Physica C 426–431 (2005) 671–675
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Temperature rise and trapped ﬁeld in a GdBaCuObulk superconductor cooled down to 10 K after
applying pulsed magnetic ﬁeld
K. Yokoyama a,*, M. Kaneyama b, H. Fujishiro b, T. Oka c, K. Noto da

National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japanb
Iwate University, 3-4-5 Ueda, Morioka, Iwate 020-8551, Japanc
IMRA Material R&D Co., Ltd., 5-50 Hachiken-cho, Kariya, Aichi 448-0021, JapandIwate Industrial Promotion Center, 3-35-2 Iioka-shinden, Morioka, Iwate 020-0852, Japan
Received 23 November 2004; accepted 22 February 2005

Abstract
In order to magnetize the bulk superconductors more strongly by pulsed-ﬁeld magnetization (PFM), the temperature rise DT(t) and the trapped ﬁeld BT of the GdBaCuO bulk cooled down to 10 K have been investigated after applying the ﬁve successive (No1–No5) pulsed-ﬁeld ranging from 4.70 T to 6.04 T. DT increased linearly with lowering initialbulk temperature Ts and with increasing strength of the applied ﬁeld Bex. From the generated heat (Q) estimation, thediﬀerence Q value, DQ = Q(No1)  Q(No5) which suggests that the DQ nearly stands for the ﬂux pinning loss Qp shows
the similar Bex dependence to the BT value.
Ó 2005 Elsevier B.V. All rights reserved.
PACS: 74.72.h; 74.25.H
Keywords: GdBaCuO bulk superconductor; Temperature measurement; Pulsed-ﬁeld magnetization; Heat generation

1. Introduction*Corresponding author. Tel.: +81 29 863 5455; fax: +81 29863 5441.
E-mail address: yokoyama.kazuya@nims.go.jp (K. Yokoyama).

Recently, the performance of REBaCuO (RE =Y, Sm, Gd, etc.) bulk superconductors such as a
critical current density Jc and the mechanicalstrength has been remarkably improved. As aresult, the bulk can trap an extremely high magnetic ﬁeld of up to 17 T at 29 K [1]. The bulk,0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2005.02.069

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cooled down to lower temperatures by a cryocooled refrigerator, can be realized as a stronger
magnet than a permanent magnet and an electromagnet and various kinds of applications such as
magnetic separation and magnetron sputtering
have been considered [2–4]. Bulk superconductors
are usually magnetized by ﬁeld-cooled magnetization (FCM). However, there are some diﬃculties
due to the use of an expensive superconducting
magnet with a higher ﬁeld and a larger bore size,
although higher trapped ﬁelds as high as the maximum ability of the bulk can be realized. Pulsedﬁeld magnetization (PFM) has been recently
investigated because the magnetizing equipment israther simple and inexpensive by use of a conventional power supply and a copper coil. The trapped
ﬁeld obtained by PFM is, however, generally about
half of that obtained by FCM at lower temperatures which results from the heat generation causedby the pinning loss and the viscous loss due to theﬂux motion such as ﬂux jump and ﬂux ﬂow.
We have investigated the time evolution and spatial distribution of the temperature rise DT(t) on thesurface of bulk superconductors after applying apulsed ﬁeld at an initial bulk temperature from
Ts = 70 K down to 40 K [5–8]. The trapped ﬁeldBT obtained by PFM increases with increasing
strengths of the pulse ﬁeld but decreases for a higher
pulse ﬁeld application due to the large heat generation. It can be clearly seen that BT increases withdecreasing Ts for a ﬁxed Bex. The monitoring oftemperature and the estimation of the generated
heat Q enabled us to elucidate the mechanisms of
PFM and to enhance BT [7]. In order to magnetize
bulk superconductors more strongly by PFM, it isdesirable to lower the temperature Ts. In this paper,
we investigate DT(t) and BT for a GdBaCuO bulk
superconductor cooled down to 10 K after applying
a pulsed ﬁeld. We discuss the possibility of the BT
enhancement by PFM at lower temperatures.

