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Thermal behavior and field-trapping property of melt-textured superconducting bulk magnets
activated by quasi-static magnetic fields

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2009 J. Phys.: Conf. Ser. 150 052195(http://iopscience.iop.org/1742-6596/150/5/052195)
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25th International Conference on Low Temperature Physics (LT25)
Journal of Physics: Conference Series 150 (2009) 052195

IOP Publishingdoi:10.1088/1742-6596/150/5/052195

Thermal Behavior and Field-Trapping Property ofMelt-Textured Superconducting Bulk Magnets Activated by
Quasi-Static Magnetic Fields
T. Okaa, K. Yokoyamab, H. Fujishiroc, and K. Notod
aNiigata University, 2-8050, Ikarashi, Niigata 950-2181 JapanbAshikaga Institute of Technology, 268-1 Ohmae-cho, Ashikaga, Tochigi, 326-8558
JapancIwate University, 3-4-5 Ueda, Morioka, Iwate, 020-8551 Japan

dEmeritus Professor of Iwate University, 3-4-5 Ueda, Morioka, Iwate, 020-8551
JapanE-mail: okat@eng.niigata-u.ac.jp
Abstract. The thermal and field trapping behaviors of melt-processed RE-Ba-Cu-O (RE=Y,Sm) bulk superconductors during their magnetization processes conducted in the zero fieldcooling (ZFC) were investigated in comparison with the field cooling (FC) operations in thetemperature range around 50 K. The temperature changes were precisely measured by fivethermocouples attached on the sample surface. The authors discuss the relationship between
the heat generation and resultant trapped magnetic field ability by means of the direct
temperature measurement during these magnetizing processes. As the applied field is notsatisfactorily given to the sample, the trapped field distributions activated in ZFC process show
trapezoid shapes, whereas they show a corn shape without any strain when magnetized in FC
process. The trapped magnetic field gradually increases even after when the applied magnetic
fields reaches the highest value. According to the balance on the heat generation and drain, thetemperature evolution shows us an almost stable state. These data suggest that theimprovement with respect to the heat propagation of the bulk material must suppress thetemperature rises and enhance the resultant flux trapping ability.

1. Introduction
The melt-textured large grain bulk superconductors composed of RE-Ba-Cu-O compounds (RE=Y,Sm, Gd, abbreviated as RE123) which includes RE211 particles perform the excellent property as
quasi-permanent magnets when they trap the applied magnetic fields [1, 2]. The maximum trappedfield has reached 16 T at 24 K by doping Zn to it, as reported by Gruss et al. [3]. The highest value of
17.24 T has been reported by Tomita et al. [4] by reinforcing the Y123 bulk sample by the resin
impregnation technique. The data thus referred as the trapping ability of bulk magnets are generally
obtained by the field cooling method (hereafter abbreviated as FC). In the magnetizing process using
the pulsed fields (PFM), it is known that the flux motion causes local heating in the sample, raises thetemperature, lowers the critical current density, and subsequently degrades the trapped field ability [5].
A smart process called MMPSC as a kind of PFM processes has recently derived the maximum
performance of trapped field of 5.2 T, which was conducted by Fujishiro et al. [6, 7] in the way thatc 2009 IOP Publishing Ltd1

25th International Conference on Low Temperature Physics (LT25)
Journal of Physics: Conference Series 150 (2009) 052195

IOP Publishingdoi:10.1088/1742-6596/150/5/052195

mm

mm

Steel ringAluminacrystal(a)mm

Copperstage

Copper rod

Bulk sample surface

Heat drain(a)(b)
Figure 1. Experimental setup shown in (a) side view of magnetic pole and (b) top view
showing the positions of thermocouples and a Hall sensor on the sample surface.

suppresses the heat generation by means of applying iterative pulsed fields at two temperatures. Since
it must be undoubtedly important for us to suppress the temperature rise even during the FC process,the sweep rates of applied fields must be carefully chosen to be slow in the FC process [8, 9].
In the study, we aim the evaluation on the temperature rises due to the heat generation by flux
motion in the bulk materials during the zero field cooling ZFC and FC processes for the Y-Ba-Cu-O
and Sm-Ba-Cu-O bulk superconductors cooled at around 50 K.
2. Experimental procedures
Schematic views of the experimental setup are shown in Fig. 1. A melt-processed Y123 bulk sample
was manufactured by Dowa Mining Co., LTD. and adapted to the FC operation [8]. The dimensions ofthe sample were 45 mm in diameter and 15 mm in thickness. A same shape of Sm123 sample wasadapted to the magnetization experiment by the ZFC process [9]. Five pairs of chromel-constantan
thermocouples (T1-T5) were glued on both positions of a grain sector region and a grain sector
boundary shown in Fig. 1(b). A Hall sensor (Bell, BHT 921) was also attached near the center of thebulk surface. In the FC process, a static field of 5 T was applied in the higher temperature ranges thanTc, and then the sample was cooled to the lowest temperature 50 K by a Gifford-McMahon refrigerator(AISIN SEIKI Co., GR-103). Then the magnetic field is gradually reduced to zero with descending
rates of -2.53, -5.06 and -11.3 mT/s. In the ZFC process, the magnetic field of 5 T was also applied
and then removed with the rates 5.06 and 11.3 mT/s at the temperature around 50 K. After themagnetization process, the trapped field distributions were measured by scanning an axial-type Hallsensor just above the vacuum chamber.
3. Results and Discussions
3.1. Trapped field distributions after ZFC and FC activation processes
The distribution of trapped magnetic flux density in ZFC process was measured by the scanning Hall
probe outside the vacuum chamber, as shown in Fig. 2. One can obviously see that the distribution
indicates concentric round contour lines of magnetic field and that the circles slightly deviate towardoutside on the lines which correspond to the growth sector boundaries (GSB). This implies that thecritical current density on GSB is rather superior to that in the growth sector regions.

