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Dive into the research topics where Johan Vanacken is active.

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Featured researches published by Johan Vanacken.


Nature Materials | 2012

Nanoscale strain-induced pair suppression as a vortex-pinning mechanism in high-temperature superconductors

A. Llordés; Anna Palau; Jaume Gazquez; Mariona Coll; R. Vlad; Alberto Pomar; Jordi Arbiol; Roger Guzmán; S Ye; V. Rouco; Felip Sandiumenge; Susagna Ricart; Teresa Puig; M. Varela; D. Chateigner; Johan Vanacken; J. Gutiérrez; Victor Moshchalkov; G. Deutscher; C. Magen; Xavier Obradors

Boosting large-scale superconductor applications require nanostructured conductors with artificial pinning centres immobilizing quantized vortices at high temperature and magnetic fields. Here we demonstrate a highly effective mechanism of artificial pinning centres in solution-derived high-temperature superconductor nanocomposites through generation of nanostrained regions where Cooper pair formation is suppressed. The nanostrained regions identified from transmission electron microscopy devise a very high concentration of partial dislocations associated with intergrowths generated between the randomly oriented nanodots and the epitaxial YBa(2)Cu(3)O(7) matrix. Consequently, an outstanding vortex-pinning enhancement correlated to the nanostrain is demonstrated for four types of randomly oriented nanodot, and a unique evolution towards an isotropic vortex-pinning behaviour, even in the effective anisotropy, is achieved as the nanostrain turns isotropic. We suggest a new vortex-pinning mechanism based on the bond-contraction pairing model, where pair formation is quenched under tensile strain, forming new and effective core-pinning regions.


Journal of Physics D | 2012

Structural transformation and magnetoelectric behaviour in Bi1−xGdxFeO3 multiferroics

Vera Lazenka; Gufei Zhang; Johan Vanacken; I.I. Makoed; A.F. Ravinski; Victor Moshchalkov

The crystal structure, dielectric, magnetic and magnetoelectric (ME) properties of Bi1?xGdxFeO3 (BGFO, x?=?0, 0.05, 0.1, 0.15, 0.2) multiferroic ceramics have been studied. The substitution of bismuth by gadolinium induces a phase structural transition at x?>?0.1, which leads to the suppression of the spiral modulated spin structure and develops weak ferromagnetic properties in the BiFeO3-based materials. Through studying the temperature/magnetic field dependence of the ME coefficient, we have revealed the effect of the substitution of Gd3+ ions on the ME properties, and have demonstrated the possibility of manipulating the electric state in BGFO multiferroics by applying magnetic field at room temperature.


Journal of Applied Physics | 2009

Intragrain defects in polycrystalline silicon layers grown by aluminum-induced crystallization and epitaxy for thin-film solar cells

Dries Van Gestel; Ivan Gordon; Hugo Bender; Damien Saurel; Johan Vanacken; Guy Beaucarne; Jef Poortmans

Polycrystalline silicon (pc-Si) thin-films with a grain size in the range of 0.1–100 μm grown on top of inexpensive substrates are economical materials for semiconductor devices such as transistors and solar cells and attract much attention nowadays. For pc-Si, grain size enlargement is thought to be an important parameter to improve material quality and therefore device performance. Aluminum-induced crystallization (AIC) of amorphous Si in combination with epitaxial growth allows achieving large-grained pc-Si layers on nonsilicon substrates. In this work, we made pc-Si layers with variable grain sizes by changing the crystallization temperature of the AIC process in order to see if larger grains indeed result in better solar cells. Solar cells based on these layers show a performance independent of the grain size. Defect etching and electron beam induced current (EBIC) measurements showed the presence of a high density of electrically active intragrain defects. We therefore consider them as the reason fo...


IEEE Transactions on Applied Superconductivity | 2008

Efficient Design of Advanced Pulsed Magnets

Tao Peng; Liang Li; Johan Vanacken; F. Herlach

The algorithms and basic equations used in the ldquoPMDSrdquo code for efficient coil design are described. The code calculates stresses and heating in the mid-plane only; there they are highest. The pulse shape is calculated for a capacitor discharge and for a voltage waveform derived from a generator. As examples, the optimization of a monolithic coil and a dual magnet are discussed, the latter based on the European ldquoARMSrdquo magnet. The influence of materials data on magnet performance is discussed.


Journal of Physics D | 2013

Effect of rare-earth ion doping on the multiferroic properties of BiFeO3 thin films grown epitaxially on SrTiO3(1 0 0)

Vera Lazenka; M. Lorenz; Hiwa Modarresi; Kerstin Brachwitz; Peter Schwinkendorf; Tammo Böntgen; Johan Vanacken; M. Ziese; Marius Grundmann; Victor Moshchalkov

High-quality epitaxial Bi1?xRExFeO3 (RE=La, Nd, Gd; x?=?0, 0.05, 0.15) thin films were prepared on SrTiO3(1?0?0) substrates using pulsed laser deposition. X-ray diffraction and RBS-channelling spectroscopy showed that the films are single-phase perovskite, free of additional phases and textured with preferential orientation along the [1?0?0] direction. The dependences of magnetization on temperature and field showed that the films exhibit weak ferromagnetic properties. Among the studied rare-earth doping ions, Bi3+ substitution by Gd3+ most considerably enhanced the ferromagnetic properties. Substitution by La3+ smoothened out the surface morphology, which is important for different potential applications. Both undoped and doped films showed clear ferroelectric response in piezoresponse force microscopy, thus confirming their multiferroic nature. The doping was found to promote a preferential ferroelectric poling of the domains.


