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End date: 2024 × Start date: 2020 × Start date: 1999 × Type: Text × Publisher: Bristol : IOP Publ. × WGL Institute: IFWD ×

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Now showing 1 - 10 of 21
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    Transition to the quantum hall regime in InAs nanowire cross-junctions
    (Bristol : IOP Publ., 2019) Gooth, Johannes; Borg, Mattias; Schmid, Heinz; Bologna, Nicolas; Rossell, Marta D.; Wirths, Stephan; Moselund, Kirsten; Nielsch, Kornelius; Riel, Heike
    We present a low-temperature electrical transport study on four-terminal ballistic InAs nanowire cross-junctions in magnetic fields aligned perpendicular to the cross-plane. Two-terminal longitudinal conductance measurements between opposing contact terminals reveal typical 1D conductance quantization at zero magnetic field. As the magnetic field is applied, the 1D bands evolve into hybrid magneto-electric sub-levels that eventually transform into Landau levels for the widest nanowire devices investigated (width = 100 nm). Hall measurements in a four-terminal configuration on these devices show plateaus in the transverse Hall resistance at high magnetic fields that scale with (ve 2 /h) -1 . e is the elementary charge, h denotes Planck's constant and v is an integer that coincides with the Landau level index determined from the longitudinal conductance measurements. While the 1D conductance quantization in zero magnetic field is fragile against disorder at the NW surface, the plateaus in the Hall resistance at high fields remain robust as expected for a topologically protected Quantum Hall phase. © 2019 IOP Publishing Ltd.
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    Magnetic granularity in pulsed laser deposited YBCO films on technical templates at 5 K
    (Bristol : IOP Publ., 2017-9-4) Lao, M.; Hecher, J.; Pahlke, P.; Sieger, M.; Hühne, R.; Eisterer, M.
    The manifestation of granularity in the superconducting properties of pulsed laser deposited YBCO films on commercially available metallic templates was investigated by scanning Hall probe microscopy at 5 K and was related to local orientation mapping of the YBCO layer. The YBCO films on stainless steel templates with a textured buffer layer of yttrium stabilized ZrO2 grown by alternating beam assisted deposition have a mean grain size of less than with a sharp texture. This results in a homogeneous trapped field profile and spatial distribution of the current density. On the other hand, YBCO films on biaxially textured NiW substrates show magnetic granularity that persists down to a temperature of 5 K and up to an applied magnetic field of 4 T. The origin of the granular field profile is directly correlated to the microstructural properties of the YBCO layer adopted from the granular NiW substrate which leads to a spatially inhomogeneous current density. Grain-to-grain in-plane tilts lead to grain boundaries that obstruct the current while out-of-plane tilts mainly affect the grain properties, resulting in areas with low . Hence, not all grain boundaries cause detrimental effects on since the orientation of individual NiW grains also contributes to observed inhomogeneity and granularity.
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    Occurrence of Flux Jumps in MgB2 Bulk Magnets during Pulse-Field Magnetization
    (Bristol : IOP Publ., 2020) Sakai, N.; Oka, T.; Yamanaka, K.; Dadiel, L.; Oki, H.; Ogawa, J.; Fukui, S.; Scheiter, J.; Häßler, W.; Yokoyama, K; Noudem, J.; Miryala, M.; Murakami, M.
    The magnetic flux capturing of MgB2 bulk magnets made by spark plasma sintering process has been precisely investigated to clarify the mechanism of flux motions during the pulse-field magnetization processes. The field trapping ratio B T/B P was evaluated as a key parameter of field trapping ability which strongly relates to the heat generation due to the rapid flux motion in the samples. The time dependence of magnetic flux density revealed the actual flux motion which penetrated the samples. The trapped fields B T and field trapping ratios B T/B P of various samples were classified into three regions of 'no flux flow', 'fast flux flow' and 'flux jump' according to the generation of heat and its propagation. A flux jump was observed late at 280 ms from the beginning of PFM process, while the field penetration B P showed its peak at 10 ms. Considering the heat propagation speed, the long-delayed flux jump should be attributed to the macroscopic barriers against the heat propagation to the surface centre of bulk magnet. © Published under licence by IOP Publishing Ltd.
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    Magnetic Flux Trapping and Flux Jumps in Pulsed Field Magnetizing Processes in REBCO and Mg-B Bulk Magnets
    (Bristol : IOP Publ., 2020) Oka, T.; Takeda, A.; Oki, H.; Yamanaka, K.; Dadiel, L.; Yokoyama, K.; Häßler, W.; Scheiter, J.; Sakai, N.; Murakami, M.
