Japanese Journal of Applied Physics
Vol. 44, No. 39, 2005, pp. L 1221–L 1224
#2005 The Japan Society of Applied Physics

Record-High Trapped Magnetic Field by Pulse Field Magnetization
Using GdBaCuO Bulk Superconductor
Hiroyuki FUJISHIRO, Masahiko K ANEYAMA, Tatsuya TATEIWA and Tetsuo O KA1
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
1
IMRA Material R&D Co., Ltd., 5-50 Hachiken-cho, Kariya, Aichi 448-0021, Japan(Received June 17, 2005; accepted August 15, 2005; published September 16, 2005)

A trapped magnetic ﬁeld BT P as high as 4.47 T, which is the highest reported using pulse ﬁeld magnetization to date, has beenrealized on the surface of a GdBaCuO bulk superconductor by a modiﬁed multi pulse technique combined with stepwise
cooling. Following an introduction of a small amount magnetic ﬂux into the bulk center by applying lower pulse ﬁelds
Bex ¼ 4:5 { 4:6 T twice at a higher starting temperature Ts ¼ 45 { 48 K, higher ﬁelds of Bex ¼ 6:6 { 6:7 T are applied threetimes at a lower Ts ¼ 28 { 29 K. The reduction in the temperature rise due to the already existing trapped ﬂux, in addition tothe optimization of the higher Bex value at the lower Ts , is a key point in enhancing BT P . [DOI: 10.1143/JJAP.44.L1221]
KEYWORDS: bulk superconductor, pulse field magnetizing, temperature rise, trapped field, multi pulse technique, step cooling

For the practical applications of high temperature superconducting bulks as a high-strength bulk magnet in amagnetic separation system, a magnetron sputtering apparatus and other magnet-based systems, static ﬁeld-cooledmagnetization (FCM) is a standard method of realizing thehighest trapped ﬁeld of the maximum trapping ability. A
trapped ﬁeld BT FC of 17.24 T at 29 K has been realized by
FCM between two resin-impregnated YBaCuO disks(26.5 mm in diameter and 15 mm in thickness).1) Recently,
pulse ﬁeld magnetization (PFM) has been intensively
investigated because of the relatively compact, inexpensive
and mobile setup of the apparatus used. The trapped ﬁeld
BT P and the total trapped ﬂux T P attained by PFM are,however, generally smaller than those attained by FCM. The
deterioration is probably due to the large temperature riseT caused by the magnetic ﬂux motion against the vortex
pinning force Fp and the viscous force Fv . In order toenhance BT P and T P , T reduction is an indispensable
issue. Yanagi et al. reported that BT P ¼ 3:8 T was attainedon the surface of the SmBaCuO bulk (36 mm in diameter,
16 mm in thickness) at 30 K by an improved iterative pulse
ﬁeld magnetization method with reduced amplitude(IMRA),2) which is the highest published BT P value to date.
Sander et al. attempted a multi pulse technique withstepwise cooling (MPSC) and conﬁrmed the enhancementof BT P .3) In this technique, the bulk was cooled down below
the superconducting transition temperature Tc and magnetic
pulses were applied several times. The bulk was further
cooled down repeatedly and magnetic pulses with the same
amplitude were applied.
We have studied the time evolution and spatial distributionof the temperature rise TðtÞ on the surface of cryocooled
REBaCuO bulks (RE ¼ Gd, Y, Sm) during PFM andestimated the total generated heat Q on the basis of the
maximum T and the speciﬁc heat C of the bulk.4–7) After
applying the successive pulse ﬁelds with the same amplitude, T decreases with increasing number of applied pulses
and then saturates to a ﬁnal value. The increment of thetrapped ﬁeld BT P also decreases with increasing number ofpulses similar to the behavior of T. In this letter, we report
the PFM procedure using the modiﬁed MPSC (MMPSC) fora cryocooled GdBaCuO bulk and the attainment of BT P ashigh as 4.47 T, the new highest record by PFM.

