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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 14, NO. 2, JUNE 2004

Flux Motion Studies by Means of Temperature
Measurement in Magnetizing
Processes for HTSC Bulks
Hiroyuki Fujishiro, Tetsuo Oka, Kazuya Yokoyama, Masahiko Kaneyama, and Koshichi Noto

Abstract—The temperature rises( ) of cryo-cooled SmBaCuO bulk superconductor have been measured after applying
the pulse magnetic field from ex = 3 01 to 5.42 T for various= 40 K
70 K). For each s
initial stage temperatures (and the trappedand ex , the observed systematic change ofmagnetic field T can be explained in terms of the diagram ofthe maximum temperature measured at the bulk center surface( 0max ) vs.
T . The maximum temperature risemax
decreases with increasing s under the identical ex .
Index Terms—High c bulk superconductors, pinning and viscous loss, pulse field magnetizing, temperature measurement.

I. INTRODUCTIONFOR the practical applications of the melt-processed REBaCuO superconductors (RE: rare earth ions), the pulse
field magnetizing (PFM) have been intensively investigated anddeveloped as well as the static field-cooled magnetizing (FCM)[1]. It is necessary to realize a wider space with higher magnetic field for the applications such as a magnetic separation.
For this purpose, new proposals using large-size bulk crystalsmm ) [2] or the assembly of the several small-size(e.g.,
bulks [3] have been made. In case of magnetizing these bulks,
it is expensive and is not functional to use a superconducting
magnet with a large bore, in which these bulks are to be inserted. The PFM has a promising potential to solve these difficulties because of the relatively compact and inexpensive setup.
in bulks by PFM is
However, the trapped magnetic fieldcurrently lower than that by FCM
FC below the liquid
nitrogen temperature [4]. The main cause has been attributed
due to the dynamical motion of theto the temperature rise
and the vismagnetic fluxes against the vortex pinning force
cous force . However, there have been few systematic experiwith thevalue. We investigatedmental studies to relate
Manuscript received October 20, 2003. This work was supported in part by
Japan Science and Technology Corporation under the Joint-Research Projectfor Regional Intensive in the Iwate Prefecture on “Development of practicalapplications of magnetic field technology for use in the region and in everyday
living.”H. Fujishiro and M. Kaneyama are with the Faculty of Engineering,
Iwate University, Morioka, 020-8551 Japan (e-mail: fujishiro@iwate-u.ac.jp;t2203004@iwate-u.ac.jp).
T. Oka and K. Yokoyama are with the Iwate Industrial Promotion
Center, Morioka 020-0852, Japan (e-mail: toka@iwate-techno.com;
yoko@iwate-techno.com).
K. Noto was with the Faculty of Engineering, Iwate University. He is now
with Iwate Industrial Promotion Center, Morioka 020-0852, Japan (e-mail:notok@abox2.so-net.ne.jp).
Digital Object Identifier 10.1109/TASC.2004.830392

on the surthe time evolution and spatial distribution offace of the cryo-cooled YBaCuO [5] and SmBaCuO [6] bulk
superconductors after applying the pulse fields, where the iniwas maintained in 38 K. It has been
tial stage temperaturefound that heat is mainly generated in the peripheral region of
the bulk disk and diffuses toward the central region. The magnetic fluxes have been confirmed to move more easily throughthe growth sector regions (GSR’s) than along the growth sector
boundaries (GSB’s). These results were consistent with the dis.
tribution of
In this paper, we investigate the time and position dependence
at various(40 K 70 K) after applying the pulseofmagnetic field fromto 5.42 T for the cryo-cooledandvalues
SmBaCuO bulk superconductor. Since both
and
, the
measurements are expected
depend on.
to offer the important clues to enhance
II. EXPERIMENTAL PROCEDURE
A highly c-axis oriented SmBaCuO bulk superconductor ofdisk shape with 46 mm in diameter and 15 mm in thickness
was fabricated by Dowa Mining Co., Ltd, which consisted of
4 growth sectors (GSR1-GSR4) [6]. The bulk was composedof SmBa Cu O (Sm123) and Sm BaCuO (Sm211) with theSm, 15.0 wt% Ag
molar ratio of Sm
powder and 0.5 wt% Pt powder. The bulk was uniformly impregnated by epoxy resin in vacuum and the epoxy resin layer
on both upper and bottom sides of the bulk disk was removed inorder to measure the precise temperature on the bulk surface
and to reduce the thermal contact resistance against the cold
stage. Fig. 1 shows the positions of the temperature measurements on the bulk, which was tightly stuck on the cold stageof a Gifford McMahon (GM) cycle helium refrigerator (AISIN,
was changed from
GR103). The initial stage temperature
40 K to 70 K. The temperatures,at the center of the bulk
at P1-P4 in the four GSR’s were measured(P0) and,by chromel-constantan thermocouples (76 m in diameter) adhered to the upper bulk surface using GE7031 varnish. P1 P4
were situated on the central lines of each GSR 9 mm apart from
P0. Each temperature was measured about 7 times/s just after
applying the pulse field.
The bulk crystal was magnetized using a pulse coil dipped. The rise time of the pulse field
was aboutin liquid
10 ms. The strength of
was determined from the current
flowing through the coil, which ranged from 3.01 T to 5.42 T.
The Hall sensor (F.W. Bell, model BHT-921) was adhered to the1051-8223/04$20.00 © 2004 IEEE

