Physica C 471 (2011) 893–896

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Physica C
journal homepage: www.elsevier.com/locate/physc

Jc distribution measurement and analysis on superconducting bulk
using ‘‘Magnetoscan’’ method
H. Fujishiro ⇑, T. Handa, T. Naito, Y. Kikuchi
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japana r t i c l ei n f o

Article history:
Available online 12 May 2011
Keywords:
Magnetoscan
Critical current density
Flux intrusion
Bulk superconductora b s t r a c t
We have investigated the detection of the crystal defects and/or inhomogeneity of critical current density(Jc) in superconducting bulk experimentally and theoretically by the ‘‘Magnetoscan’’ technique. To detect
them sensitively, the Hall probe located near the permanent magnet should be scanned as near as possible on the surface of the superconducting bulk. The scan of the permanent magnet causes the ﬁeld trapon the bulk, which seriously inﬂuences the magnetoscan signal detected by Hall probe.
Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction
The critical current density (Jc) is one of the most importantparameters for high temperature superconducting bulks, tapes
and thin ﬁlms. The contact-less and non-destructive techniques
to measure the Jc value and its distribution are desired for thepractical application as a substitute for a direct measurement by
a four-probe method. For example, a third harmonics method
was developed for the superconductors to measure the Jc distribution [1]. When an ac magnetic ﬁeld is applied to the superconductor by a multi-turn coil, the third harmonics generated by theshielding current are suddenly exited and the Jc distribution wasdetected. Ohshima et al. proposed the permanent magnet method
for measurement of Jc distribution [2]. After approaching a permanent magnet closer to the superconductor, the maximum of theelectromagnetic force is found to be approximately proportional
to Jc. Eisterer et al. proposed a ‘‘Magnetoscan’’ technique to detect
the local inhomogeneities directly without destroying the bulk
crystal [3].
For a superconducting bulk, the Jc value cannot be measured directly using a conventional four-probe method because of large
heat generation at the electric contacts. As a result, the Jc value isusually estimated by a magnetization method, that is M-H curve,
using a small peace cut from the bulk crystal [4]. On the otherhand, the Jc distribution for the whole bulk disk is commonly estimated by scanning the remnant magnetic ﬂux proﬁle afterﬁeld-cooled magnetization (FCM), which is called ‘‘Hall scan’’. This
technique is rather insensitive to the material inhomogeneity,
since all parts of the sample volume contribute to the magnetic⇑ Corresponding author. Tel./fax: +81 19 621 6363.
E-mail address: fujishiro@iwate-u.ac.jp (H. Fujishiro).
0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2011.05.082

ﬁeld at each measuring point, although the melt processed bulks
are strongly inhomogeneous. We have investigated the enhancement of the trapped ﬁeld BT magnetized by pulsed ﬁeld magnetization (PFM) and the relation between the trapped ﬁeld proﬁle and
the Jc distribution [5]. The trapped ﬁeld proﬁle on the bulk magnetized by PFM was fairly inhomogeneous because of the complicated ﬂux motion and the instantaneous and local temperaturerise. The development of the measuring technique to connect thetrapped ﬁeld proﬁle by PFM with the Jc distribution is desired.
In this paper, we constructed the apparatus of the ‘‘Magnetoscan’’ technique to detect the Jc distribution and/or crystal defects
on the superconducting bulk. We investigated the optimum conﬁguration between the permanent magnet, the Hall probe and thebulk surface experimentally and theoretically.
2. Experimental setup
Fig. 1a shows the experimental setup of the Magnetoscan method, which was proposed by Eisterer et al. [3]. A Hall probe (AREPOC, HHP-VP) with an active area of 0.05  0.05 mm2 was ﬁxed
on the edge of the small NdFeB permanent magnet (u 6 mm,20 mm in height; the surface magnetic ﬁeld of 0.43 T). The permanent magnet and Hall probe were mounted in the GFRP holder inwhich the distance between the center of the permanent magnet
and that of the active region of the Hall probe was ﬁxed
L0 = 5.7 mm. The holder was scanned with a step of 0.5 mm alonga ‘‘meander line’’ just above the bulk dipped in liquid nitrogenand the magnetoscan signal Bz along the z-direction, which was detected by the Hall probe, was recorded. The permanent magnet always follows the Hall probe. By scanning the Hall probe
together with the magnet slightly above the bulk surface, local differences in Jc, associated with material inhomogeneity or crystal

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H. Fujishiro et al. / Physica C 471 (2011) 893–896

Fig. 2. The magnetoscan signals Bz for the Bulk-A at 77 K. The scan was performed
along the y-direction.

Fig. 1. (a) The experimental setup of the ‘‘Magnetoscan’’ method. The measured
samples of (b) Bulk-A and (c) Bulk-B are indicated.

