IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009

3545

Pulsed Field Magnetization for GdBaCuO Bulk
With Stronger Pinning Characteristics
Hiroyuki Fujishiro, Takuya Hiyama, Takashi Miura, Tomoyuki Naito, Shinya Nariki, Naomichi Sakai, andIzumi Hirabayashi

Abstract—The GdBaCuO superconducting bulk with stronger
at 77 K by the field cooled
pinning characteristics ( Tmagnetization (FCM)) has been magnetized at sby the pulsed field magnetization (PFM) techniques; a sequential pulsed-field application (SPA) and a modified multi pulse
technique with stepwise cooling (MMPSC). With decreasing s ,the pinning force P increases especially at the growth sector
boundaries (GSBs), and then the nonuniformity of the trappedfield profile becomes more and more conspicuous on the bulk
surface. At low s , the SPA technique is not necessarily a suitable
technique to enhance the trapped field T and the total trapped
flux T . However, these values can be enhanced by the MMPSC
method. The properties of the bulk with stronger pinning characteristics during PFM are discussed.

= 18T= 70 20 K

Index Terms—MMPSC method, pinning force, pulsed field magnetization, trapped field.

I. INTRODUCTIONFOR the practical application of superconducting bulks as astrong quasi-permanent magnet such as a magnetic separation for environmental cleaning [1] and a drug delivery system(DDS) for medical applications [2], the magnetizing technique
is very important. Recently, a pulsed field magnetization (PFM)
has been developed instead of a conventional field-cooled magnetization (FCM) because of a compact, mobile and inexpenby PFM was, however, pretty
sive setup. The trapped fieldsmall, compared with the trapped fieldby FCM due to thelarge temperature rise by the dynamical motion of the magnetic
ever reported had been as low as 3.8 T
fluxes. The maximum
at 30 K by an iteratively magnetizing pulsed-field method with
reducing amplitude (IMRA) [3]. We have systematically studied,
the time and spatial dependences of the temperatureand the trapped fieldon the surface of crylocal fieldocooled REBaCuO (RE: rare earth element) bulks during PFM, andfor various starting temperatures and applied fields

Manuscript received August 16, 2008. First published June 05, 2009; current version published July 15, 2009. This work was supported in part by aGrant-in-Aid for Scientific Research from the Ministry of the Education, Culture, Sports, Science and Technology, Japan 19560003.
H. Fujishiro, T. Hiyama, T. Miura, and T. Naito are with the Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan (e-mail: fujishiro@iwate-u.ac.jp).
S. Nariki, N. Sakai, and I. Hirabayashi are with the Superconductivity Research Laboratory (SRL), International Superconductivity Technology Center(ISTEC), 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan (e-mail:nariki@istec.or.jp).
Digital Object Identifier 10.1109/TASC.2009.2018081

have suggested the importance of the precise measurements ofon the bulk surface [4], [5].
, the reduction in temperature riseis an
To enhance
to 10–20 K, which isindispensable issue. The lowering ofeffective for FCM due to the enhancement of the critical current
density , is not necessarily effective for PFM because of thelarge heat generation due to the increase of pinning loss and thedecrease in the heat capacity of the bulk. Taking the obtained experimental results into consideration, we proposed a new PFM
technique named a modified multi pulse technique with stepwise cooling (MMPSC) [6], and have succeeded the highest
on the GdBaCuO bulk?45 mm
field trapping of
in diameter, on which only 3.6 T was trapped by a single pulse
is a record-high value
application at 40 K [7].
by PFM to date. We applied the technique to other bulks withdifferent pinning ability and confirmed that the MMPSC was a[8].
universal and effective method to enhance
Another approach to enhance
is to use the bulk withhigher . Nariki et al. have fabricated the high-performance
were as high as
GdBaCuO bulks, on which trapped fields
2.1–2.2 T (48 mm in diameter) [9] and 3.05 T (65 mm in diamand highereter) at 77 K [10]. If the bulk with higher
is magnetized by PFM such as the MMPSC method, higher
value is expected. In this paper, we applied the PFM techniques
to the GdBaCuO bulk with higher trapped field characteristics
at 77 K). The trapped field, the total trapped(, and the temperature riseare investigated for the
flux
sequential pulse field application (SPA) and MMPSC methods.
II. EXPERIMENTAL PROCEDURE
The GdBaCuO superconducting bulk disk (45 mm in diameter and 18 mm in thickness) used in this study was fabricated by
SRL-ISTEC, Japan. The melt processing was performed underin Ar [9]. The
controlled oxygen partial pressure of 1%
trapped field
was as high as 1.8 T at 77 K. The bulk was
mounted on soft iron yoke cylinder and tightly anchored onto
the cold stage of a Gifford-McMahon (GM) cycle helium refrigerator. The experimental setup around the bulk was described
elsewhere [8]. The magnetizing solenoid coil, which generated
with a rise time of 12 ms andthe pulse field up toa duration of 100 ms, was placed outside the vacuum chamber,in which the central axis of the resulting field coincides withof the bulk wasthat of the bulk. The starting temperature
controlled over the range from 70 K to 20 K. Three magnetic
were applied sepulses (Nos. 1–3) with the same amplitude
quentially after re-cooling to . We abbreviate this technique
as a sequential pulsed-field application (SPA) method. The time1051-8223/$25.00 © 2009 IEEE