Fig. 1. Experimental equipment.

helium refrigerator (Sumitomo Heavy Industries,Ltd, SRDK-408D) with a cooling ability of down
to 4 K. The bulk fabricated by Nippon Steel Co.,
Ltd. consists of GdBa2Cu3Oy (Gd123), Gd2BaCuO5 (Gd211) with a molar ratio of 1.0:0.4,0.5 wt.% Pt powder and 10 wt.% Ag2O. The epoxy
resin layers impregnated upper and bottom surfaces of the bulk were removed in order to measure
the surface temperature precisely and to reduce
thermal contact resistance to the cold stage. TheSUS304 ring was set onto the bulk disk formechanical reinforcement and for DT(t) reduction[8]. The bulk was cooled down to 10 K with a radiation shield set on the ﬁrst stage of the refrigerator.
A Teﬂon-coated chromel–constantan thermocouple, 76 lm in diameter, was adhered to the centerof the bulk surface using the GE7031 varnish
and the time dependence of temperature T(t) wasmeasured. A Hall sensor (F.W. Bell, BHT-921)
was also adhered near the bulk center in order tomeasure the trapped ﬁeld BT. A magnetizing copper coil dipped in liquid nitrogen was set outside
the vacuum vessel and an exciting pulse current
was supplied from a condenser bank of 60 mF.
After the GdBaCuO bulk was cooled down to
the initial temperature Ts = 10, 20, 30, 40 and
60 K, ﬁve successive pulsed ﬁelds (No1–No5) of
4.70, 5.53 and 6.04 T with a rise time of 12 ms
were applied and the time dependence of temperature T(t) and BT were measured at each stage.

2. Experimental
Fig. 1 illustrates the experimental equipment. A
highly c-axis oriented GdBaCuO bulk superconductor, 46 mm in diameter and 15 mm in thickness, was set on the cold stage of a GM-cycle

3. Results and discussion
Fig. 2 shows the time responses of temperatureT(t) for various initial temperatures Ts after apply-K. Yokoyama et al. / Physica C 426–431 (2005) 671–675673

Fig. 2. The time responses of temperature T(t) at the initialtemperatures Ts ranging from 10 to 60 K after applying the ﬁrst
pulsed ﬁeld of 5.53 T.

Fig. 3. The initial temperature Ts dependence of temperaturerise DT after applying the ﬁrst (No1) and the ﬁfth (No5) pulsed
ﬁeld for various applied ﬁelds Bex.

ing the ﬁrst (No1) pulse of 5.53 T. T(t) for each Ts
rises up, takes a maximum within 5 s and thenslowly recovered to the initial Ts for 15–20 min.
In order to enhance the BT value by PFM, thereduction of Tmax is an indispensable issue. Themaximum temperature Tmax reaches 62 K for
Ts = 20 K and Tmax is 60 K for Ts = 10 K. Tmax
slightly decreases and shows a saturation tendencywith decreasing Ts. Tmax of the bulk is expected todecrease with decreasing Ts for the same input heatgeneration. However, the eﬀect was very small,even if Ts decreased down to 10 K. Because thespeciﬁc heat C of the bulk decreases and approaches zero with decreasing temperatures and/or the cooling power of the GM refrigerator becomes small in this lower temperature region.
Fig. 3 displays the Ts dependence of the temperature rise DT after applying the No1 and No5
magnetic pulses for each applied ﬁeld Bex. For all
the Ts and Bex, the DT is the largest for the No1pulse, and decreases with increasing pulse numbers, approaching a ﬁxed ultimate behavior for
the No4 and No5 pulses. This behavior is quite
reasonable because the largest amount of the ﬂux
penetrates into the virgin state bulk and further
ﬂux penetration for the succeeding pulses should
be limited due to the presence of the alreadytrapped ﬂux. These saturation tendencies in T(t)are closely related with those of the trapped ﬁeld
BT. For the No1 pulse, DT increases with an in-crease in Bex and monotonically decreases withincreasing Ts. The DT values seem to approachzero at the superconducting transition temperature
Tc. For the No5 pulse application, the magnetic
ﬂux previously intrudes into the bulk and the DTvalues are smaller than those for the No1 pulse.
It should be noticed that the DTTs curve is nota straight line, contrary to that of the No1 pulse.
The heat generation Q in PFM can be estimatedby the following equation:
Z T s þDT max
Q¼
C ðT ÞV dT ¼ Qp þ Qv ;
ð1Þ
Tswhere C(T) is the speciﬁc heat and V is the volumeof the bulk. Q is regarded as the sum of the pinning loss Qp, which is closely related with the ﬂux
trapping, and the viscous loss Qv which is relatedwith the ﬂux ﬂow (Q = Qp + Qv). Since no additional ﬂux trapping takes place for the No5 pulse,
it is reasonable to assume that Q(No5) for the No5
pulse mainly consists of Qv. The diﬀerence DQ(=Q(No1)  Q(No5)) roughly stands for the pinning loss correlated with the ﬂux trapping for theNo1 pulse [7].
Fig. 4(a) shows the generated heat Q(No1) andQ(No5) for various Ts after applying the No1 andNo5 pulsed ﬁelds as a function of Bex. The Q values were calculated using Eq. (1). The Q(No1) values at all the Ts are nearly equal for Bex = 4.70 T,
and the values increase linearly with increasing Bex

