Institute of Physics Publishingdoi:10.1088/1742-6596/43/1/101

Journal of Physics: Conference Series 43 (2006) 405–408
7th European Conference on Applied Superconductivity

Trapped Field over 4 Tesla on GdBaCuO Bulk by Pulse Field
Method and Magnetizing Mechanism
Hiroyuki Fujishiro1, Masahiko Kaneyama1, Tatsuya Tateiwa1 and Tetsuo Oka2
1
2

Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, JapanIMRA Material R&D Co. Ltd., 5-50 Hachiken-cho, Kariya 448-0021, Japan

E-mail: fujishiro@iwate-u.ac.jp
Abstract. We report a new pulse field magnetizing (PFM) method using the modified multipulse technique combined with a stepwise cooling (MMPSC) to obtain the trapped field BTPover 4 T on a bulk superconductor. The BTP value as high as 4.47 T has been attained on thesurface of the GdBaCuO bulk, which is the present highest using PFM. The key points toobtain the higher BTP are the introduction of a small amount magnetic flux (BTP~1 T) into thebulk with the “M-shaped” trapped field distribution at a higher starting temperature Ts~45 K,
and the following application of the optimum stronger pulse fields of Bex~6.7 T at a lower
Ts~30 K. The reduction in the temperature rise DT due to the already existing trapped flux is avery important issue.

1. Introduction
The superconducting bulk magnet using the melt-textured REBaCuO superconducting bulk is one of
the exemplary models for practical applications. Recently, the pulse field magnetizing (PFM) has been
intensively investigated because of the relatively compact and mobile setup. The trapped field BTP by
PFM is, however, generally smaller than that attained by the field-cooled magnetizing (FCM) at low
temperatures, possibly due to the large temperature rise DT caused by the dynamical motion of themagnetic fluxes. The highest BTP value ever reported was 3.80 T on the SmBaCuO bulk at 30 K by theiteratively magnetizing pulsed-field method with reducing pulse amplitude (IMRA) [1].
DT during PFM results from the sum of the pinning loss Qp related to the flux trap and the viscous
loss Qv related to the flux movement. After applying the successive pulse fields with the same
amplitude, BTP increases with increasing number of the magnetic pulse and then saturates to a final
value. DT decreases and saturates, then the main contribution to DT being due to Qv because of theabsence of the increment of BTP [2,3]. In order to reduce DT and to enhance BTP during PFM, the metalring setting onto the bulk disk is very effective due to the heat transfer from the bulk to the ring [4].
Sander et al. attempted a multi pulse technique with stepwise cooling (MPSC) and confirmed theenhancement of BTP [5]. The lowering of the starting temperature Ts of the bulk is also effective toenhance BTP because of the increase in the pinning force Fp.
Taking these results into consideration, we recently developed a new PFM method using themodified MPSC technique (MMPSC) and the attained BTP as high as 4.47 T on the surface of the
GdBaCuO bulk superconductor, whch is the present highest using PFM [6]. After a small amount ofmagnetic flux has been trapped into the bulk center at a higher Ts, the higher pulse field is applied at alower Ts. The reduction in DT due to the already trapped flux and the optimization of higher Bex at the© 2006 IOP Publishing Ltd

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Table 1. The conditions of MMPSC and SPA processes and the trapped fields for each step.
Run
MMPSC1

MMPSC2

MMPSC3SPA1

Ts (Bex)
BT(C)
BT(E)
Ts (Bex)
BT(C)
BT(E)
Ts (Bex)
BT(C)
BT(E)
Ts (Bex)
BT(C)
BT(E)

Pulse No.1
45 K (4.54 T)
0.89 T
2.49 T
63 K (3.84 T)
0.93 T
1.82 T
45 K (4.03 T)
0.15 T
0.45 T
31 K (6.61 T)
2.36 T
1.74 T

Pulse No. 2
48 K (4.60 T)
1.00 T
2.20 T
63 K (3.97 T)
1.00 T
1.64 T
45 K (3.96 T)
0.11 T
0.48 T
32 K (6.73 T)
2.78 T
2.39 T

Pulse No. 3
29 K (6.72 T)
2.61 T
2.27 T
24 K (6.73 T)
3.57 T
2.16 T
24 K (6.65 T)
2.79 T
1.34 T
31 K (6.60 T)
2.93 T
2.42 T