A GdBaCuO bulk superconductor disk (45 mm in diameter and 15 mm in thickness) with a highly c-axis oriented
structure was used, which was fabricated by Nippon Steel.
The bulk is composed of GdBa2 Cu3 Oy (Gd123) and Gd2 BaCuO5 (Gd211) with a molar ratio of Gd123 : Gd211 ¼
1:0 : 0:4, 10.0 wt % Ag2 O and 0.5 wt % Pt. The bulk was
initially impregnated in vacuum by epoxy resin, and theupper and bottom resin layers were removed from the bulk
surfaces in order to enhance the thermal response. The inset
of Fig. 1(a) shows the experimental setup around the bulk.
The bulk was magnetized while being sandwiched by two
soft-iron yokes; the bulk was tightly stacked on the lower
soft-iron yoke, (a disk 40 mm in diameter and 20 mm inthickness) on the cold stage of the helium refrigerator in thevacuum atmosphere and was magnetized using a pulse coildipped in liquid N2 with the upper soft-iron yoke (a cylinder
40 mm in diameter and 65 mm in thickness). The rise time ofthe pulse ﬁeld was 12 ms and the pulse duration was
120 ms. The time evolution of temperature TðtÞ was
measured using a chromel-constantan thermocouple (76 mm
in diameter) adhered to the bulk surface using GE7031
varnish at a 2 mm distance from the bulk center. Two Hallsensors (F.W. Bell, model BHA 921) were adhered topositions C (bulk center) and E (5 mm away from the bulkedge) and the time evolutions of the local ﬁelds BL (C)(t) andBL (E)(t) were monitored using a digital oscilloscope. The
trapped ﬁeld BT P was determined at position C. The timedependence of the applied ﬁeld 0 Ha ðtÞ was monitored by
the current IðtÞ ﬂowing through the shunt resistor. The
maximum strength of the pulse ﬁeld 0 Ha ðtÞ was deﬁned asBex .
We performed the modiﬁed MPSC (MMPSC); ﬁve
magnetic pulses were applied at diﬀerent initial temperatures Ts on the bulk surface. A pulse ﬁeld Bex  4:5 T wasapplied twice at Ts  45 K in order to produce a small
magnetic ﬁeld (1 T) trapped at the bulk center. Hereafter,
we refer to these two applied pulses as No. 1 and No. 2,
respectively. Then the bulk was cooled down to Ts  29 K
and a higher Bex of 6:6 T was applied three times (No. 3,
No. 4 and No. 5), while keeping Ts at 29 K.
Figure 1 shows the time dependences of the applied ﬁeld
0 Ha ðtÞ and of the local ﬁelds [BL (C)(t), BL (E)(t)] forapplied pulses No. 1 to No. 4. The inset of Figs. 1(b)–1(d)L 1221

L 1222

Jpn. J. Appl. Phys., Vol. 44, No. 39 (2005)

H. FUJISHIRO et al.

77(a) pulse No. 1
6 T =45K, B =4.54T
E

bulk
yoke

coil

T

µ0Ha

L

L

3

BL(C)

2

0

0

20

40
60
Time (ms)

80

BL(C)
BL(E)

3

0

100

0

20

80

100

s

80

100

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

6

s

ex

a

B , µ H (T)

5

ex

5
4

0

0

4

L

3
0

a

B (C)
L

1

3
µH

2

µH

2

0

L

1

B (E)

0

20

40
60
Time (ms)

a

B (C)
B (E)
L

L

0

40
60
Time (ms)7(c) pulse No. 3
T =29K, B =6.72T

6

a

µ0Ha

4

1

7

L

ex

2

BL(E)

1

B , µ H (T)

s

5

0

4

0

a

B , µ H (T)

C

a

ex

B , µ H (T)s5(b) pulse No. 2
T =48K, B =4.60T

6

yoke

80

100

0

0

20

40
60
Time (ms)Fig. 1. Time evolutions of applied ﬁeld 0 Ha ðtÞ and local ﬁelds BL (C)(t) and BL (E)(t) at positions C and E, for (a) pulse No. 1(Ts ¼ 45 K, Bex ¼ 4:54 T), (b) pulse No. 2 (Ts ¼ 48 K, Bex ¼ 4:60 T), (c) pulse No. 3 (Ts ¼ 29 K, Bex ¼ 6:72 T) and (d) pulse No. 4(Ts ¼ 28 K, Bex ¼ 6:59 T). The inset of (a) shows the experimental setup and those of (b)–(d) show the trapped ﬁeld distribution
scanned 3.5 mm above the bulk surface.

