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Physica C 460–462 (2007) 803–804
www.elsevier.com/locate/physc
Magneto-optical investigations of Ag-sheathed Bi-2223 tapes
with ferromagnetic shielding
V.V. Yurchenko a, D.V. Shantsev a, Y.M. Galperin a, A.K.M. Alamgir b,
Z. Han b, T.H. Johansen a,*
b
a
University of Oslo, Department of Physics, P.O. Box 1048, Blindern, 0316 Oslo, Norway
Applied Superconductivity Research Center, Department of Physics, Tsinghua University, Beijing 100084, China
Available online 14 April 2007
Abstract
An increase in the critical current and suppression of AC losses in superconducting wires and tapes with soft magnetic sheath have
been predicted theoretically and confirmed experimentally. In this work we present the results of magneto-optical investigations on a
series of Ag-sheathed Bi-2223 tapes with Ni coating. We visualize distributions of magnetic field at increasing external field and different
temperatures, demonstrating a difference between the flux propagation in the superconductor with Ni rims and a reference sample without Ni coating.
Ó 2007 Published by Elsevier B.V.
Keywords: Bi-2223; Tapes; Ferromagnetic shielding; Magneto-optical imaging
1. Introduction
Advantageous effects produced by a soft ferromagnetic
(FM) sheath on a superconductor (SC) have been described
in a series of recent theoretical papers [1–8]. Subsequent
experimental spatially resolved studies on YBa2Cu3O7 d
thin films [8,9] and MgB2 wires [10,11] proved the enhanced
screening properties and increase of the critical current in
SC/FM structures.
In this work we present the result of a magneto-optical
(MO) investigation of magnetic field distributions in
Bi2Sr2Ca2Cu3O10 x/Ag/FM (Bi-2223/Ag/FM) multi-filamentary tapes developed for electrical power applications.
Ni coating was electroplated along the lateral sides of the
tape forming a U-shaped magnetic rim around both edges
of the tape. Details of the sample preparation can be found
in the paper by Alamgir et al. [13]. In order to show directly
the effect of the magnetic sheath the Ni rims were fully
removed from one part of the sample, and kept on the
*
Corresponding author. Tel.: +47 22856481; fax: +47 22856422.
E-mail address: tomhj@fys.uio.no (T.H. Johansen).
0921-4534/$ - see front matter Ó 2007 Published by Elsevier B.V.
doi:10.1016/j.physc.2007.04.122
other except on one face of the tape, i.e., the Ni rim became
L-shaped. Then, MO indicator films [12] were placed over
each of the two parts, as shown in Fig. 1, allowing us to
observe the flux penetration in both parts simultaneously
and make comparisons.
2. Experimental results
Shown in Fig. 2a and b are MO images taken at two
consecutive field values during ramping up after zerofield-cooling (ZFC) down to 4 K. The darker horizontal
stripes represent the individual filaments which shield the
magnetic field. The FM rims (right part) appear brighter
because they effectively anchor the external magnetic field.
As a consequence, the penetration of magnetic field is
strongly hampered in the part with FM edge coating. We
find that this part has always an overall lower flux density,
thus giving direct evidence that the ferromagnetic rim offers
a clear additional shielding of the tape. Fig. 2c shows the
subsequent remanent state, where the stray field of the
trapped flux in the central region remagnetizes the tape in
the outer region where the field is reversed. This also
Author's personal copy
804
V.V. Yurchenko et al. / Physica C 460–462 (2007) 803–804
Fig. 1. Sketch of the experimental set-up. It allows simultaneous
observation of flux distributions in the Ni-coated and bare SC tape.
Fig. 3. MO images of Bi-2223/Ag tapes with and without Ni edge coating.
The images were taken at 77 K in remanent state after applying a
maximum field of B = 85 mT.
At higher temperatures the stray field of the trapped flux
becomes weaker, and eventually its magnitude becomes
insufficient to remagnetize the FM. Unlike at 4 K, at
77 K the rims give very low contrast, see Fig. 3 where only
a small part of the lower rim shows remagnetization. This
is a clear indication that the total effective field on the edge
is strongly suppressed, resulting from the superposition of
the positive coercive field of the magnetic rims and negative
field induced by the currents flowing in the superconductor.
Since the critical current depends on magnetic field, such a
field cancelling should increase jc(B), particularly when the
polarity of the field (current) reverses.
Acknowledgement
The work was supported by the Research Council of
Norway, Grant No. 158518/431 (NANOMAT). V.V.Y.
acknowledges literature overview provided by A. Snezhko.
References
Fig. 2. MO images of Bi-2223/Ag tapes with and without Ni edge coating.
The images were taken after ZFC to 4 K in magnetic fields Ba = 20 mT
(a), 40 mT (b) and 0 mT (c).
applies to the part with Ni rim, since at 4 K the reverse field
on the edges exceeds the coercive field of the ferromagnet.
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