Magnetic properties of superconductors
A study the magnetic properties of superconductors has been performed with the aim to improve their technological applicability and increase the understanding of the basic mechanisms underlying high-Tc superconductivity (HTS).
The project aims to examine the influence of magnetic fields on superconducting properties. In particular it will be studied how the magnetic flux is expelled from and can be pinned in the superconductor. The purpose is to enhance the critical current density J_c in technological applications by identifying weak links in the current path and finding effective methods for flux pinning. A magneto-optical setup will be designed and constructed for the studies and supplemented by existing equipment for high field magnetisation, AC-susceptibility and determination of the magnetic flux line lattice by neutron diffraction and decoration technique. Focus will be on examination and optimisation of superconducting BiSCCO/Ag tapes produced by Nordic Superconductor Technology (NST) for applications in power transmission lines and magnetic field coils. However, also the properties of YBa_2Cu_3O_7 (YBCO) high-T_c material produced by the company Haldor Topsøe will be studied with the aim to optimise and apply it for applications in magnetic bearings for fly-wheels, current feedthroughs and current limiters. The studies will rely on the basic microscopic knowledge about the properties of magnetic flux line lattices that has been established in recent years and will be furhter developed at Risø. The project comprises the following activities: 1) Design, construction and development of magneto-optic equipment, technique and methods of analysis. 2) Determination and improvement of weak current paths in BiSCCO/Ag tapes. 3) Enhancement of the critical current density of BiSCCO/Ag tapes in applied magnetic fields. 4) Examination and improvement of AC-losses in BiSCCO/Ag tapes. 5) Improvement of flux pinning and critical current in YBCO high-T_c materials. 6) Studies of magnetic flux line lattice and magnetic phases in superconductors
As a supplement to existing techniques, based on neutron and x-ray scattering, transport, susceptibility and magnetization measurements, a major task of the project has been to establish a modern PC-controlled magneto-optic (MO) system with special resolution in the micrometer range, operating at temperatures 4 - 300 K and magnetic fields up to 0.15 T. Design, construction and calibration have been performed and methods for image analysis and numerical conversion between local magnetic field and critical current density have been established. Using the available techniques, in particular the MO-equipment, the magnetic flux penetration has been studied in BSCCO/Ag (BSCCO = (Bi,Pb)2Sr2C2Cu3010) superconductor tapes produced by Nordic Superconductor Technologies A/S, and weak current paths and filaments in the tapes have been identified. The current carrying capability of YBCO (YBa2Cu306+x), produced by Haldor Topsøe A/S has been improved by a factor of ten by fast neutron irradiation, and the relation between local texture and critical current has been studied using a combination of synchrotron x-ray and magnetization measurements. An unusual tetragonal (should be orthorhombic) superconducting phase in YBCO with chi = 0.62 has been identified and characterized using ac-susceptibility, MO and synchrotron x-ray scattering techniques. The exotic co-existence of superconducting and magnetic phases in some of the borocarbides, RENi2B2C (RE = Y, Lu, Dy, Ho, Er, Tm) has been studied by small angle neutron scattering (SANS) and neutron diffraction. The phase diagram of the magnetic flux line lattice has been established for several of the systems, and for the magnetic systems with Er and Tm a significant coupling between superconductivity and magnetism has been established. The static magnetic phases and the magnetic fluctuations in superconducting YBCO, and in materials where Y is replaced by magnetic ions like Nd and Pr, have been studied by neutron scattering. While Nd may replace Y without deterioration of superconductivity (Nd orders magnetically below 0.62 K), substitution by Pr suppresses superconductivity for all oxygen compositions chi. The neutron scattering studies have revealed a strong coupling between the Cu and Pr ions, which may be related to the current theory in literature suggesting that the holes doped into the CuO2 planes are localized in bonds close to the Pr-ions, thus enhancing the coupling to the Cu-ions, rather than forming itinerant superconducting pairs. A highly surprising result is the observation by neutron diffraction of quasi-static magnetic ordering in the Cu02 planes of YBCO
Key figures
Category
Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| Nordic Superconductor Technologies A/S |
Contact
P.O. Box 49
DK-4000 Roskilde, Denmark
Hessel Andersen, Niels (seniorforsker), 46774711, niels.hessel@risoe.dk
Øvr. Partnere: Nordic Superconductor Technology A/S