Pulse No. 4
28 K (6.59 T)
4.47 T
2.42 T
33 K (6.78 T)
3.27 T
2.31 T
29 K (6.72 T)
3.01 T
2.42 T
29 K (6.67 T)
2.98 T
2.42 T

lower Ts are key points to enhance BTP. The higher BTP than 4 T, however, could not be exactly
predicted only from Bex and Ts just before applying the field. Another parameters such as the
microscopic BTP distribution may critically govern the BTP value by PFM. In this paper, we survey the
MMPSC method under several conditions and discuss what are the important factors to enhance BTP.
2. Experimental
A highly c-axis oriented GdBaCuO bulk disk (45 mm in diameter and 15 mm in thickness, Nippon
Steel Co., Ltd.) was used. The inset of figure 1 shows the experimental setup around the bulk. The
bulk was magnetized under the condition sandwiched by two soft-iron yokes; the bulk was tightly
stacked on the soft-iron disk on the cold stage of the helium refrigerator and was magnetized using apulse coil dipped in liquid N2 with a soft-iron cylinder. Two Hall sensors (F.W. Bell, model BHA 921)
were adhered to the position C (bulk center) and E (7 mm distant from the bulk edge) and the time
evolutions of the local field BL(C)(t) and BL(E)(t) were monitored using a digital oscilloscope. The
applied field m0Ha(t), of which the maximum strength was defined as Bex, was monitored by the current
I(t) flowing through the shunt resistor. The temperature T(t) was monitored at the position T.
We have performed three MMPSC runs. Table 1 summarizes the conditions [Ts and Bex] and thetrapped fields [BT(C) and BT(E)] for each run. For the MMPSC1 run, the pulse field of Bex~4.5 T wasapplied twice (pulse No. 1 and No. 2) at Ts~45 K. Then the bulk was cooled down to Ts~29 K and thehigher Bex~6.6 T was applied twice (pulse No. 3 and No. 4). The highest BTP=BT(C)=4.47 T was
attained for the pulse No. 4 [6]. The MMPSC2 run was performed at Ts~63 K for the pulses No. 1 andNo. 2 with Bex~3.9 T, in which BT(C)~1 T could be attained similar to the MMPSC1 run. This process
enabled us to clarify the effect of the higher Ts. The MMPSC3 run was performed at Ts~45 K for thepulses No. 1 and No. 2 with lower Bex~4.0 T in order to clarify the effect of the BT(C) value before
applying the No. 3 pulse. For comparison, the results for four successive magnetic pulse applications
with the same strength (SPA) are also shown for Bex~6.6 T at Ts~30 K (SPA1).
3. Results and Discussion
Figure 1 pictures BT(C) at the bulk center as a function of the maximum temperature Tmax after
applying the each pulse field for each run. The trapped field BTFC by FCM is also presented, whichcorresponds to the possible maximum of the trapped field in this bulk [3]. Figure 2 shows the timedependences of the applied field m0Ha(t) and the local fields [BL(C)(t), BL(E)(t)] after applying thepulse No. 3 (or No. 4) for each run. For the MMPSC1, the bulk temperature rises from Ts=45 K to
Tmax=61 K for the pulse No. 1, but Tmax decreased to 52 K for the pulse No. 2 due to the absence of theadditional flux trap. For the pulse No. 3, BT(C) increases to 2.61 T and the temperature rises from
Ts=29 K to Tmax=54 K (DT=25 K). In figure 2(a), BL(C)(t) and BL(E)(t) rise up, take a maximum (~5.2
T) and then slowly decrease to 2.0~2.5 T. For the pulse No. 4, as shown in figure 2(b), the maximum

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6
MMPSC1
MMPSC2
MMPSC3
SPA1

5

BT(C) (T)

No.4

4
No.3

3

B

yoke

bulk
yoke

coil

1
0
40

FC

No.1

2

E

T

T

No.3

No.4
No.3

C

No.1

No.1
No.2

No.1

50

60

70
Tmax (K)80

90

Figure 1. Trapped field BT(C) at the bulk centre as a function of the
maximum temperature Tmax after applying the pulse field for each run.
77(a) MMPSC1 (pulse No. 3)
T =29K, B =6.72T
BL(C)

4

BL(E)

3

L

L

s

m0Ha
a

5

1

0

0

100

ex

6

BL(C)

5

a

BL(E)

B , m H (T)

s

5

m0Ha

BL(C)
BL(E)

0

50
Time (ms)100(d) MMPSC3 (pulse No.3)
T =24K, B =6.65T
s

ex

m0Ha
BL(C)
BL(E)

4

0

4

0

a

m0Ha

7
(c) MMPSC2 (pulse No.3)
T =24K, B =6.73T

6

3

L

L

3

1

7

B , m H (T)

4

2

50
Time (ms)

ex

5

2

0

(b) MMPSC1 (pulse No. 4)
T =28K, B =6.59T

6

0

ex

0

a

B , m H (T)s

B , m H (T)

6

2

2
C

1
0

3

E

0

C

1

T

E

T

50
Time (ms)

100

0

0

50
Time (ms)100

Figure 2. The applied field m0Ha(t) and the local fields (BL(C)(t), BL(E)(t)) for the (a) pulse No. 3 of
MMPSC1, (b) pulse No. 4 of MMPSC1, (c) pulse No. 3 of MMPSC2 and (d) pulse No. 3 of MMPSC3.

