Physica C 412–414 (2004) 688–694
www.elsevier.com/locate/physc

Temperature measurement of RE123 bulk superconductors
on magnetizing process
K. Yokoyama a,*, M. Kaneyama b, T. Oka a, H. Fujishiro b, K. Noto aaIwate Industrial Promotion Center, 3-35-2 Iioka-shinden, Morioka, Iwate 020-0852, Japanb
Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan
Received 29 October 2003; accepted 19 January 2004

Abstract
We study on the magnetization behavior of to magnetize RE123 bulk superconductors to apply it as strong magnets.
Through magnetizing process, the temperature of bulk superconductors is raised by pinning loss caused by the magnetic
ﬂuxes motion (e.g. ﬂux jump of ﬂux ﬂow), and the trapped ﬁeld is decreased. This paper presents the measurement of
temperature changes of Sm123 bulk superconductors during the exciting process by iteratively magnetizing pulsed-ﬁeld
operation with reducing amplitudes (IMRA) method. Five thermocouples are put on the surface of Sm123 bulk
superconductor of 46 mm in diameter. The temperatures at the center, on the growth sector boundary (GSB) line and in
the sector region surrounded by GSB’s line (inter-GSB region) are monitored. The temperature at a cold stage is alsomeasured. A Hall sensor is attached near the center thermocouple to measure the trapped ﬁeld. After a bulk superconductor is cooled by the GM type refrigerator until 40 K, iterative pulsed-ﬁelds of 2.32–5.42 T are applied by amagnetizing coil. When high magnetic ﬁeld of 5.42 T is applied, a temperature of bulk superconductor reaches to 72.4 K
and the magnetic ﬁeld distribution has C form with which a part of circle is dented, and then, a trapped ﬁeld is 2.28 T.
When a lower magnetic ﬁeld of 4.64 T is applied, a maximum temperature is 68.3 K and a trapped ﬁeld is raised to 2.70
T, and moreover, the distribution becomes round shape like ﬁeld-cooling method (FC). We showed clearly that heatgeneration by pinning loss was related to the mechanism of magnetic ﬁeld capture.
Ó 2004 Elsevier B.V. All rights reserved.
PACS: 74.72.)h; 74.25.H
Keywords: RE123 bulk superconductor; Temperature measurement; Pulsed-ﬁeld magnetization; IMRA method; Pinning loss

1. Introduction*

Corresponding author. Tel.: +81-19-635-7220; fax: +81-19631-1610.
E-mail address: yoko@iwate-techno.com (K. Yokoyama).

The performance of high-Tc REBaCuO(RE ¼ Y, Sm, Gd, etc.) bulk superconductors has
improved remarkably [1–3]. Since a critical current
density, Jc , increased and the mechanical strength
was improved, a bulk superconductor can trap ahigher magnetic ﬁeld. In addition, the use of the0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2004.01.091

K. Yokoyama et al. / Physica C 412–414 (2004) 688–694

cryocooled refrigerators allowed us to easily generate the stronger magnetic ﬁeld than permanent
magnets and electromagnets. At the present, amagnetic ﬁeld generation system in which a bulk
superconductor is used as a permanent magnet
is developed [4–6], and various applications ofsuperconducting bulk magnets such as magnetic
separation and magnetron spattering are considered [7–9].
Bulk superconductors are mostly magnetized bya ﬁeld-cooling method (FC) and by a pulsed-ﬁeld
magnetization (PFM). In FC, although highmagnetic ﬁeld can be trapped, the place in whicha magnetization is possible is restricted due to useof a superconducting magnet. In PFM, althougha trapped ﬁeld is lower than FC, magnetizing
equipment is simple due to the use of a condenserbank and a copper coil. Therefore, PFM is an
important magnetizing method towards the utilization of bulk superconductors. Since a trapped
ﬁeld of usual PFM generally is a half of that ofFC, the development of magnetizing method toimprove the trapped ﬁeld in PFM has been considered. The iteratively magnetizing pulsed-ﬁeld
operation with reducing amplitudes (IMRA)
method is to impress pulsed-ﬁelds several times
changing strength of magnetic ﬁelds and higher
magnetic ﬁeld can be trapped than usual PFM[5,6].
During magnetization, bulk superconductors
generate heat and thus the trapped ﬁeld is decreased due to reduction in a critical current density Jc , where it is supposed that heat generation
occurs by pinning loss when magnetic ﬂuxes moveinto the bulk (e.g. ﬂux jump and ﬂux ﬂow). For
applying bulk superconductors as strong magnets, it is important to elucidate the magnetizing
mechanism and to improve the magnetizing eﬃciency [5,10]. At present, various researches aredone by experiments and simulations [11,12].
Ikuta et al. measured the time dependence of themagnetic ﬂux density using pickup-coils in PFM of
Sm123 bulk superconductor and clariﬁed the motion of ﬂux lines. Moreover, the velocity of ﬂux
lines is calculated using the experimental data andtemperature changes are evaluated [11]. Ohsaki
et al. studied PFM of a ring-shaped bulk superconductor and then estimated the current density

