Physica C 426–431 (2005) 794–799
www.elsevier.com/locate/physc

Temperature changes in a melt-processed
YBCO superconductor activated by ﬁeld coolingmagnetizing process
T. Oka a,*, K. Yokoyama b, H. Fujishiro c, M. Kaneyama c, K. Noto cba
IMRA Material R&D Co., Ltd, 5-50 Hachiken-cho, Kariya, Aichi 448-0021, Japan
National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japanc
Iwate University, 3-4-5 Ueda, Morioka, Iwate 020-8551, Japan

Received 23 November 2004; accepted 24 January 2005
Available online 26 July 2005

Abstract
The temperature rise during the ﬁeld cooling (FC) magnetizing process with various sweep rates by using a 10 T
superconducting solenoid magnet was systematically evaluated with respect to an YBCO single domain trapped-ﬁeld
magnet cooled to 57–75 K by a GM refrigerator. The maximum temperature change has reached 6 K when the sweep
rate was 11.3 mT/s and initial temperature of 57 K. The recent elevation of the sweep rate of superconducting magnet
and the critical current density of bulk samples have made the heat generation serious even in the FC operation byquasi-static magnetic ﬁeld.
Ó 2005 Elsevier B.V. All rights reserved.
PACS: 74.25.Ha; 74.81.Bd
Keywords: Melt-process; Bulk superconductor; Trapped ﬁeld magnet; Field cooling

1. Introduction
It is well-known that the melt-processed REBa2Cu3Oy (RE = Y, Sm, Gd, abbreviated as RE123)
bulk superconductors including RE2BaCuO5*Corresponding author. Tel.: +81 566 24 9382; fax: +81 566
24 9370.
E-mail address: toka@ai-i.aisin.co.jp (T. Oka).

(RE211) precipitates act as permanent magnets
when they capture the magnetic ﬁeld [1,2]. As
was reported by Gruss et al. [3], the performancesof the ﬁeld-trapping ability of a Y123 compound
has been greatly improved by doping Zn to it.
The maximum trapped ﬁeld has reached 16 T at
24 K. Recently, 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

0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2005.01.085

T. Oka et al. / Physica C 426–431 (2005) 794–799

technique. The data thus referred as the trapping
ability of bulk magnets are generally obtained by
the ﬁeld cooling method (hereafter abbreviatedas FC).
Some experimental results on the heat generation due to the ﬂux penetration during the pulsed
ﬁeld magnetization process (PFM) have been reported elsewhere [5]. The ﬂux motion causes local
heating in the sample, raises the temperature, lowers the critical current density, and subsequently
degrades the ﬁeld trapping ability. Ikuta et al. [6]reported that the temperature rise during thePFM at 77 K reaches up to 5 K. On the otherhand, Yanagi et al. [7] reported they reached
17 K when the PFM was operated at 35 K. Various eﬀorts such as ‘‘IMRA’’ method [8] or magnetizing method on the way of cooling the sample [9]have been made to suppress the heat generation.
Fujishiro et al. [10,11] reported on the relationshipbetween temperature-rises during the PFM process
and suggested the possible improvement with respect to the ﬁeld-trapping ability of the bulkmaterials.
We have considered it very important to suppress the temperature rise even during the FC process as well as the PFM. The sweep rates of appliedﬁelds are chosen to be suﬃciently slow in the FCprocess. However, since the sweep rates of superconducting solenoid magnets have been improved
to be faster, in conjunction with the enhancementof the critical current density of the bulk samples,the heat generation is no longer negligible even in
the process. Tomita et al. have succeeded to suppress the temperature rise less than 1 K by slowing
the ﬁeld descending speed less than 5.8 mT/s. Inthe report, the temperature changes during the FC
process as functions of the temperatures and thesweep rates on the reducing ﬁeld are estimatedfor an YBCO bulk superconductor cooled around57 K by the GM refrigerator.

2. Experimental details
Fig. 1 shows the schematic views of the experimental setup. A melt-processed Y–Ba–Cu–O bulk
sample manufactured by Dowa Mining Co. consisted of YBa2Cu3Oy (abbreviated as Y123) and795

Fig. 1. Experimental setup for the temperature measurement
for the Y-system bulk sample sustained on the cold stage of thecryostat (a) and positions of the temperature sensors on thesample surface (b).