    Pulsed-field magnetization technique (PFM) is expected as a cheap and an easy way for HTS bulk materials for utilizing as intense magnets. As the generation of heat due to magnetic flux motion in bulk magnets causes serious degradation of captured fields, it is important to investigate the flux motions during PFM in various field applications. The authors precisely measured the magnetic flux motion in the cryocooled MgB2 bulk magnets containing various amount of Ti. We classified the motions to "no flux flow (NFF)", "fast flux flow (FFF)", and "flux jump (FJ)" regions. The results showed that addition of Ti shifts the field invasion area to high field areas, and expands the NFF regions. The highest field-trapping appears at the upper end of the NFF region. Since the heat generation and its propagation should attribute to the dissipation of magnetic flux, FFF leads to FJ. Compared with MgB2, we referred to GdBCO as for the flux motion. A flux jump was observed at 30 K when the pulse field of 7 T was applied to the preactivated sample, showing its stability against FJ. © Published under licence by IOP Publishing Ltd.
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    A review on stretchable magnetic field sensorics
    (Bristol : IOP Publ., 2020) Melzer, M.; Makarov, D.; Schmidt, O.G.
    The current establishment of stretchable electronics to form a seamless link between soft or even living materials and the digital world is at the forefront of multidisciplinary research efforts, bridging physics, engineering and materials science. Magnetic functionalities can provide a sense of displacement, orientation or proximity to this novel formulation of electronics. This work reviews the recent development of stretchable magnetic field sensorics relying on the combination of metallic thin films revealing a giant magnetoresistance effect with elastomeric materials. Stretchability of the magnetic nanomembranes is achieved by specific morphologic features (e.g. wrinkles or microcracks), which accommodate the applied tensile deformation while maintaining the electrical and magnetic integrity of the sensor device. The entire development, from the demonstration of the world's first elastically stretchable magnetic sensor to the realization of a technology platform for robust, ready-to-use elastic magnetosensorics is described. Soft giant magnetoresistive elements exhibiting the same sensing performance as on conventional rigid supports, but with fully strain invariant properties up to 270% stretching have been demonstrated. With their unique mechanical properties, these sensor elements readily conform to ubiquitous objects of arbitrary shapes including the human skin. Stretchable magnetoelectronic sensors can equip soft and epidermal electronic systems with navigation, orientation, motion tracking and touchless control capabilities. A variety of novel technologies, like electronic skins, smart textiles, soft robotics and actuators, active medical implants and soft consumer electronics will benefit from these new magnetic functionalities. © 2019 IOP Publishing Ltd.
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    Voltage-controlled on switching and manipulation of magnetization via the redox transformation of β-FeOOH nanoplatelets
    (Bristol : IOP Publ., 2020) Nichterwitz, Martin; Neitsch, Sabine; Röher, Stefan; Wolf, Daniel; Nielsch, Kornelius; Leistner, Karin
    Redox-based metal/metal oxide transformations achieved via electrolytic gating recently emerged as a novel, magneto-ionic route for voltage control of magnetism. So far, mainly metal or oxide thin films and nanoporous metal alloy structures are used as starting materials. The present study demonstrates a magneto-ionic transformation starting from a stable electrodeposited FeOOH nanoplatelet structure. The application of a low voltage in a Li-based electrolyte results in the reduction of the virtually non-magnetic FeOOH into ferromagnetic Fe, yielding an ON switching of magnetization. The magnetization can be tuned in a large range by the time of voltage application and remains stable after voltage-switch off. A reversible magneto-ionic change of magnetization of up to 15% is achieved in the resulting iron films with a thickness of about 30 nm. This large magneto-ionic effect is attributed to the enhanced roughness of the iron films obtained from the nanoplatelet structure. The robust, voltage-controlled, and non-volatile ON switching of magnetism starting from a stable oxide structure is promising for the development of energy-efficient magnetic switches, magnetic actuation and may offer new avenues in magnetoelectronic devices. © 2019 IOP Publishing Ltd.
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    Impact of Mn-Pn intermixing on magnetic properties of an intrinsic magnetic topological insulator: the µSR perspective
    (Bristol : IOP Publ., 2023) Sahoo, M.; Salman, Z.; Allodi, G.; Isaeva, A.; Folkers, L.; Wolter, A.U.B.; Büchner, B.; De Renzi, R.
    We investigated the magnetic properties of polycrystalline samples of the intrinsic magnetic topological insulators MnPn2Te4, with pnictogen Pn = Sb, Bi, by bulk magnetization and μSR. DC susceptibility detects the onset of magnetic ordering at TN = 27 K and 24 K and a field dependence of the macroscopic magnetization compatible with ferri- (or ferro-) and atiferro- magnetic ordering, respectively. Weak transverse field (wTF) Muon Spin Rotation (μSR) confirms the homogeneous bulk nature of magnetic ordering at the same two distinct transition temperatures. Zero Field (ZF) μSR shows that the Sb based material displays a broader distribution of internal field at the muon, in accordance with a larger deviation from the stoichiomectric composition and a higher degree of positional disorder (Mn at the Pn(6c) site), which however does not affect significantly the sharpness of the thermodynamic transition, as detected by the muon magnetic volume fraction and the observability of a critical divergence in the longitudinal and transverse muon relaxation rates.
