Physica C 445–448 (2006) 399–402
www.elsevier.com/locate/physc

Development of ﬁve aligned superconducting bulk
magnets by pulse ﬁeld magnetizing
A. Fujiwara a, T. Tateiwa a, H. Fujishiro a,*, H. Hayashi b, T. Nagafuchi b, T. Oka ca

Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japanb
Kyushu Electric Power Co. Inc., 2-1-47 Shioharu, Fukuoka 815-8520, Japanc
IMRA Material R&D Co. Ltd., 5-50 Hachiken-cho, Kariya 448-0021, Japan
Available online 9 June 2006Abstract
A new high-Tc bulk magnet system, which consists of ﬁve aligned superconducting bulks, has been developed for the magnetic separation to purify waste water. Five GdBaCuO bulk blocks are cryo-cooled down to 40 K from the side face (along ab-plane) and aremagnetized in turn by the pulse ﬁeld magnetizing using a split-type pulse coil with the applied ﬁeld of Bex = 5.0–5.5 T. The trapped ﬁeld
BPT  2:0 T can be attained on the bulk surface and the B6T mm , 6 mm distant above the bulk surface in the open space, is 1.0–1.2 T for eachbulk. The results for the temperature rise and trapped ﬁeld are compared with those for the same bulk cooled down along the c-axis andmagnetized using a solenoid-type pulse coil.
 2006 Elsevier B.V. All rights reserved.
PACS: 74.25.Bt; 74.80.Bj; 74.60.Ge
Keywords: Superconducting bulk magnets; Multi-bulk; Pulse ﬁeld magnetizing; Trapped ﬁeld

1. Introduction
For the practical application of high-Tc superconducting
bulks, a high-strength bulk magnet in a magnetic separation system for waste water etc., is one of the typicalmodels [1]. On magnetizing the bulks, a ﬁeld cooled magnetizing (FCM) is usually used to obtain the maximum abilityof the trapped ﬁeld. However, the superconducting magnet
with a large bore must be used to magnetize the bulk byFCM. Recently, a pulse ﬁeld magnetizing (PFM) has been
investigated and developed because of a compact and inexpensive experimental setup. The trapped ﬁeld BPT by PFM
is, however, generally smaller than that attained by FCM
in terms of the large temperature rise DT due to the dynamical ﬂux motion. Several approaches have been performed
in order to reduce DT and to enhance BPT [2–5]. Recently,*Corresponding author. Tel./fax: +81 19 621 6363.
E-mail address: fujishiro@iwate-u.ac.jp (H. Fujishiro).

0921-4534/$ - see front matter  2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2006.04.026

we have attained the trapped ﬁeld of BPT ¼ 4:47 T by themodiﬁed multi pulse technique combined with stepwise
cooling (MMPSC), which is the present highest value by
PFM [6].
In order to realize a superconducting bulk magnet, thebulk is usually cooled down below the superconducting
transition temperature (Tc) along the c-axis on the cold
stage of the helium refrigerator, and is magnetized along
the c-axis using a solenoid-type pulse coil. In this setup,
only one bulk can be utilized for one refrigerator and only
the upper surface of the bulk can be utilized as a magnetic
pole in open space. Oka et al. constructed a multi-bulk
magnet system consisting of seven bulks and attained
0.9 T at the center of the pole surface in open space byFCM [7]. In this setup, however, it is diﬃcult to magnetize
the multi-bulks by PFM at a time because of the insuﬃcient mechanical strength of the supporting parts and soon. We have developed a prototype of multi-bulk magnet
system consisting of two aligned REBaCuO bulk disks
cooled down from the side face (along ab-plane) and400