225th International Conference on Low Temperature Physics (LT25)
Journal of Physics: Conference Series 150 (2009) 052195

SmZFC5T-11.3mT/s(N) T60
50Position (mm)

0.3

1.1
1.0
0.8
0.6
0.4
0.2
0400.9300.6

2001000102030

4050

IOP Publishingdoi:10.1088/1742-6596/150/5/052195

Figure 3 indicates the trappedfield distribution along the dashed line inFig. 2 with various sweeping rates ofapplied field in comparison with those inanother ZFC and FC processes. As theapplied field was not satisfactorily given
to the sample, the trapped field
distributions activated in ZFC process
show a trapezoid shape, whereas theyshow a corn shape without any strain
when the sample is magnetized in FC
processes.
3.2 Temperature evolutions in the ZFC
process

60Position (mm)0.
3Figure 4 shows the temperature changes
on its way of ZFC process with a sweep
Figure 2. Trapped field distribution after 5 T ZFC
rate of 11.3 mT/s for Sm123 sample
process at 50 K with a sweep rate of 11.3 mT/s.
operated at initial temperatures around
50 K, whereas the cold head of thecooler was kept at 38 K. The profiles
FC2.56mT/s
show that the temperatures began to rise
50K
FC11.3mT/s
immediately after when the magnetic
ZFC5.06mT/s
field started rising at all the measured
ZFC11.3mT/s
points. The temperature rises have soon
saturated when the external field reached
5 T and stopped growing. The averaged
temperature rise has reached 7.5 K until
the field started decreasing. It is notedthat the trapped field gradually keepincreasing for about two hundred
minutes even after the field stopped
growing. This indicates that the flux
distribution rearranges for a long time
Position (mm)even in the static field and fluxes in thebulk sample still keep moving. The
Figure 3. Trapped field distributions after 5 T FC andsaturation of the temperatures implies
ZFC processes with various sweep rates.
that the heat generation and draining
heat are almost balanced in the presentcondition. It is also suggested that theimprovement with respect to the heat propagation of the bulk material must suppress the temperaturerises and enhance the resultant flux trapping ability.
According to the detailed thermal changes measured at the time of 200 minutes, as shown inthe circle in Fig. 4, all the temperature data measured at all thermocouples rise together while the flux
lines keep moving in the sample on the field descending stage. This is apparently attributed to the factthat the amount of generating heat is greater than draining one when the sweep rate is high. And thetemperatures steeply start decreasing when the descending field reaches zero. The schematic
illustration which explains the flux motion in the sample is shown in Fig. 5. The magnetic fluxes keep
penetrating into the sample even after when the external field stops increasing, and fall into the central
portion like a snow slide showing a flux creeping phenomenon.
0.
25.
10.
15.
00.
0

Magnetic Flux Density (T)

5.
20
60
40205.
0-3

25th International Conference on Low Temperature Physics (LT25)
Journal of Physics: Conference Series 150 (2009) 052195

IOP Publishingdoi:10.1088/1742-6596/150/5/052195-11.3mT/s

T1
T2T3
T4
T5

ZFC 38K -11.3mT/s

Time (min)Figure 4. Temperature and trapped field evolution in ZFC operation at around 50 K with asweep rate of 11.3 mT/s. Cold head temperature was set at 38 K.
4. Conclusions
The temperature rises due to the heat generation ofRE-Ba-Cu-O single domain bulk superconductors
during the ZFC process have reached 7.5 K when thesample was magnetized with the sweeping rate of 11.3
mT/s. The invading flux exhibits the flux creeping due
to the rearrangement of flux distribution even after
when the external field stops increasing, and falls into
the central portion like a snow slide. We have so faremphasized that the temperature change is never
negligible even in the ZFC magnetizing operation as
well as FC.

5. Acknowledgement
The works have been partially supported by grantsin-aid from Japan Science and Technology Agency(JST) and Iwate Industrial Promotion Centre.

Ｂ
μ0HM①③④②-ａ0aFigure 5. Illustration of invading flux in theSm123 bulk sample in ZFC operation.

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Letters 79 3131
[4] Tomita M and Murakami 2003 M Nature 421 517[5] Itoh Y and Mizutani U 1996 Jpn. J. Appl. Phys. 35 2114
[6] Fujishiro H, Kaneyama M, Tateiwa T and Oka T. 2005 Jpn. J. Appl. Phys. 44 L1221[7] Fujishiro H, Tateiwa T, Fujiwara A, Oka T and Hayashi H 2006 Physica C 445-448 334-338[8] Oka T, Yokoyama K, Fujishiro H, Kaneyama M and Noto K 2005 Physica C 426-431 794[9] Oka T, Yokoyama K, Fujishiro H and Noto K 2007 Physica C 460-462 7484