Y2BaCuO5 (Y211) by a molar ratio of 5:2 withaddition of 0.5 wt.% Pt. The dimensions of thesample were 45 mm in diameter and 15 mm inthickness. The sample was impregnated by epoxy
resin reinforced by glass ﬁbers. The trapped ﬁeld
distribution measured at 77 K showed a single domain shape with the maximum value of 0.9 T at
the center of the bulk surface. The top resin layer
was carefully removed as to measure the accurate
thermal data by the thermocouples attached on
the sample surface, and the bottom layer was also
removed to improve the thermal conduction fromthe cold head of refrigerator.
The sample was placed on the cold stage with asapphire plate, as shown in Fig. 1(a). Five pairs ofchromel–constantan thermocouples (T1–T5) witha size of 0.76 mm in diameter for the shielded lines
were glued on the positions shown in Fig. 1(b) by

T. Oka et al. / Physica C 426–431 (2005) 794–799

GE 7031 varnish. A Hall sensor (Bell, BHT 921)
was attached just near the center of the sample
to monitor the applied magnetic ﬁeld. A Pt–Co
thermometer embedded in the cold head of therefrigerator controlled the temperatures of the coldstage.
The FC process was operated after setting theequipment in the room temperature bore of a
10 T superconducting solenoid magnet, which
was manufactured by Japan Superconductor
Technology Company. In the FC process, a staticﬁeld of 5 T was applied in the higher temperature
ranges than Tc, and the sample was cooled to thelowest temperature by a Giﬀord-McMahon typerefrigerator (AISIN Seiki Co., GR-103). Thenthe magnetic ﬁeld is reduced to zero with various
descending rates that do not lead to quench thesuperconducting coil. The sweep rates of magnetic
ﬁeld on the descending stage were 2.53, 5.06,
and 11.3 mT/s.
The trapped ﬁeld distribution after the FC process was measured by scanning an axial-type Hallsensor just above the vacuum chamber. The direction of measurement was parallel to the bulk axis.
The distance between the sensor and the bulk sample surface was 3.5 mm.

64-11.3mT/s-5.06mT/s-2.53mT/s

63

Temperature (K)7966261605958570300600

900

1200

1500

1800

2100

2400

Time (s)Fig. 2. Time evolutions of the temperatures measured at thecenter of the bulk sample surface (T1) with various sweep rates
during the FC process operated form 5 to 0 T.

scatter from the brink of the sample at the very
beginning of the ﬁeld reducing. The heat generation starts in the periphery region and graduallypenetrates into the central portion. It arrives tothe maximum value when the ﬁeld change reaches
the center of the sample.
Fig. 3 indicates the temperature changes measured in the FC at various temperatures as afunction of ﬁeld descending rates. All of the3. Results and discussions
7-11.3Fig. 2 shows the temperature changes of thesample for various descending rates of the magnetic ﬁeld, 11.3, 5.06 and 2.53 mT/s. The data
were measured by the thermocouple T1 at the surface center during FC magnetization process. The
magnetic ﬁelds started decreasing from 5 T at t = 0and arrived at 0 T showing their steep peaks ofmaximum temperatures. One can obviously seethat the faster the speed of descending ﬁeld thehigher the temperature rises. The maximum temperature raise was observed as 5.9 K when thesweep rate was 11.3 mT/s.
The curves shown in Fig. 2 appear to consist oftwo changing rates in each rate. According to thecritical state model, the magnetic ﬂuxes start toTemperature Change (K)3.1. Temperature changes after FC magnetization
process

6-5.065-2.53

4mT/s

32
1050

60

70

80

90

Temperature (K)Fig. 3. The temperature changes at the bulk magnet surface forthe initial temperatures before lowering ﬁelds as a function ofsweep rates of the reducing magnetic ﬁeld. The averaged
temperature data derived from T1 to T5 were used.

T. Oka et al. / Physica C 426–431 (2005) 794–799
7797

457.5
-11.3

65.1

Magnetic Flux Density Bz (T)Temperature Change (K)

674.2

582.9
4
3
2
10-15

-10-5-5.06
3-2.53mT/s
210

Sweep Rate (mT/s)

temperature data plotted in Fig. 3 were averaged
T1–T5 before and after FC. One can see the apparent dependency of temperature rise on the initialtemperature before FC. The lower the initial temperature, the higher the maximum temperaturerises. It is attributed to the facts that the speciﬁcheat of the sample decreases whereas the pinning
force increases when it is cooled to the lower temperatures [10]. The maximum temperature rise of
6 K was observed when the FC was operated withthe sweeping rate of 11.3 mT/s at the initial temperature of 57 K.
The sweep rate dependent of temperature rise isshown in Fig. 4. The faster descend on the appliedﬁeld causes the faster motion of ﬂuxes. As was reported by Ikuta et al. [6]., the heat generation isproportion to the ﬂux speed, viscous force is proportion to the square of that. Hence the pinning
force is strongly responsible to the heat generation.
3.2. Trapped magnetic ﬁeld
Trapped magnetic ﬂux density is plotted as afunction of sweep rates in Fig. 5. As the positionof Hall sensor is slightly put aside from the centerof sample surface, the plotted points do not represent the ﬁeld trapping ability of each FC operation,
however they have exhibited the superior values ofover 3 T. An apparent scatter of data was observed