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distribution and temperature distribution bycomputer simulations using experimental results[12]. These reports are based on numerical analysis
and those quantitative evaluations are not reported. We measure temperature changes ofRE123 bulk superconductors using thermocouples
directly and guess behavior of magnetic ﬂuxes
from experimental results [13,14].
This paper presents the measurement of temperature changes of Sm123 bulk superconductors
during the magnetization process by IMRA
method. Five thermocouples and a Hall sensor are
attached on the surface of bulk superconductor,
and temperatures and trapped ﬁelds are monitored on the each magnetizing process. Thenmotion of magnetic ﬂuxes is discussed using thedata of temperature rise and magnetic ﬂux distribution.

2. Experimental
Fig. 1(a) shows a schematic of temperature
measurement equipment. A Sm123 bulk superconductor of 46 mm in diameter and 15 mm inthickness with epoxy impregnation is set on acopper base connected to a cold head of GM
refrigerator (AISIN SEIKI CO., LTD, GR301).
The epoxy resin on both upper and lower sides ofthe bulk is however removed in order to measure
the precise temperature and to reduce a thermal
contact resistance to the cold head. The bulk
superconductor is a highly c-axis oriented crystal
and consists of SmBa2 Cu3 Oy (Sm123), Sm2 BaCuO5 (Sm211) with the molar ratio of 1.0:0.3, 0.5
wt% Pt powder and 1.5 wt% Ag2 O addition (DowaMining Co., Ltd.). A heater coil is wound arounda thermal conduction bar in order to adjust an
initial temperature of bulk superconductor and it
is controlled by a thermo-controller connected
outside.
The surface of bulk crystal can be classiﬁed into
two regions: One is the growth sector boundary(called as GSB) and another is sector region surrounded by GSB lines (called as inter-GSB region). The magnetic ﬂuxes were conﬁrmed moreeasily to move through inter-GSB regions than
GSB lines [11] and we have also obtained the same

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Fig. 1. Experimental equipment and arrangement of thermocouples. (a) A schematic of measurement equipment. SmBaCuO bulk
superconductor is set on a copper base connected to a cold head of GM refrigerator and they are covered with a vacuum vessel. The
bulk superconductor is magnetized by an inducting magnet. (b) Arrangement of thermocouples. T1 is put in the center and otherthermocouples of T2–T4 are attached to each four inter-GSB regions, respectively.

results in the past experiment [13,14]. Teﬂoncoated chromel-constantan thermocouples of 76
lm in diameter are adhered on the surface of bulk
superconductor using the GE7031 varnish. The
arrangement of ﬁve thermocouples (T1,
T2, . . . , T5) is displayed in Fig. 1(b). T1 is put inthe center where two GSB lines cross. Other
thermocouples of T2–T5 are put in each four interGSB regions, respectively. Another thermocouple
T6 is attached to the cold stage as shown in Fig.
1(a). A Hall sensor (F.W. Bell, BHT-921) is put
near T1 to measure a magnetic ﬂux density of
trapped ﬁeld. A superinsulation is wound aroundthe bulk superconductor and they are covered witha vacuum vessel.
A magnetizing coil, which is solenoid coilwinding copper wire and dips in liquid nitrogen, isarranged outside the vacuum vessel in Fig. 1(a)and exciting current is supplied by a condenserbank of 60 mF. A rising time of the pulsed-ﬁeld is
10 ms and a maximum ﬁeld of 7 T is generated.
After a bulk superconductor is cooled down to 40
K, pulsed-ﬁelds of 2.32–5.42 T are applied. Eachthermoelectric voltage of the thermocouples and atrapped ﬁeld are measured about 7 times/s justbefore applying a pulsed-ﬁled. After the magnetization, the magnetizing coil is removed and thenthe distribution of magnetic ﬂux density at 3.5 mmabove the bulk surface is scanned.