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Fig. 5. The Bex dependence of the trapped ﬁeld BT for each Ts.

Fig. 4. (a) A generated heat Q(No1) and Q(No5), and (b)DQ = Q(No1)  Q(No5) at Ts = 10–60 K as a function of Bex.

and the Q(No1) increases with lowering Ts. On theother hand, the Q(No5) values are almost equal
independently of Bex except for the value atTs = 60 K and are always smaller than Q(No1).
Fig. 4(b) presents the Bex dependence of DQ ateach Ts. The DQ value for Bex = 5.53 T and
6.04 T, which stands for Qp decreases with increasing Ts. However, it can be seen that the DQ forBex = 4.70 T slightly increases with increasing Ts
except for Ts = 60 K. The DQ increases withincreasing Bex at Ts = 10 K, but it changes moderately for a higher Bex at other temperatures.
Fig. 5 shows the Bex dependence of the trapped
ﬁeld BT for each Ts. At Ts = 10 K, BT increases
linearly with increasing Bex. On the other hand,for Ts = 60 K, BT monotonically decreases withincreasing Bex. These Bex dependences are similar

to the DQ behaviors as shown in Fig. 4(b). Theseresults support the fact that the DQ value should
be closely related with the ﬂux trapping. For alower Bex = 4.70 T, BT is the smallest for Ts =10 K because the magnetic ﬂux can hardly intrude
into the bulk due to the strong pinning potential.
On the other hand, for a higher Bex = 6.04 T, BT
decreases with increasing Ts. The Bex = 6.04 Tvalue for the bulk cooled to Ts = 60 K is too highto trap the magnetic ﬂux. These results suggestthat there may be an optimal applied ﬁeld Bex(opt)
for each Ts. If we apply a magnetic ﬁeld Bex higher
than 6.04 T at Ts = 10 K, the BT value may enhance. The maximum BT value was 1.6 T at Ts =10 K after applying Bex = 6.04 T in this study.

4. Conclusions
We investigated the possibility of the BT
enhancement in the GdBaCuO bulk by PFM atlower temperatures down to 10 K. The temperature rise (DT) increased linearly with a lowering initial bulk temperature Ts and with increasing
strengths of the applied ﬁeld Bex. From the generated heat estimation, the DQ = Q(No1)  Q(No5)value which could be conﬁrmed that the DQ valuenearly stood for the ﬂux pinning loss Qp showed
the similar Bex dependence of the BT value. Furthermore, from the Bex dependence of the BT, it
was clariﬁed that there was an optimal applied ﬁeldBex(opt) for BT becoming maximum for each Ts.

K. Yokoyama et al. / Physica C 426–431 (2005) 671–675

AcknowledgmentThis work is partially supported by the IWATE
CREAT project (October 1999–September 2004)
by the Japan Science and Technology Agency.
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