60708090

Temperature (K)Fig. 5. Trapped magnetic ﬂux density measured on the surfaceof the bulk magnet for the temperature as a function of sweep
rates of the reducing magnetic ﬁeld. The position of the Hallsensor is shown in the Fig. 1.

at 57 K. It suggests that the trapped ﬁeld is strongly
inﬂuenced by the sweeping rate during FC. The decline was estimated to be 11.7% of the maximum
data 3.415 T. As shown in Fig. 6, the sweep ratedependency is quite clear at every temperature.
That implies that the sweeping rate is really necessary to be carefully controlled so as to suppress the4

Magnetic Flux Density (T)Fig. 4. The temperature rise measured on the bulk magnet
surface as a function of sweep rates of reducing magnetic ﬁeld.
The averaged temperature data (derived from T1 to T5) werecited in the ﬁgure.

050

32

57.5
165.1
74.2
82.9

0-15

-10-50Sweep Rate (mT/s)
Fig. 6. Sweep rate dependent for the trapped magnetic ﬂux
density as a function of the averaged temperatures.

798

T. Oka et al. / Physica C 426–431 (2005) 794–799

heat generation due to the ﬂux motion in the highﬁeld trapping bulk sample.
3.3. Position dependence of temperature change
Fig. 7 shows the temperature development measured at every thermocouple T1–T5. The sweep
rate was 11.3 mT/s at 57 K. The proﬁles reachthe maximum values at the same time. It indicatesthat the generating heat diﬀuses in a short time toall the portions of the sample. The fact that thetemperatures at T4 and T5 are slightly higher than
the rests may possibly suggest that the heat generation does not uniformly occurs and that isresponsible for the variety of critical current density that is originated from the microstructure,such as the dispersion of pinning centers. It is
not clear at present because of the variationof the initial temperatures on the surface andfurther investigation is necessary to clarify thephenomena.
3.4. Trapped ﬁeld distribution
Fig. 8 shows a trapped ﬁeld distribution map
magnetized with a reducing rate of 11.3 mT/s
at 57 K, and the temperature rise during FC was
6 K. The map was measured in the horizontal
plane 3.5 mm above the bulk surface. The distribution indicates concentric round contour lines ofmagnetic ﬁeld and that no serious cracks exist inTemperature Change (K)

8

T1
T2
T3
T4
T5

6

Sweeprate(mT/s)

2

0300

600

900

1200

the sample. One can note that the circles on thedashed-lines which corresponds to the growth sector boundaries (GSB) slightly deviates outside. Itshows that the critical current density on the GSBs
is rather superior to that in the growth sector regions. The inhomogeneous heat generation is regard to be less inﬂuential to the whole trapped
ﬁeld distribution map that was measured above
3.5 mm from the surface, in spite of the temperature rise of 6 K. All the data were listed inTable 1. It is worthwhile mentioning that eventhe slowest sweeping rate of 2.53 mT/s resultsin the temperature rise of 3 K at 57 K.
Table 1
Temperature and trapped ﬁled data for the ﬁeld cooling process

40Fig. 8. Trapped ﬁeld distribution map of bulk sample measured
by a scanning Hall probe 5 mm above the vacuum surface.
Dashed lines show the growth sector boundaries.

Initialaveragedtemperature (K)Averagedtemperaturechange dT (K)Trappedﬁeld Bz (T)

2.53

57.2
65.1
73.6

2.8
1.3
1.1

3.415
2.453
1.341

5.06

57.6
64.9
74.3
82.9

4.1
2.8
1.9
1.2

3.319
2.340
1.313
0.665

11.3

57.6
65.2
74.8

5.9
4.5
2.4

3.025
2.098
1.219

1500

Time (s)Fig. 7. Normalized temperature changes of various positions
on the bulk sample.

T. Oka et al. / Physica C 426–431 (2005) 794–799

799

4. Conclusions

References

The temperature rise of a YBCO single domainbulk superconductor in FC process has reached
6 K when the sample was magnetized with the fastest descending speed of 11.3 mT/s. Strong dependency on the resultant trapped magnetic ﬁeld andsweep rate during FC operation was certiﬁed. It
was found that the planar variation of the criticalcurrent density on the sample surface has an eﬀect
on the performance as a trapped-ﬁeld magnet. We
have so far emphasized that the temperaturechange is never a negligible phenomena even in
the FC magnetizing operation using quasi-static
magnetic ﬁeld.

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Acknowledgement
This study was supported by IWATE CREATE
project driven by Japan Science and Technology
Agency.