3. Results and discussion
Fig. 2 shows the relationship between appliedﬁelds l0 H and trapped ﬁelds BT in magnetizing
process by IMRA method, where BT is the value
measured by a Hall sensor on the surface of thebulk. Magnetic ﬁelds are impressed 14 times inorder of 1, 2, . . . , 14 changing strength of applied

Fig. 2. The relationship between trapped ﬁeld and applied ﬁeld
in the magnetizing process by the ‘‘IMRA’’ method. Trapped
ﬁelds are measured with a Hall sensor attached at the center of
the top surface of Sm123 bulk superconductors. Magnetic ﬁelds
are impressed 14 times in order of 1; 2; . . . ; 14 changing strengthof applied ﬁeld, where it is increased from nos. 1–5 and decreased after no. 6.

K. Yokoyama et al. / Physica C 412–414 (2004) 688–694

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ﬁeld, where l0 H is increased from nos. 1 to 5 anddecreased after no. 6. Temperatures and themagnetic ﬁeld distributions are measured for every
one excitation. Also, the following pulsed-ﬁeld is
excited after the temperature is stabilized at 40 K
and then the interval is about 40–60 min.
In the increasing process of applied ﬁelds, BT
increases with increase of l0 H and takes the local
maximum of 1.86 T when l0 H ¼ 4:64 T. When astronger magnetic ﬁeld l0 H ¼ 5:42 T is impressed,
it decreases to 1.58 T. Figs. 3 and 4 show (a) thetime dependence of temperatures and (b) magnetic
ﬂux distribution when l0 H ¼ 3:10 T (no. 2) and5.42 T (no. 5), respectively.

Fig. 4. Experimental results in the exciting ﬁeld of 5.42 T. (a)Time responses of temperatures at T1–T6 and trapped ﬁled.
The left and right axes express the temperature and trapped
ﬁeld, respectively. A rising time of T4 is the fastest and maximum temperature reaches to 66 K. (b) Trapped ﬁeld distribution 3.5 mm above the bulk surface. The distribution is indented
at the portion of T4 and then it is the so-called C type distribution.

Fig. 3. Experimental results in the exciting ﬁeld of 3.10 T. (a)Time responses of temperatures at T1–T6 and trapped ﬁled.
The left and right axes express the temperature and trapped
ﬁeld, respectively. The rising time of temperature change at T4
is the fastest of the other part and peak temperature is thehighest, too. (b) Trapped ﬁeld distribution 3.5 mm above thebulk surface. Magnetic ﬂuxes are trapped only at the portionof T4.

In Fig. 3(a), a rising time of T4 is the fastest anda maximum temperature is the highest of otherparts. This shows that magnetic ﬂuxes invade only
from the portion of T4 though they are shielded
inside the bulk superconductor since the appliedﬁeld is weak. On the other hand, a temperature riseof T1 is the slowest and it is considered that a heatgeneration of T1 is caused by transmitting the heatgenerated on the perimeter. These results correspond well with the magnetic ﬁeld distribution
illustrated in Fig. 3(b) and magnetic ﬂuxes aretrapped only at the portion of T4 though they aresmall.

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Fig. 5. Time dependence of temperature changes at T4 whenmagnetic ﬁeld of 4.26 T is applied 4 times (pulse nos. 8–11). A
temperature rise is about 21 K at no. 8 and it becomes small
whenever a magnetic ﬁeld is impressed and it is 10 K at no. 11.
Also, the local maximum does not appear in no. 11 though it
appears in nos. 8–10.