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009

Fig. 1. The trapped field B at the bulk center for the No. 1 and No. 3 pulses
in the SPA method as a function of the applied pulsed field B for various
starting temperatures T . In the figure, [a]–[d] denote the typical conditions, atwhich the B (0.5 mm) profiles are mapped as shown in Figs. 5(a), 5(b), 6(a),and 6(b), respectively.

dependence of the local field
was measured at the bulk
center 0.5 mm above the bulk surface, and the temperatureon the bulk surface was also measured using a fine thermocouple(0.5 mm) was mapped
during PFM. The trapped field profile
0.5 mm above the bulk surface, stepwise with a pitch of 1.2 mm
by scanning an axial-type Hall sensor (F.W. Bell, BHA 921) instage controller with aside the vacuum chamber using an
flexible bellows. The total amount of the trapped magnetic flux
(0.5 mm) was calculated by integrating thedensity(0.5 mm) over the region where it wasmagnetic flux density
positive. The MMPSC method was also applied to the bulk. The
detailed sequence of the method was described in Section 3.3.
III. RESULTS AND DISCUSSION
A. Sequential Pulsed-Field Application (SPA)
Fig. 1 shows the trapped field(0.5 mm) at the bulk
center after the No. 1 and No. 3 pulses in SPA, as a function offor various starting temperatures
the applied pulsed field. For the No. 1 pulse,
atstarts to increase, takes a maximum atand thenfor. The
curve can be
decreases with increasing
usually observed for the PFM technique, which is attributed toa temperature rise in the bulk due to the heat generated by thefast motion of flux lines in the presence of resistive forces; thepinning force and the viscous force. The applied pulse field, atwhich the magnetic flux starts to be trapped at the bulk center,and further increases withincreases to 4.5 T atdecreasing . This comes from the increase of the shielding
current in the superconductor with decreasing . The peak inthecurve cannot be confirmed atbecause. For the No. 3 pulse,of the experimental limit ofslightly increases as shown in the dotted lines.
after the No. 1 and No.
Fig. 2 shows the total trapped flux
3 pulse as a function of the applied fieldfor various starting
curves take a maximum.
temperatures . All thevalue increases and thevalue, at which
The maximum
takes a maximum, shifts to the higher value with decreasing. It should be noted that thevalue fairly increases for the

Fig. 2. The total trapped flux 8 for the No. 1 and No. 3 pulses in the SPA
method as a function of the applied pulsed field B for various starting temperatures T . [a]–[d] denote the conditions, at which the B (0.5 mm) profiles
are mapped as shown in Figs. 5(a), 5(b), 6(a), and 6(b), respectively.

Fig. 3. The time evolution of applied field  H (t) and the local fields B (t)after applying the magnetic pulse of B = 6:25 T at T = 50, 30 and 20 K.

SPA technique; thevalue for the No. 3 pulse is 20 40%
larger than that for the No. 1 pulse, and that the increment rateincreases with decreasing . In this way, the SPA technique iseffective for the enhancement ofrather than that of.
Fig. 3 shows the time evolution of the applied field
and the local fieldat the bulk center after applying the No.
at various .
was monitored
1 pulse ofby the current
flowing through the shunt resister using astarts to increase for, takes
digital oscilloscope.
a maximum at 15 ms with a time delay and then decreases to a
final value due to the flux flow. The maximum of theis smaller than that of the, which suggest thatis not the enough strength to make the magnetic fluxes.
intrude into the bulk center at
Fig. 4(a) shows the time dependence of the temperatureon the bulk surface after applying the pulse fields
changesharply rises up, takes
from 5.00 T to 6.32 T at 30 K.
a maximum at 5 s and then gradually decreases. It should betakes a maximum, is innoticed that the time, at which
the order of seconds, which is clear contrast with the local
shown in Fig. 3; the time, at which
takes
field
a maximum, is in the order of milliseconds. The maximum
increases with increasing
due to thetemperature
increase of pinning loss and viscous loss.
reaches 70 K

FUJISHIRO et al.: PULSED FIELD MAGNETIZATION FOR GDBACUO BULK WITH STRONGER PINNING

3547

Fig. 4. (a) Time dependence of the temperature change T (t) on the bulk surface
after applying the pulse fields from 5.00 T to 6.32 T at 30 K. (b) The pulse
number dependence of T (t) on the bulk surface for B = 6:32 T at 30 K.