A. Fujiwara et al. / Physica C 445–448 (2006) 399–402

attained the trapped ﬁeld BT = 1.97 T by PFM on the surface of the vacuum sheath [8].
In this paper, we develop the ﬁve-aligned superconducting bulk magnet system using rectangular-shaped high-Tc
superconducting bulks for the magnetic separation system,
which are cooled down along the ab-plane and magnetizedusing a split-type pulse coil. The superiority to the coolingalong the ab-plane is presented and the possibility of theextension of the number of the bulks is discussed.
2. Experimental
Highly c-axis oriented ﬁve rectangular-shaped GdBaCuO bulks (34 · 34 · 20 mm3, Nippon Steel Co., Ltd.)
were used, which showed identical trapped ﬁeld by FCM,
BFC
T ¼ 1:26–1:30T at 77 K. The bulks were uniformly
impregnated by epoxy resin in vacuum. Fig. 1 shows theexperimental setup around the bulks. Five bulk blocks(bulk #0–bulk #4) were tightly fastened with the brassmetal holder from the side face (along the ab-plane) andthe holder was attached to the cold stage of a Giﬀord
McMahon (GM) cycle helium refrigerator. Hereafter, weabbreviate this type of magnet system as an ‘‘ab-plane
cooled type’’. The initial temperature of the cold stage Tswas ﬁxed at 40 K. The time evolutions of temperature Tand trapped magnetic ﬁeld BPT were monitored on the central three bulks (bulk #1, #2 and #3) using the Cernox
thermometer and the Hall sensor (F.W. Bell, ModelBHA921) adhered on the center of each bulk. The bulk
was magnetized using the split-type pulse coil dipped inliquid N2. The pulse ﬁeld of Bex = 5.0 T is applied to thevirgin state bulk as the pulse No. 1 and then the pulse ﬁeldsof Bex = 5.5 T were applied as the subsequent pulses (No.
2–No. 5) after recovering to the initial temperature. Afterthe completion of the magnetizing a bulk, the split-type coil
was moved parallel and another bulk was magnetized inturn in a same manner. The trapped ﬁeld BT(z) along thez direction away from the vacuum sheath surface was measured using an axial-type Hall sensor. We compare theresults of the ‘‘ab-plane cooled type’’ system with thoseof the ‘‘c-axis cooled type’’ system; the GdBaCuO rectangular bulk is cooled down along the c-axis and is magnetized using a solenoid-type pulse coil with a magnetic
ﬁeld Bex = 4.8 T or 5.5 T as the pulse No. 1.
3. Results and discussion
Figs. 2(a) and (b) show the time evolutions of temperature T(t) after applying the pulse ﬁeld of Bex = 5.0 T to the‘‘ab-plane cooled type’’ bulk (#1) and of Bex = 4.8 T or
5.5 T to the ‘‘c-axis cooled type’’ bulk, respectively. Inthe ‘‘ab-plane cooled type’’, T(t) sharply rises up(DT = 16.7 K) and then returns to the initial temperature
within 10 min. In the ‘‘c-axis cooled type’’ bulk for
Bex = 5.5 T, T(t) also sharply rises up with a larger DT(=18.5 K) due to the slightly larger applied ﬁeld Bex, butrather moderately decreases and returns to the initial temperature in 20 min. For Bex = 4.8 T, T(t) cannot return tothe initial temperature within 10 min, although DT is smaller than that in the ‘‘ab-plane cooled type’’ bulk for
Bex = 5.0 T. These results mainly come from the diﬀerencein the cooling direction for the bulk; the thermal conductivity jab in the ab-plane is much higher than that along thec-axis jc and the propagating speed of the generated heat(a)"ab-plane cooled type"

T (K)

60

No.1
Bex=5.0 T

5040(b)"c-axis cooled type"

T (K)

60

No.1
5.5 T
4.8 Tyokebulkyoke
Solenoidcoil

50

400Fig. 1. The experimental setup around the GdBaCuO bulks and split-type
pulse coil.

246Time (min)810Fig. 2. The time evolution of temperature T(t) after applying the pulse
ﬁeld of (a) Bex = 5.0 T for the ‘‘ab-axis cooled type’’ bulk and (b)Bex = 5.5 T and 4.8 T for the ‘‘c-axis cooled type’’ bulk.