In Fig. 4(a), a rising time of T4 is the fastest andmaximum temperature reaches to 66 K. Unlike
l0 H ¼ 3:10 T, temperatures of the other parts
including T1 are simultaneously rising and itmeans that magnetic ﬂuxes invade to the center ofthe bulk since applied ﬁeld is large. On the other
hand, BT decreases after the excitation and themagnetic ﬁeld distribution shown in Fig. 4(b) is
indented at the portion of T4. It means thatmagnetic ﬂuxes escape through T4.
From these results, it is considered that theplace of low-Jc is located locally in the portion ofT4 and that becomes the channel along whichmagnetic ﬂuxes pass. When applied ﬁeld is weak,since pinning loss is small and heat generation also
is little, magnetic ﬂuxes invade from T4 and they
are trapped there. When applied ﬁeld is large, on
the other hand, since pinning loss becomes large

Fig. 6. Trapped ﬁeld distributions when magnetic ﬁeld of 4.26 T is applied 4 times (pulse nos. 8–11). Magnetic ﬂuxes are gradually
added for the dent portion of the portion of T4, and at last, it becomes the round distribution which is alike in FC.

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and heat generation also is large, Jc is lowered andmagnetic ﬂuxes go away.
In IMRA method, when a trapped ﬁeld is decreased, the applied ﬁeld is lowered. In Fig. 2, itlowers 0.39 T of applied ﬁelds at a time after pulse
no. 6. Since heat generation also becomes little
because the applied ﬁeld is small, trapped ﬁeld
increases gradually. When the magnetic ﬁeld of
4.26 T is impressed (pulse no. 8), BT increases
greatly compared with pulse nos. 6 and 7. Then,the magnetic ﬁelds of the same strength are impressed iteratively.
Figs. 5 and 6 show the time dependence oftemperature changes at T4 and the magnetic ﬁeld
distributions when magnetic ﬁeld of 4.26 T is applied 4 times (pulse nos. 8–11), respectively. Although the magnetic ﬁeld distribution has C form
where the portion of T4 is dented in pulse no. 8,
magnetic ﬂuxes are gradually added for the dent
portion, and at last, it becomes the round distribution which is alike in FC. In the time dependence of temperature change, on the other hand, atemperature rise is about 21 K at no. 8 and it becomes small whenever a magnetic ﬁeld is impressed and it is 10 K at no. 11. Also, the local
maximum of temperature does not appear in no.
11 though it appears in nos. 8–10. It is consideredthat pinning loss decreases since magnetic ﬂuxes
are trapped almost to the limit in bulk superconductors and thus the pinning center which participates in capture of a magnetic ﬁeld is lost. Fromthese results, a temperature rise is suppressed although magnetic ﬂuxes invade the inside of bulk
when the magnetic ﬁeld of suitable strength isimpressed. Therefore, heat generation is little after
second excitation of 4.26 T and thus magnetic
ﬂuxes are able to be trapped at the position of T4.
Since a trapped ﬁeld and temperature hardly
change by pulse nos. 10 and 11 because magnetic
ﬂuxes are trapped in the bulk fully, strength ofapplied ﬁeld is lowered. In pulse nos. 12–14, temperature changes are almost ﬁxed and trapped
ﬁelds do not increase, since applied ﬁeld is weak
and thus they cannot invade the inside of bulk.
However, magnetic ﬂuxes are added to theperimeter of bulk without heat generation, andtherefore, total amount of magnetic ﬂuxes increases and a strong magnetic ﬁeld is formed.

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4. Conclusions
We investigated temperature changes of Sm123
bulk superconductor on the magnetizing process
by IMRA method and considered the magnetizing mechanism. Temperatures were monitoredby thermocouples attached at four inter-GSB
regions and at the center on the surface of thebulk, and the trapped ﬁeld distributions were
scanned.
When the applied ﬁeld is large, pinning loss
becomes large and heat generation also is large,
and thus, Jc is lowered. Moreover, the place of
low-Jc was located locally and that became thechannel along which magnetic ﬂuxes past. Thus,
the magnetic ﬁeld distribution had C form withwhich a part of circle was dented. Next, when aweak magnetic ﬁeld was impressed, pinning loss
became small and heat generation was little.
Then, magnetic ﬂuxes were gradually trapped forthe dent portion and it became the round distribution which was alike in FC. That is, the
IMRA method was the magnetizing process
which utilized heterogeneity of bulk superconductors.
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