Physica C 460–462 (2007) 748–749
www.elsevier.com/locate/physc

Temperature rise in melt-textured large grain superconducting
bulk magnets during their magnetizing operations
Tetsuo Oka a,*, Kazuya Yokoyama b, Hiroyuki Fujishiro c, Koshichi Noto db

aNiigata University, 2-8050 Ikarashi, Niigata 950-2181, JapanAshikaga Institute of Technology, 268-1 Ohmae-cho, Ashikaga, Tochigi 326-8558, JapancIwate University, 3-4-5 Ueda, Morioka, Iwate 020-8551, JapandIwate University, 3-19-27 Chomeigaoka, Izumi-ku, Sendai 981-3212, JapanAvailable online 3 April 2007

Abstract
The temperature rises in the melt-processed RE–Ba–Cu–O (RE = Y, Sm, Gd) bulk superconductors were evaluated when the magnetizing operations were performed by the ﬁeld cooling (FC), the zero ﬁeld cooling (ZFC) and the pulsed ﬁeld magnetization (PFM)
processes. It is known that the ﬁeld-trapping ability of the superconducting bulk magnets is restricted by the heat generation due tothe ﬂux motion inside the sample not only in the PFM but also even in the FC process. In this paper, the authors discuss the mechanismof heat generation and the ﬂux motions by means of the temperature and resultant trapped ﬁeld measurement during three magnetizing
processes. The sample was cooled in a 5 T static ﬁeld and then the ﬁeld was removed with various descending rates (2.53–11.3 mT/s) in
FC method. The ﬁelds were also applied and then removed with the same rates after setting the sample in the superconducting state inZFC. These data were compared with the results obtained in the PFM. The temperature evolutions have shown apparent peaks that
correspond to the heat generation in their magnetizing processes. The maximum temperature rises have reached 5.9 K in FC and7.6 K in ZFC operated at 57.6 K and 51.5 K, respectively. The temperature rise exhibited 2 K even when the slowest rate of
2.53 mT/s was applied in FC mode. These data suggest it is crucial to take the heat generation into account in estimating the ﬁeld-trapping ability of the high performance bulk magnets.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Melt-process; Bulk superconductor; Trapped ﬁeld; Heat generation; Magnetization

1. Introduction
We know that the melt-processed REBa2Cu3Oy(RE = Y, Sm, Gd, abbreviated as RE123) bulk superconductors including RE2BaCuO5 (RE211) precipitates act
as permanent magnets when they capture the magnetic
ﬁeld. The ﬁeld-trapping ability of the compounds has been
greatly improved through various fabrication processes.
The maximum trapped ﬁeld has reached 17.24 T (reportedby Tomita et al. [1]) for Y123 sample with the size 26.5 mm
in diameter. The data was obtained by the ﬁeld coolingmethod (abbreviated as FC) and the referred temperature

has risen up to 30 K from initial temperature 29 K before
starting the magnetizing operation in spite of careful lowering of sweeping rate of provided magnetic ﬁeld. The ﬂux
motion during the magnetization process, even in the FC,
causes local heating in the sample, raises the temperature,
lowers the critical current density, and subsequently
degrades the trapped ﬁeld.
In the report, the temperature changes during the FC andzero ﬁeld cooling (ZFC) processes are estimated as functions of temperatures and the sweeping rates for bulk superconductors cooled by a Giﬀord–McMahon refrigerator.
2. Experimental*Corresponding author. Tel.: +81 25 262 7668; fax: +81 25 262 7010.
E-mail address: okat@eng.niigata-u.ac.jp (T. Oka).

0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2007.03.368

Fig. 1 shows the schematic views of the experimental
setup. A bulk sample manufactured by Dowa Mining Co.

T. Oka et al. / Physica C 460–462 (2007) 748–749

749

Fig. 2. Temperature change of Sm123 bulk during ZFC process operated
up to 5 T with a sweep rate 5.03 mT/s.

3.2. Temperature changes in ZFC process

Fig. 1. Schematic views of experimental setup.

consisted of RE123 and RE211 with addition of Pt, andwas impregnated by epoxy resin with glass ﬁbers.
The temperature data were measured by ﬁve sets of ﬁne
chromel–constantan thermocouples with 76 lm in diameter attached on the sample surface. The heat capacities ofthem do not need to be taken into account.
In the FC process, a static ﬁeld of 5 T was applied in thenormal state, and the sample was cooled to the lowest
temperature by a refrigerator, and then the magnetic ﬁeld
is removed with descending rates, 2.53, 5.06, and11.3 mT/s. In the ZFC process, the magnetic ﬁeld of
5 T was applied after cooling the sample below Tc andwas subsequently removed. The data were compared withthose obtained by the PFM experiments [2].
3. Results and discussion
3.1. Temperature changes in FC process
When the magnetic ﬁelds started decreasing from 5 T to0 T, the temperatures (T1–T5) of an Y123 sample drastically rose showing steep peaks of the temperatures. One
can see that the faster the speed, the higher the temperaturerises. The maximum averaged temperature rise was 5.9 K
when the descending speed was 11.3 mT/s.

Fig. 2 shows the temperature changes in the ZFC process
for Sm123 operated at 50 K. The proﬁles show that the temperatures at all the measured points rose for 7–8 K. The
averaged temperature rise reached 7.6 K when the sweep
rate was 5.06 mT/s. It is noted that the temperatures andthe trapped ﬁeld kept increasing for about 200 min afterstopping the increase of ﬁeld at 5 T. This indicates thatthe ﬂux distribution reconstructs even in the static state
for a long time. It suggests that heat propagation must be
improved to suppress the temperature rises.
The temperature rises thus obtained are substantial incomparison with the data reported as about 21 K during
PFM process when the pulsed ﬁeld of 5 T was applied tothe Sm123 sample at 40 K [2].
4. Conclusions
The temperature measurements revealed that the temperature rises reached 5.9 and 7.9 K in FC and ZFC magnetization processes operated at 50 K, respectively. They
are not negligible in estimating the ﬁeld-trapping abilityof bulk superconductors.
Acknowledgments
This study was supported by IWATE CREATE projectdriven by Japan Science and Technology Agency and Iwate
prefecture authorities.
References[1] M. Tomita, M. Murakami, Nature 421 (2003) 517.
[2] K. Yokoyama, M. Kaneyama, T. Oka, H. Fujishiro, K. Noto, Physica
C 412–414 (2004) 688.