BL(E) decreases to 4.6 T, while the maximum BL(C) remains at 5.1 T and BTP=BT(C) survives at 4.47 T.
These results suggest that DT is suppressed during the pulse No. 4 and that this small DT (~19 K)
obstructs the escape of the magnetic fluxes for the pulse No. 4.
In the MMPSC2 run, BT(C)~1 T was obtained after the pulse No. 2 similar to that for the MMPSC1.
For the pulse No. 3, BL(E)(t) takes a maximum ~5.5 T, but BL(C)(t) takes a lower maximum of 4.3 T
and then approaches to BT(C)=3.57 T, in contrast to the MMPSC1 run. Ts for the pulse No. 3 is 24 K
and is by 5 K lower than that for the MMPSC1. The Fp value increases with decreasing T and themagnetic fluxes cannot fully penetrate into the bulk center for the pulse application of Bex=6.73 T at
this low Ts. The minute control of Ts is very important together with that of Bex. For the MMPSC3 run,the smaller trapped field BT(C)~0.1 T was provided after the pulse No. 2. When the pulse field of408

Bex=6.65 T was applied at Ts=24 K as the pulse No. 3, the larger DT occurred and Tmax reached 61 K.
In figure 2(d), it should be noticed that a sharp drop in BL(C)(t) suddenly occurred at the bulk center inthe descending stage of PFM due to the large DT. The smaller BT(C) after the pulse No. 2 introduces
the large DT and promotes the escape of the trapped fluxes. For the SPA1 run, DT for the pulse No. 1
is larger than that for the pulse No. 3 of the MMPSC runs. DT decreases and BT(C) increases slightly
with increasing number of applied filed. For the virgin state bulk, the pulse field of Bex~6.6 T at Ts~30
K may be too large to enhance the trapped field by the following pulses. The similar sudden drop in
BL(C)(t) as figure 2(d) was also monitored.
Figure 3 shows the one-dimensional BT profile on the bulk surface through the center for the pulses
No. 2, No. 3 and No. 4 of each MMPSC run, in which the measured BT(C) and BT(E) values are used
and BT=0 at the bulk edge is assumed. The BT values between the points C, E, and the bulk edge arelinearly interpolated and the BT profile is assumed to be symmetrical along the circumferential
direction. For the MMPSC procedures, it is to be emphasized that the BT profile for the pulse No. 2
shows “M-shaped” one. The BT barrier in the periphery region (=BT(E)) is the highest for the
MMPSC1 and the lowest for the MMPSC3. For all the MMPSC runs, the “M-shaped” profile changes
to the “cone-shaped” profile for the pulse No. 3 (or No. 4) with Bex~6.6 T at Ts~30 K. However, it
should be noticed that the height of the cone (=BT(C)) depends on the height of the “M-shaped”
profile; BT(E) should be higher and BT(C) should be lower. On the other hand, the BT profile of SPA1
is “cone-shaped” one from the pulse No. 1. In this “cone-shaped” profile, it seems that the sizableenhancement of BT(C) cannot be realized by the succeeding pulse.
In summary, we performed the MMPSC method for the GdBaCuO bulk under several conditions
and clarified what are the important factors to enhance BTP over 4 T. At the first stage, the “M-shaped”
trapped field profile should be realized on the bulk; BT(C) at the bulk center should be as low as ~1 T
and the barrier height BT(E) at the periphery region must be enhanced to 2.5~3 T. At the final stage atlower Ts, the appropriate higher field Bex, should be applied. The optimum Bex and Ts at each stage
depend on the bulk used and should be precisely controlled. The reduction in the temperature rise DT
due to the already existing trapped flux is also an important issue.
5

diameter of bulk

No.4

C

4
BT(E)

3

No.4

No.4

No.3

T

B (T)

E

No.4

2

No.3

BT(C)
No.3

1
No.2

No.2

No.2

MMPSC1

MMPSC2

MMPSC3

No.1

0
SPA1

Figure 3. The estimated trapped field distribution for No. 2, No. 3 and No. 4 pulses.

References[1] Yanagi Y, Itoh Y, Yoshikawa M, Oka T, Hosokawa T, Ishahara H, Ikuta H and Mizutani U
2000 Advances in Superconductivity XII (Tokyo: Springer-Verlag) p 470[2] Fujishiro H, Yokoyama K, Oka T and Noto K 2004 Supercond. Sci. Technol. 17 51[3] Fujishiro H, Yokoyama K, Kaneyana M, Oka T and Noto K 2004 Physica C 412-414 646[4] Fujishiro H, Yokoyama K, Kaneyama M, Oka T and Noto K 2005 IEEE Trans. Appl.
Supercond. 15 3762
[5] Sander M, Sutter U, Koch R and Kläser M 2005 Supercond. Sci. Technol. 13, 841[6] Fujishiro H, Kaneyama M, Tateiwa T and Oka T 2005 Jpn. J. Appl. Phys. 44 L1271