Group of High-Temperature Superconductivity

D e p a r t m e n t o f M a g n e t i s m a n d L o w T e m p e r a t u r e s
S i t e S l o v a n k a


Dr. Milos Jirsa, DSc. head of the group Phone: (+420)266052718 E-mail: jirsa@fzu.cz	Michal Rames, MSc. doctoral student Phone: (+420)266052723 E-mail: ramesm@fzu.cz

Research

Our lab is dealing with magnetic relaxation fenomena in high-temperature superconductors. The complex magnetic behavior of the superconductors originates in the behaviour of vortices that penetrate the superconducting matter in presence of sufficiently large external magnetic field (exceeding the lower critical value, B_c1). If vortices can freely move, resistivity appears in the superconductor. In type-II superconductors quenched inhomogenities represent defects pinning vortices and internal field gradients are formed in the sample. Zero resistance is preserved until the current density associated with the internal field gradients exceeds some critical value.

As vortex relaxation processes opose pinning forces and participate thus on formation of field gradients in the superconductor, measurements of relaxation phenomena gives us an opportunity to study effects of pinning, which are of an essential importance for the industrial use of superconductive materials.

Magnetic relaxation phenomena are studied in different ways:

Measurement of time dependence of the total induced current in a fixed magnetic field (conventional relaxation) by means of the vibrating sample magnetometer (VSM), torsion magnetometer (TM), extraction magnetometer (EM), and SQUID magnetometer (SQUID).
Measurement of magnetic hysteresis loops with different field sweep rates (dynamic relaxation) by means of VSM, TM, and EM.
Measurement of transport currents in the superconductor. The transport IV curves are equivalent to magnetic hysteresis loops as the measured transport current I is proportional to the critical current density associated in magnetic measurements with the total induced magnetic moment and the transport voltage V is proportional to the derivative of the current density with time or, alternatively, to the ramping field rate dB/dt.
Measurement of time dependence of the local currents in a fixed magnetic field by a miniature Hall probe.

Facility available

Vibrating sample magnetometer PAR 4500:

bi-directional electromagnet with horizontal +/- 2 Tesla field ramping at the maximum constant rate of 87 mT/s
helium cryostat down to 2.5 K
oven for high-temperature measurement (up to 1000 K) - not yet good for superconductors, unfortunately
vertical sample rotator

SQUID magnetometer QD MPMS-5S

superconducting magnet with field up to +/- 5 T
helium cryostat to 2 K
reciprocating sample option (RSO) and a new temperature sweep option

PPMS (Physical Property Measurenent System) of QD:

vertical superconducting magnet with field up to +/- 14 Tesla ramping at the maximum rate of 12 mT/s
helium cryostat down to 2 K
torsion magnetometer
ac susceptibility and dc magnetisation (extraction magnetometer)
ac transport measurement system
temperature heat capacity system (high pressure measurement available)
resistivity option
horizontal sample rotator

Research

REBaCuO thin films, substituted YBCO ceramics, REBaCuO single crystals, REBaCuO melt-textured samples, irradiated REBaCuO single crystals

In these samples, we are mainly interested in the interaction of vortices with different types of pinning defects, like random point-like pinning disorder, twin planes, and columnar defects produced by irradiation by heavy ions. This interaction is reflected in the shape of magnetic hysteresis loop, in transport characteristics, ac susceptibility measurements, and in relaxation phenomena. A special interest is paid to the "fishtail" or "secondary peak" effect (enhanced current density at intermediate magnetic fields) that promises an interesting use of bulk superconductors in practice. Besides the fishtail peak itself also the central peak of the hysteresis loop has been investigated in connection with its interference with the fishtail maximum at low fields, in particular at low temperatures.

Bi-based superconducting tapes, Bi-2212 ceramics and single crystals, and patterned YBaCuO thin films

Bi-based tapes are at the moment best candidates for high-power cables and wires. We are studying the interplay between intergranular and intragranular critical currents, anomalous central magnetization and hysteretic behavior of induced intergranular currents. These effects arise from the granular nature of these materials. To simplify the complex conditions inside the superconducting tape we proposed a structured sample of YBaCuO thin film, which should model conditions in the polycrystaline Bi-2223 and Bi-2212 tapes. Such samples were produced at Chalmer's University, Göteborg, and Institut für Schicht- und Ionentechnik of the Forschungszentrum in Jülich, Germany. The really exhibited the behavior similar to that of Bi-2223 and Bi-2212 tapes as predicted.

Cooperations and working contacts

ISTEC, Tokyo, Japan
Institute for Materials Research, Tohoku University, Sendai, Japan
Blackett Laboratory, London, UK
Institute of Electrical Engineering SAS, Bratislava, Slovakia
University of Oslo, Oslo, Norway
Forschungszentrum Jülich, Jülich, Germany
Shanghai Jiao Tong University, Shanghai, China
Saarland University, Saarbrücken, Germany