IEEE Transactions on Applied Superconductivity | 2008

The Pulsed High Magnetic Field Facility at HUST, Wuhan, China and Associated Magnets

Liang Li; Hongfa Ding; Tao Peng; Xiaotao Han; Zhengcai Xia; Jin Chen; Xianzhong Duan; Cheng Wang; Yuan Pan; Johan Vanacken; F. Herlach

A pulsed high magnetic field laboratory has been funded to be established at the Huazhong University of Science and Technology (HUST), Wuhan, China by the National Development and Reform Committee. The facility is planned to be open for external users in 2011 with the implementation of various experimental techniques in pulsed magnetic fields up to 80 T. Pulse durations are in the range from 15 to 1000 ms with the magnet bore sizes from 12 to 34 mm. The pulsed power supplies are a 12 MJ, 25 kV capacitor bank and a 100 MVA/100 MJ flywheel pulse generator. The design and analysis of the power supplies and the magnets of short and long pulse durations and the multi-stage pulsed magnets with controllable waveforms are presented in this paper.


Review of Scientific Instruments | 2006

Synchrotron X-ray powder diffraction studies in pulsed magnetic fields

P Frings; Johan Vanacken; C. Detlefs; F. Duc; J. E. Lorenzo; M. Nardone; J. Billette; A. Zitouni; Wim Bras; Glja Rikken

X-ray powder diffraction experiments under pulsed magnetic fields were carried out at the DUBBLE beamline (BM26B) at the ESRF. A mobile generator delivered 110kJ to the magnet coil, which was sufficient to generate peak fields of 30T. A liquid He flow cryostat allowed us to vary the sample temperature accurately between 8 and 300K. Powder diffraction patterns of several samples were recorded using 21keV monochromatic x-rays and an on-line image plate detector. Here we present the first results on the suppression of the Jahn-Teller structural distortion in TbVO4 by magnetic field. These data clearly demonstrate the feasibility of x-ray powder diffraction experiments under pulsed magnetic fields with relatively inexpensive instrumentation.


Journal of Applied Physics | 2012

Weak ferromagnetism in La-doped BiFeO3 multiferroic thin films

Vera Lazenka; A.F. Ravinski; I.I. Makoed; Johan Vanacken; Gufei Zhang; Victor Moshchalkov

Bi1−xLaxFeO3 thin films (x = 0.0, 0.3, 0.5) were grown on glass substrates by thermal physical vapor deposition. The monoclinically distorted crystal structure of the films was revealed by x-ray diffraction at room temperature. Field and temperature (up to 1000 K) dependences of magnetization were studied. Saturation of the room temperature magnetic hysteresis loop has been observed at magnetic field above 0.15 T, demonstrating the weak ferromagnetic nature of the thin films. Our magnetic force microscopy results show clearly the presence of magnetic domains in BFO thin films. These structural and magnetic properties suggest the absence of magnetic spiral spin structure in monoclinically distorted BFO-based thin films.


Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms | 1990

Formation of buried and surface cosi2 layers by ion-implantation

Mf Wu; André Vantomme; Hugo Pattyn; Guido Langouche; Karen Maex; Jan Vanhellemont; Johan Vanacken; H Vloeberghs; Yvan Bruynseraede

Abstract The formation is studied of buried CoSi 2 layers and surface CoSi 2 layers formed by high-dose ion implantation in an energy range of 30 to 160 keV combined with conventional furnace annealing or with rapid thermal processing. The crystalline quality, the phase of the buried and surface silicide layers, the abruptness of the interfaces and the electrical transport properties are studied by RBS, channeling, cross-sectional TEM, Mossbauer spectroscopy and resistivity measurements. The stability of the buried and surface CoSi 2 layers at temperatures between 1000°C and 1200°C is also studied and compared with the results for MBE-grown samples.


Journal of Solid State Chemistry | 2003

Charge and orbital order in rare-earth and Bi manganites: a comparison

J.L. García-Muñoz; Carlos Frontera; M.A.G. Aranda; Clemens Ritter; A. Llobet; M. Respaud; M Goiran; H. Rakoto; O Masson; Johan Vanacken; J.M. Broto

A comparative investigation of the charge modulation tendencies in R–M–MnO3 and Bi–Sr–MnO3 manganites reveals remarkable differences in these series of materials. By means of magnetic, magnetotransport, synchrotron X-ray and neutron diffraction, as well as pulsed high magnetic fields data, we compare the systematic electronic and magnetic properties of several rareearth-based manganites and bismuth-based Sr manganites. The differences between these types of systems are rationalized in terms of the role that the different electronic structure of bismuth and rare earth plays in these materials.

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Dive into the Johan Vanacken's collaboration.

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F. Herlach

Katholieke Universiteit Leuven

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Lieven Trappeniers

Katholieke Universiteit Leuven

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Yvan Bruynseraede

Université catholique de Louvain

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Gufei Zhang

Katholieke Universiteit Leuven

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Y. Bruynseraede

Katholieke Universiteit Leuven

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Kris Rosseel

Katholieke Universiteit Leuven

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Willy Boon

Katholieke Universiteit Leuven

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Liesbet Weckhuysen

Katholieke Universiteit Leuven

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P. Wagner

The Catholic University of America

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