shows the trapped ﬁeld distribution after each pulse, which
was monitored 3.5 mm above the bulk surface in open space.
For pulse No. 1 in Fig. 1(a), Bex ¼ 4:54 T was applied to thevirgin-state bulk at Ts ¼ 45 K. BL (C) and BL (E) rise up, takea maximum within 15 ms and then slowly decrease due tothe ﬂux creep to the ﬁnal stable value. The maximum valueof BL (E) is 4.3 T, nearly equal to Bex . On the other hand,
BL (C) reaches only about 0.9 T. For pulse No. 2 shown in
Fig. 1(b), the pulse ﬁeld of Bex ¼ 4:60 T was applied to thebulk at Ts ¼ 48 K. Now the maximum BL (E) is 2.9 T, which
is far smaller than 4.3 T for pulse No. 1, suggesting that thealready trapped ﬂuxes obstruct the intrusion of the magnetic
ﬂux.6) BL (C) and the total trapped ﬂux T out 3.5 mm above
the bulk surface slightly increase for pulse No. 2.
Figure 2 shows the time evolution of temperature TðtÞafter each pulse ﬁeld. For pulse No. 1, TðtÞ gradually
increases, takes a maximum of 60 K at 10 s, and thengradually decreases to the initial temperature in 20 min. The
temperature rise T (15 K) for pulse No. 1 to the virginstate mainly comes from the heat generated by ﬂux
trapping.6) For pulse No. 2, T markedly decreases to4 K as a result of the very small additional ﬂux trap, andviscous loss should be the dominant origin of the heat as
analyzed in our previous paper.6)

Fig. 2. Time evolutions of temperature TðtÞ on bulk surface after eachpulse ﬁeld application. The arrow shows the origin of the time t.

After cooling the bulk down to Ts ¼ 29 K, a higher
magnetic pulse No. 3, Bex ¼ 6:72 T was applied. In
Fig. 1(c), BL (C)(t) and BL (E)(t) rise up, take a maximum(5:2 T) and then slowly decrease. The rise time of BL (C)(t)is long (19 ms) compared with that of BL (E)(t) (14 ms).
This time delay of BL (C)(t) roughly corresponds to the ﬂux
traveling time from points E to C.7) In contrast to Figs. 1(a)Jpn. J. Appl. Phys., Vol. 44, No. 39 (2005)

H. F UJISHIRO et al.

L 1223and 1(b), the maximum values of BL (C) and BL (E) arealmost the same, which suggests that the ﬂux fully reaches
the central region of the bulk for this strength of Bex . For
pulse No. 4 shown in Fig. 1(d), the pulse ﬁeld of Bex ¼
6:59 T was applied after cooling to Ts ¼ 28 K. The maximum BL (E) decreases to 4.6 T, again suggesting theobstruction of the ﬂux intrusion by the trapped ﬂux. On
the other hand, the maximum BL (C) remains at 5.1 T and thetrapped ﬁeld BT P [¼ BL (C) (t ! 1)] settles at 4.47 T. In
Fig. 2, the maximum T is 54 K for pulse No. 3 and decreases
to 49 K for pulse No. 4 in spite of the similar strength of Bex .
This suppressed T for pulse No. 4 obstructs the outﬂow of
the magnetic ﬂux. After removing the pulse coil with theupper yoke from the bulk in order to measure the trapped
ﬁeld distribution, BT P decreased to 4.27 T. The measured
BT P is slightly enhanced by the upper yoke, which ismagnetized by the trapped ﬁeld of the bulk. The trapped
ﬁeld BT out and the total trapped ﬂux T out in open space at
3.5 mm above the bulk was about 2.45 T and 17:02  104
Wb, respectively. For pulse No. 5 with Bex ¼ 6:80 T at Ts ¼
28 K, BT P remains at 4.47 T, while T out slightly increases to
17:35  104 Wb. Actually, pulse No. 5 is not necessary toeﬃciently realize the maximum BT P .
Figures 3(a) and 3(b) show the pulse number dependencesof the maximum temperature rise Tmax and the trapped
ﬁeld BT P for the present MMPSC method. For comparison,
the results of ﬁve successive magnetic pulse applications(SPA) with the same strength are also shown for Bex ¼ 4:5 T
at Ts ¼ 45 K and for Bex ¼ 6:6 T at Ts ¼ 29 K. In the SPA