for. The maximum temperature riseincreased with decreasing(not shown) due to the enhancement of pinning loss and the decrease in the heat capacity ofthe bulk. Fig. 4(b) shows the pulse number dependence ofin the SPA method on the bulk surface forat.
gradually decreases with increasing pulse
number and then saturates due to the decrease of the pinningloss of the already trapped fluxes. The decrease of theenhances theandvalues as shown in Figs. 1 and 2.
Figs. 5(a) and 5(b) show the trapped field profiles(0.5 mm) after applying the No. 1 pulse field of
at
and
at
, respectively. For
relatively higher
and lower, as shown in Fig. 5(a), thenonuniformity of the trapped field profile is not so large, except
for the region near the position A. The result roughly suggeststhat the distribution of in the bulk is relatively small at higher. On the other hand, at lower
and relatively lower, asshown in Fig. 5(b), the position dependence of the trapped fieldat the positions B, C and D increases
is quite remarkable;at the position A (bulk center) remains
over 1.5 T. However,
0.5 T. The positions B, C and D are not in the growth sector
boundaries (GSBs) but in the growth sector regions (GSRs).
The pinning force in the GSBs is usually stronger than thatin the GSRs due to the large number of crystal defects. The
nonuniformity of the trapped field might be a characteristic feature for the bulk with strong pinning force. Figs. 5(c) and 5(d)show the cross sections of the trapped field profiles for typicalconditions along the - and -axes, respectively, in which thecharacteristic positions from A to D are indicated.
at thepositions B, D and around C increases with decreasing . For, the magnetic fluxes can be preferentially trapped
the lower
at the weak pinning centers in the GSRs.
(0.5 mm) profile after apFigs. 6(a) and 6(b) show theatplying the higher pulse field (No.1 pulse) of
and 30 K, respectively. In Fig. 6(a), theshowsnearly the cone-shaped profile and the magnetic fluxes are preferentially trapped along the GSBs rather than the positions B,
C and D. As a result, the center of the trapped field shifts toward the bulk center (position A). In this way, for the higher

Fig. 5. The trapped field profiles B (0.5 mm) after applying the pulse field(No.1 pulse) of (a) B = 3:4 T at T = 70 K and (b) 5.2 T at T = 30 K.
The cross sections of the trapped field profiles for typical conditions along the(c) x- and (d) y -axes, respectively

Fig. 6. The trapped field profiles B (0.5 mm) after applying the pulse field(No.1 pulse) of (a) B = 6:2 T at T = 50 K and (b) 6.2 T at T = 30 K.
The cross sections along the x-axis of the trapped field profiles after applying
the (c) No. 1 and (d) No. 3 pulse field of B = 6:2 T, respectively.

, the magnetic fluxes can be preferentially trapped at thestrong pinning centers in the GSBs due to the large temperature
profilerise. Figs. 6(c) and 6(d) show the cross section of theatalong the -axis for No. 1 and No. 3 pulses ofand 30 K, respectively. It should be noted that thetrapped fields are enhanced especially at the bulk periphery by
the SPA method.

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009

magnetization (FCM)) has been magnetized by a sequential
pulsed-field application (SPA) and a modified multi pulse
technique with stepwise cooling (MMPSC). With decreasing
increases especially at thetemperature, the pinning force
growth sector boundaries (GSBs) rather than the growth sector
regions (GSRs), and then the nonuniformity of the trappedfield distribution becomes more and more conspicuous on
the bulk surface. It was found that the MMPSC method wasand thevaluable technique to enhance the trapped fieldalso for the bulk with stronger pinning
total trapped fluxes
characteristics. However, since the optimum condition for the
MMPSC method strongly depends on the used bulk crystal andthe experimental apparatus, it is not easy to apply the MMPSC
method. The advent of a new PFM technique is anticipated tomagnetize sufficiently the superconducting bulks with stronger
pinning characteristics.
ACKNOWLEDGMENT
Fig. 7. (a) The estimated time dependence of temperature T (t) in the MMPSC
method. (b) The measured cross section of the B (0.5 mm) profiles for eachstep in the MMPSC method. The B (0.5 mm) profiles after applying the(c) No. 2 pulse and the (d) No. 3 pulse in the MMPSC method.

The authors would like to thank Y. Yanagi and Y. Itoh of
IMRA Materials R&D Co., Ltd., Japan, for valuable discussion
and for the technical support for the measurement apparatus of
the trapped field distribution.

B. MMPSC Method
Fig. 7(a) shows the conception of the MMPSC method [6].
Four magnetic pulses were applied at different initial temperaand. At the first stage, a lower pulse field oftures
was applied twice (No. 1 and No. 2) atin order to realize the “ -shaped” trapped field profile on thebulk surface, which means the lower trapped field at the bulk
center than that at the bulk periphery, and is necessary to en[6], [7]. At the second stage, the bulk washance the final
and a higher pulse field ofcooled down to
was applied twice (No. 3 and No. 4). A small and properamount of magnetic fluxes should be trapped on the bulk peat a first stage at, and theriphery for the reduction inshould be applied at a second
strong and optimum pulse field.
stage atFig. 7(b) shows the cross section of the(0.5 mm) profile
along the -axis for each step in the MMPSC method. At the firststage, the “ -shaped” trapped field profile can be obtained, andas high as 2.6 T andat the second stage,
were realized, which were higher than those obtained by the SPA method as shown in Fig. 1. Figs. 7(c) and(0.5 mm) profiles of the No. 2 pulse and the7(d) show theNo. 3 pulse in the MMPSC method, respectively. After the No. 3
pulse application, the magnetic fluxes are preferentially trapped
along the GSBs rather than along the GSRs, where the positions
B, C and D exist.

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