defect, are detected. The gap between the Hall probe and the bulk
surface was Z0 = 0.25 mm and between the face of the permanent
magnet and the bulk surface Z1 = 1.5 mm.
We measured the bulk sample (Bulk-A) shown in Fig. 1b, inwhich the acrylic plate with 2 mm in thickness was sandwichedby two GdBaCuO bulk blocks (9  18  3 mm3) using epoxy resin.
After the ‘‘Magnetoscan’’, the remnant magnetic ﬁeld Bz was measured along a same meander line using the another Hall sensor(Bell, BHA921; active area u 0.51 mm) without magnet, which
was named the ‘‘Hall scan’’. The ‘‘magnetoscan’’ signal was alsomeasured using a half GdBaCuO disk (Bulk-B) (u 45 mm and5 mm in thickness) as shown in Fig. 1c, in which the two ditches(0.8 mm in width and 1.3 mm in depth) were scribed on the surface as quasi-crystal defects.
3. Results and discussion
3.1. Results of experiments
Fig. 2 depicts the magnetoscan signals Bz scanned along they-direction for the Bulk-A at 77 K. At the edge of the bulk, the signal shows a dip for the scan from epoxy to bulk and shows a peakfor the scan from bulk to epoxy. It should be noted that the shapeof the magnetoscan signal changes with increasing the number ofscanning; the depth of the dip and the peak height in the Bz–y proﬁle become broad.
Fig. 3a presents the proﬁles of the magnetoscan signal Bz
scanned along the x-direction across the acrylic plate for theBulk-A at 77 K. The magnetoscan signal at the bulk edge shows asimilar shape to the y-direction scan, and a new dip and peak proﬁle can be seen around the acrylic plate, which suggests that thecrystal defect or inhomogeneity of Jc as small as 2 mm in size canbe detected by the magnetoscan signal. It should be noted thatthe shape of the magnetoscan signal around the acrylic platechanges with increasing number of scanning, similarly to that atFig. 3. The (a) ‘‘magnetoscan’’ signals Bz and the (b) ‘‘Hall scan’’ signals Bz for theBulk-A at 77 K. The scan was performed along the x-direction across the acrylicplate.

the edge of the bulk. Fig. 3b shows the ‘‘Hall scan’’ proﬁles scannedalong the x-direction after the magnetoscan for the Bulk-A. The
Hall scan proﬁles suggest that the magnetic ﬂux was not trappedon the acrylic plate, but on the superconducting bulk clearly during
the magnetoscan process. This result suggests that the Hall scan
after the scanning of the permanent magnet is a possible technique
to detect the inhomogeneity of Jc and/or the crystal defect on thebulk surface.
Fig. 4 shows the magnetoscan signals Bz scanned along thex-direction for the Bulk-B. The anomaly of the magnetoscan signals
can be detected both at the ditches 0.8 mm in width and at theedge of the bulk.

H. Fujishiro et al. / Physica C 471 (2011) 893–896

Fig. 4. The magnetoscan signals Bz for the Bulk-B at 77 K. The scan was performed
along the x-direction.

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Fig. 6. The simulated magnetoscan signal Bx along the x-direction as a function of L0
under the conditions of constant Z0 = 0.1 mm and Z1 = 1.0 mm values. The scan wasalso performed along the x-direction.

the Hall probe were calculated in the gap between the coil andthe bulk for each position.
Fig. 5a shows the simulated magnetoscan signal Bz as a functionof Z0 under the conditions of constant L0 = 3.2 mm and Z1 = 1.0 mm
values. The simulated proﬁle shows a peak for the scan from bulk
to ditch (x = 0 mm) and shows a dip for the scan from ditch to bulk
(x = 1 mm), which reproduce the experimental results as shown in
Fig. 3a. The signal becomes sharp and large drastically withdecreasing Z0. Fig. 5b shows the magnetoscan signal Bz as a function of L0 under the conditions of constant Z0 = 0.1 mm andZ1 = 1.0 mm values. The Bz value becomes sharp and large withdecreasing L0. However, the change in Bz(L0) is rather moderatecompared with the Z0 dependence of the Bz signal. The Z0 and L0
dependences of the magnetoscan signal were also conﬁrmed
experimentally (not shown).
Fig. 6 shows the simulated magnetoscan signal Bx along thex-direction as a function of L0 under the conditions of constantZ0 = 0.1 mm and Z1 = 1.0 mm values. The magnitude of By decreases
with increasing L0, which is a similar behavior to Bz as shown inFig. 5b. The results of the simulation are consistent with those reported by Zehetmayer et al. [7].
4. Summary
We have investigated the detection of the crystal defects and/orinhomogeneity of critical current density (Jc) in superconducting
bulk experimentally and theoretically by the ‘‘Magnetoscan’’ technique. The important results and conclusions obtained from thestudy are summarized as follows,Fig. 5. (a) The simulated magnetoscan signal Bz as a function of Z0 under theconditions of constant L0 = 3.2 mm and Z1 = 1.0 mm values. (b) The magnetoscan
signal Bz as a function of L0 under the conditions of constant Z0 = 0.1 mm andZ1 = 1.0 mm values.

3.2. Results of simulation
We have simulated the shape and the magnitude of the magnetoscan signal as parameters of the conﬁgurations between permanent magnet, Hall probe and surface of the bulk. Let us
consider a superconducting bulk (Jc = 1.0  108 A/m2 at 77 K) witha ditch 1 mm in width and 0.5 mm in depth as a quasi-crystal defect on the bulk surface. In the simulation, a permanent magnet
was replaced with a small coil equivalently and the current was induced in the coil. Physical phenomena during magnetoscan weredescribed using electromagnetic equation [6], and the magnetic
ﬂux distribution and the magnetoscan signals (Bz, Bx) detected by(1) The quasi-crystal defects about 1–2 mm in size can bedetected by the Magnetoscan technique experimentally.
(2) The magnitude of the magnetocan signal Bz increased sensitively with decreasing the gap Z0 between the Hall probe andthe surface of the bulk superconductor, which is also conﬁrmed by the numerical simulation. The magnetoscan signal
Bx parallel to the bulk surface is also sensitive to detect thequasi-crystal defect.
(3) The scan of the permanent magnet causes the ﬁeld trap on
the bulk surface, which seriously inﬂuences the magnetoscan signal detected by Hall probe. Therefore, the ‘‘Hall scan’’after the scanning of the permanent magnet is a possible
technique to detect the inhomogeneiety of Jc and/or thecrystal defect on the bulk surface. The position and the magnitude of the permanent magnet must be optimized toenhance the sensitivity and the reliability of the magnetoscan signal.

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