders do not accommodate sample having a larger size than few mm³. Therefore samples have to be cut from a larger pellet to be characterized. We designed an insertion tool for a Physical Property Measurement System (PPMS) that allows to rotate a sample of 9 mm diameter and up to 8 mm height inside the sample chamber from a parallel to a perpendicular direction with respect to the magnetic field direction. A Pt100 temperature sensor and a Hall probe were placed in the sample holder in order to measure the local temperature and the magnetic induction at the centre of the sample surface. A GdBCO sample was characterized between 50 K and 85 K by measuring first the penetration and the remnant fields through hysteresis loops measurements. Then transverse magnetic fields of different amplitudes were applied at constant sweep rates. The effect of the transverse magnetic field on the trapped field was studied and compared to the natural relaxation due to the flux creep. Results show that the application of a transverse magnetic field of the same percentage of the axial penetration field at several temperatures causes the same relative demagnetization (corrected from the flux creep relaxation). For a relatively small number of cycles, this demagnetization is found to be a logarithmic function of the time. Advantage was also taken from the possibility to change the temperature by measuring the crossed field effect of a magnetized sample which temperature was lowered to freeze the natural relaxation. [less ▲]

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