A. Fujiwara et al. / Physica C 445–448 (2006) 399–402

is higher in the ab-plane [9]. The ‘‘ab-plane cooled type’’
has a merit to shorten the cooling time for the heatgeneration.
Figs. 3(a) and (b) present the pulse number dependenceof the maximum temperature rise DT and the trapped ﬁeld
BPT of the ’’ab-plane cooled type’’ bulks (#1 and #2). DT is
the largest for the pulse No. 1 and monotonically decreases
and approaches a steady state value with increase numberof the magnetic pulse. The largest amount of the ﬂuxes istrapped during the application of pulse No. 1 and BPT is
gradually enhanced with increasing pulse number. The BPT
value of the bulk #2 is larger than that of the bulk #1 inspite of the similar BFC
T characters. These results may result
from diﬀerence in temperature rise; the DT of the bulk #2 issmaller than that of the bulk #1. In Fig. 3, DT and BPT ofthe ‘‘c-axis cooled type’’ bulk are also shown for
Bex = 5.5 T. For the ‘‘c-axis cooled type’’ bulk, the similarpulse number dependence of DT and BPT can be seen, butthe BPT is larger than that of the ‘‘ab-plane cooled type’’bulks. These results suggest that the GdBaCuO bulk has
a possible potential to trap the BPT value up to 2.5 T at
40 K by PFM and that BPT can increase by the optimum
application of Bex. The detailed investigation for the ‘‘abplane cooled type’’ bulk is in progress.
Fig. 4 shows the line scan proﬁle of the trapped ﬁeld
B6T mm , 6 mm distant above the bulk surface in the openspace, along the direction through the bulk centers after
the magnetization of the ﬁve bulks. The measurement20(a)"ab-plane cooled" #1"ab-plane cooled" #2"c-axis cooled"

ΔT (K)

15105

B =5.0 T
ex

401Fig. 4. The line-scan proﬁle of the B6T mm value along the direction throughthe bulk centers on the sheath surface. The BPT values for the bulks #1 to#3 are also shown.

was performed after the magnetic ﬁelds of pulse No. 1(Bex = 5.0 T) and No. 2 (5.5 T) were applied for each bulk[8]. In this ﬁgure, the ‘‘front’’ means the side, on which theHall sensor was adhered and the ‘‘back’’ means the opposite side. The BPT values for the bulks #1, #2 and #3 are alsoshown. The maximum B6T mm values on the ‘‘front’’ and‘‘back’’ sides are between 0.9 T and 1.2 T, and the very similar maximum B6T mm values can be seen on both sheath surfaces. The slight diﬀerence in the maximum B6T mm value isdue to the diﬀerence of crystalline property of the bulkon which the seed crystal was attached during the crystal
growth. This type of bulk magnet system can be eﬀectively
used on both surfaces as magnetic poles in open space,which may be a great merit for a new application.
Fig. 5 presents the trapped ﬁeld BT(z) at the center of
each bulk as a function of the distance z from the bulk
surface. In this ﬁgure, z = 6 mm means the surface on thevacuum sheath. TheBT(z) value sharply decreases with

B =5.5 T
ex

0BTP (T)(b)

2

B =5.5 T

1ex"ab-plane cooled" #1"ab-plane cooled" #2"c-axis cooled"

B =5.0T
ex012345Pulse number
Fig. 3. The pulse number dependences of (a) the maximum temperaturerise DT and (b) the trapped ﬁeld BPT for the ‘‘ab-plane cooled type’’ bulks(#1 and #2). The results for the ’’c-axis cooled type’’ are also shown.

Fig. 5. The trapped ﬁeld BT(z) at the center of each bulk as a function of
the distance z away from the bulk surface.

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A. Fujiwara et al. / Physica C 445–448 (2006) 399–402

increasing z. For example, for the bulk #3, the BT(6 mm)value ð¼ B6T mm Þ is 50% of BT ð0Þ ¼ BPT , and BT(17 mm) is
15% of BPT . These results suggest that, on the application
for the magnetic separation, it is necessary to minimize
the gap between the bulk and the vacuum sheath in order
to eﬀectively take out the magnetic ﬁeld trapped in thebulk.
4. Summary
We have constructed ﬁve-aligned superconducting bulk
magnet system magnetized by pulse ﬁeld magnetizing.
The cooling method along the ab-plane, which is adopted
in this study, has a merit to shorten the cooling time forthe heat generation, compared with that along the c-axis.
The trapped ﬁeld BPT as high as 2.0 T is attained on thebulk surface and the trapped ﬁeld B6T mm on the vacuum
sheath surface is 1.0–1.2 T for each bulk. The optimizationof the conditions such as applied ﬁeld and the bulk stage
temperature is necessary in order to enhance the trapped

ﬁeld for the ﬁve-aligned bulks, compared with two aligned
bulk magnet system.
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