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    Towards a reliable bridge joint between REBCO coated conductors
    (Bristol : IOP Publ., 2020) Kirchner, A.; Nielsch, K.; Hühne, R.
    REBa2Cu3O7-x (REBCO; RE = Y, Gd) coated conductor wires are commercially available up to a length of about 1 km. However, for large-scale devices like superconducting coils for high-field magnets several kilometres of a coated conductors are required. Therefore, it is desirable to use joints, which exhibit similar superconducting properties as the coated conductor itself. In this study, we jointed commercial REBCO coated conductors by a two-step procedure. At first, a superconducting soldering solution was developed and deposited on unstabilized coated conductors via chemical solution deposition. The soldering precursor is based on a Cu-rich solution with a metal cation ratio Y:Ba:Cu of 1:2:4. Secondly, a piece of the coated conductor was delaminated between the superconducting and the buffer layer and used as bridge between two other conductors covered with the soldering layer. Annealing the resulting bridge joint under pressure results in a mechanical stable configuration. © Published under licence by IOP Publishing Ltd.
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    On the growth of Co-doped BaFe2As2 thin films on CaF2
    (Bristol : IOP Publ., 2019) Langer, Marco; Meyer, Sven; Ackermann, Kai; Grünewald, Lukas; Kauffmann-Weiss, Sandra; Aswartham, Saicharan; Wurmehl, Sabine; Hänisch, Jens; Holzapfel, Bernhard
    The competition between phase formation of BaF2 and Ba(Fe1-xCox)2As2 on CaF2 single crystals has been analysed. Ba(Fe0.92Co0.08)2As2 thin films have been deposited by pulsed laser deposition. X-ray diffraction, atomic force microscopy and scanning electron microscopy studies have revealed that the formation of secondary phases and misorientations as well as the growth modes of the Ba(Fe0.92Co0.08)2As2 thin films strongly depend on the growth rate. At high growth rates, formation of BaF2 is suppressed. The dependency of the Ba(Fe0.92Co0.08)2As2 lattice parameters supports the idea of fluorine diffusion into the crystal structure upon suppression of BaF2 formation similar as was proposed for FeSe1-xTex thin films on CaF2. Furthermore, a growth mode transition from a layer growth mechanism to a three-dimensional growth mode at high supersaturation has been found, suggesting similarities between the growth mechanism of iron-based superconductors and high-T c cuprate thin films. © 2019 Published under licence by IOP Publishing Ltd.
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    Influence of the magnet aspect ratio on the dynamic stiffness of a rotating superconducting magnetic bearing
    (Bristol : IOP Publ., 2020) Espenhahn, Tilo; Wunderwald, Florian; Möller, Marcel; Sparing, Maria; Hossain, Mahmud; Fuchs, Günter; Abdkader, Anwar; Cherif, Chokri; Nielsch, Kornelius; Hühne, Ruben
    Rotating superconducting bearings promise great potential in applications due to their frictionless operation. However, these bearings show a lower dynamic stiffness and damping coefficient compared to ball bearings. In this paper we studied a bearing consisting of a fixed YBCO ring and a rotating magnet above the superconductor. The influence of the magnet aspect ratio on the dynamic stiffness of the bearing was investigated in order to find an optimized size. To change the aspect ratio, we kept the inner diameter of the ring constant and reduced the outer diameter while increasing the ring height. In addition to these magnets, one magnet with a reduced cross-sectional area was studied. The aspect ratio selection was based on preliminary magnetic flux density simulations, which compared the magnetic flux density distribution and the potential radial force for different aspect ratios. To conduct the measurements, the field-cooled magnets were displaced in a lateral direction and then released, resulting in a damped oscillation. The dynamic stiffness constants were calculated for each bearing from the relation of three axis acceleration measurements for different field cooling heights. The comparison of the stiffness constants for the different bearings revealed an optimal aspect ratio for the given YBCO ring. This optimum is almost independent from the cooling height. The comparison between the two magnet rings with similar diameters and different heights was similar for the bearing characteristics at a low cooling height, whereas a significant reduction of stiffness was observed with a larger cooling distance. The difference is bigger for the magnet with a reduced height. The optimal aspect ratio as well as the stiffness dependence on the cross-sectional area was confirmed by simulations of the magnetic flux density distribution. © 2019 IOP Publishing Ltd.