Magneto-Optical Characterization of Superconductors for Large Scale Applications
Anatolii Polyanskii, Matt Feldmann and David Larbalestier
Applied Superconductivity Center, University of Wisconsin, 1500 Engineering Dr. Madison WI 53706 USA
Abstract—The magneto-optical method based on the
Faraday Effect in ferrimagnetic Bi-substituted garnet
films with in-plane magnetization has been used for the
investigation of BSCCO monocore and multifilamentary
tapes manufactured by different technological processes.
It was found that pressing and rolling creates in tapes a
defect structure oriented along the tapes axis or perpendicular to it, respectively. These defects effect the value of
the critical current very dramatically.
Polarized light
z
αF=V Bz 2d
β
GGG
Ba
M
(Bi,Lu)3(Fe,Ga)5O12
Sample
I. INTRODUCTION
During the last ten years after the discovery of high temperature superconductors (HTS) many up to date materials
for large-scale applications have been found. These new materials are very promising for technical applications in the
near future in such fields as transmission lines and transformers. While superconductors have the ability to carry far more
current with much less dissipation than conventional conductors they still are very far away off from optimal conditions. To understand the sources of this problem many investigations have been done on HTS using different modern
techniques. The magneto-optical method, employing garnet
films as indicators of magnetic flux behavior in high temperature superconductors, is one of them. Developed 10 years
ago, after the discovery of HTS, it gave athe capability to
investigate materials over a large temperature range from 4 K
to above room temperature. It has been used for observation
of not only static flux distribution in HTS at low temperatures, but for imaging dynamic flux and current behavior at
temperature of liquid nitrogen and higher. Such properties of
this technique together with possibility to observe technological damaging areas of superconductors through nonsuperconducting coatings make MOI an irreplaceable tool for
testing superconductors. Below, the application of MOI technique for the investigation of flux behavior in superconductors is presented.
II. MAGNETO-OPTICAL TECHNIQUE.
The fundamental principle behind this technique is the
ability of various magneto-active materials, placed on the
surface of a sample, to rotate the polarization
This work was supported by EPRI, DOE and AIR Force Office of Scientific research and benefited from facilities supported by NSF-supported
MRSEC at the University of Wisconsin.
d
Reflective layer
Protective layer
Fig. 1. Geometry of MO imaging with in-plane indicators.
plane of the light passed through them. At present aBi- substituted iron garnet films with the planer orientation of magnetization have been actively used for imaging HTS. The
thickness of these films is 3-5 µm and the angle of Faraday
rotation is 1-2 0/µm at fields perpendicular to the surface. The
dynamic range of magnetic field that can be detected by indicators is limited by the field of saturation (600-2000 Oe) and
by the film composition. Fig. 1 shows the arrangement of a
magneto-optical indicator with planer magnetization on the
sample surface. The magneto-optical setup consists of a polarizing microscope, working in reflective mode, and a continuous-flow optical cryostat operating down to 6 K with a
system of two copper electromagnetic coils to generate magnetic fields normal to the sample surface and in-plane. The
magnetic field is stabilized by a power supply and a temperature controller stabilizes the temperature. An analog color
camera and a digital black and white one connect the setup to
a computer via two different frame grabber cards.
III. MAGNETO-OPTICAL IMAGING OF BSCCO
At present BSCCO tapes are the most promising conductors for a large-scale application. The properties of both
monocore tapes and for multifilamentary tapes have been
improved significantly during last few years. However, the
critical currents of these conductors still are very far from
optimal. Understanding the sources of restrictions, which
play strong role in limiting of the current carrying capability
these tapes, are vital to widespread application of HTS. The
magneto-optical technique works well on these length scales,
which cover the very important range of defects introduced
by mechanical
Ha
I
MO indicator film
C-axis
BSCCO
a
b)
a)
1 mm
c)
Fig. 4 MO images of BSCCO tapes fabricated by pressed and rolled:
a) sample pressed at 2 Gpa, Jc=27kA/cm2; b) sample pressed at 1Gpa
Jc=22kA/cm2 and c) rolled sample, Jc=10kA/cm2
c)
100 µm
Fig. 2. a) Geometry of experiment and MO images of longitudinal
sections of BSCCO tape for regions with b) higher Jc and c) lower Jc
fabrication processes, namely on a special scale from several
microns to a few millimeters. As well known magneto-optical
technique is able to image only upper layer of the sample
surface not deeper than 5-10 µm. To investigate the magnetic
flux behavior in different layers of superconducting tapes two
assemblies of magneto-optical experiments has been used.
One of them, shown on Fig. 2, using for imaging of a longitudinal section of a tape. Seeing the thickness of monocore
tape is enough for MO imaging it was found that the AgBSCCO interface layers have higher critical current and correlate with the colonies of well-aligned grains as the middle
portion of tape carries a much smaller current than interfaces
(Fig.2 b) [1]. While the next section of tape with lower Jc exhibit the poor magneto-optical contrast across tape and also
percolation current flow dominates in this area. Such sections
of tape with a small value of Jc control the transport current of
Ha
MO indicator film
b)
a whole tape. A significant improvement of the quality of
tapes can be achieved by using different thermomechanical
treatments of tapes in a fabrication procedure. To study the
influence of manufacture on the defect structure of tapes we
have imaged monocore tapes on their broad surface. The geometry of this experiment showed in Fig.3 and MO images
for pressed and rolled samples showed on Fig.4. As seen
from MO images processing techniques affect the defect
structure of BSSSCO tapes very significantly. The cracks
running across rolled tape decrease the critical current to the
lower value. However, in pressed tapes all cracks run along
the tape core and hence parallel to the direction of transport
current. Such orientation of cracks in pressed tapes has a
smaller influence on the Jc than in rolled tape.
One of the advantages of the MO technique is its ability to
image magnetic flux distribution in BSSCO directly through
silver sheath. Even in this case the sensitivity of MO in-plane
indicators and the spatial resolution of this technique are
good enough to reveal the general defect structure in most
multifilamentary tapes [2]. It was found that periodic pressing
[3] damages BSCCO filaments less than rolling. Even in 85filament rolled tapes with Jc =54kA/cm2 [2] a very pronouncing network of cracks was observed underneath the silver
sheath. These cracks run transverse to the direction of the
filaments and inhibit current flow.
BSCCO
REFERENCES
[1]
A.E. Pashitski, A. Polyanski, A. Gurevich, J.A. Parell, D.C. Larbalestier. Physica C 246, pp. 133-144, 1994.
[2] X.Y. Cai, A. Polyanskii, Q. Li, G.N. Riley and D.C. Larbalestier. Nature,
vol 392, pp. 906-909, April, 1998.
Ag
Rolling direction
[3] F. Marty, G. Grasso, Y.B. Huang and R. Flukiger Supercond.Sci. &
Technol. Vol. 11, pp. 1251-1254, 1998.
Fig. 3. Geometry of MO experiment for plan of view of BSCCO tape.
The silver layer etched away from the topside of tape.