process, as we mentioned beforehand, Tmax decreases and
BT P increases gradually with increasing number of pulses
and then both saturate to their ﬁnal values. These results
indicate that the heat generation is directly related with theincrement of BT P . The Tmax and BT P values after pulse
No. 3 (Bex ¼ 6:6 T at Ts ¼ 29 K) are almost the same both in
the MMPSC and SPA processes. However, the BT P values
after pulse No. 4 are quite diﬀerent; 4.47 T in MMPSC and2.93 T in SPA, respectively, despite nearly equal Bex and Ts
for both No. 4 pulses. At the present stage, BT P cannot be
exactly predicted only from the macroscopic conditions of
the bulk such as the BT P and Ts just before applying the pulse
ﬁeld. There seems to be a saddle point for the BT P
enhancement, and another parameter such as the minutetrapped ﬁeld distribution in the bulk may critically govern
the ﬁnal BT P value attained by PFM. The suitable choice of
the initial temperature Ts and the optimization of Bex foreach step are of crucial importance to attain a BT P value
higher than 4 T. The successive pulse application with thesame strength is very eﬀective in reducing the temperaturerise, and at least two pulses with the same Bex are necessaryfor each Ts .
For the FCM method, the maximum BT is expected to be
proportional to the diameter of the bulk, if Jc of each bulk
superconductor is identical. In the PFM method, however,
BT P is governed by a complicated ﬂux motion and a resultant
local temperature rise. Taking account of the results in SPA,
however, the attainment of the BT P ¼ 4:47 T value is mainly
attributable to the present MMPSC technique, and the eﬀectof the size of the bulk disk may be of secondary importancefor the present results. We tested the reproducibility by
performing the exact same procedure and conﬁrmed thatBT P  4:2 T can be consistently trapped at position C by thistechnique.
In summary, a trapped magnetic ﬁeld BT P as high as
4.47 T has been attained on the surface of a GdBaCuO
superconducting bulk (45 mm in diameter and 15 mm inthickness) sandwiched by two soft-iron yokes using themodiﬁed multi pulse technique with stepwise cooling(MMPSC). After a small amount of magnetic ﬂux (Bex 
4:5 T) is applied into the bulk center twice at Ts  45 K, thebulk is cooled down to Ts  29 K and then a higher Bex of
6:6 T is applied three times. The introduction of thetrapped ﬁeld of about 1 T near the bulk center at a high Ts ,which results in the reduction of the temperature rise, andthe optimization of Bex at a low Ts are very important inenhancing the trapped ﬁeld. A BT P of 4.47 T is attained forpulse No. 4, which is the highest value ever reported as
being attained by the pulse ﬁeld magnetization method. Inorder to clarify the relevant physical mechanisms, detailed
studies are in progress.
Valuable suggestions by Professor M. Ikebe and Professor
K. Noto of Iwate University are greatly acknowledged. The
technical support by Mr. A. Fujiwara of Gradual School of
Iwate University is also acknowledged. This work is
supported in part by the Iwate Prefecture Project, Japan.

Fig. 3. Pulse number dependences of (a) maximum temperature riseTmax and (b) trapped ﬁeld BT P for MMPSC. The results of ﬁve SPA
with the same strength are also presented for Bex ¼ 4:5 T at Ts ¼ 45 K
and for Bex ¼ 6:6 T at Ts ¼ 29 K.

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