FUJISHIRO et al.: FLUX MOTION STUDIES BY MEANS OF TEMPERATURE MEASUREMENT

Fig. 1. The positions for the temperature and trapped field measurements on
the surface of the SmBaCuO bulk. P0–P4 for the temperature measurementof T 0 T 4 are indicated. The trapped fields B mm 0 B mm 4 are alsomeasured at 3 mm just above P0–P4 by a scanning Hall sensor. The trappedfield B on the bulk surface is measured using a fixed Hall sensor at PH.

00position PH, 2.5 mm apart from P0 as shown in Fig. 1, which
value. The two-dimensional distribution ofmonitored the
mm was monitored using an axial
the trapped magnetic field
type Hall sensor, which scanned 3 mm above the bulk surface
mm values at P0 P4
stepwise with a pitch of 1.2 mm. The
mm
mm were extracted from the distribution.
FC by FCM was also measured at sevThe trapped fielderal temperatures using a cryo-cooled superconducting magnet.
During FCM, the static magnetic field of 5 T was decreased
down to 0 T in 18 min.
III. RESULTS AND DISCUSSION
Figs. 2(a) and 2(b) show the time evolutions of temperatures
for P0 P4 at
K after applying the pulse
field of 3.01 T and 4.64 T, respectively. Figs. 2(c) and (d) show
at
K after applying the pulse field of
mm
3.01 T and 4.64 T. The insets of each figure present thevalues at P0 P4. It can be seen that the maximum temperaturemmrise, the time evolution of temperature, and thevalue for each position change depending on theandvalues. In Fig. 2(a) for
at
K,
and
rise up first withK, followed by
and
, and
rise up latest.
andshow a char, but the others increase moderately
acteristic peak at
K.
and then reach a maximum at
All the temperatures return to the initial ones in about 10 minandwith a characterutes. The rapid and large rise of
istic peak means that the powerful heat source is located in thevery vicinity of P2 and P3. Since the flux motion necessarily
generates heat inside the bulk through the pinning power loss
and the viscous flow power loss , such a rapid rise suggests the existence of the easy path for the flux motion dueof GSR2 and GSR3. In
to relatively weaker pinning force,
andrise up faster with a clear peakFig. 2(b),K , followed by, andrise up latest
K.
increases with increasing
and.
the spatial distribution of the generated heat depends on
In Fig. 2(c) for
at
K,
,
and
rise up first with a clear peak at, followed by, and thenrise up latest.
is 8 K for ,1055

and , which is similar to that foratK’s forshown in Fig. 2(a). In Fig. 2(d), on the other hand,P1 P4 rise up simultaneously and steeply and do not show aclear peak.
is 22 K and slightly smaller than those forat
K shown in Fig. 2(b). The rapid risemeans that the fluxes penetrate pretty uniformlyinthrough the surfaces of the circumference of the 4 GSR’s. In all
K to 70 K,
the cases of the flux penetration fromat the center of the bulk rise up latest, because the main heatgeneration always takes place in the peripheral region where themagnetic fluxes move faster.
mm also changes depending on and
The distribution ofmm. In the inset of Fig. 2(a) for
at 40 K,mmmm
is larger for P2
and P3,mm andmm
is also larger. However,where
are quite small andandare small as well. The
mm andsuggest that the surbehaviors offace barrier against the flux penetration has been destroyed in. On the other hand, the barGSR2 and GSR3 forriers are kept intact in GSR1 and GSR4. In the inset of Fig. 2(b)mm is the largest for the GSR1
at 40 K,forand the smallest for the GSR3 with a V-shaped distribution,. This result is consistent withmaking a contrast to
the existence of hard path through P1 and the easy path throughK , the behaviors
P3 for the flux motion. In the higher
mm change. In the inset of Fig. 2(c) forofmm andmm take similar values to those forat 50 K,mm increases from 0.36 T
at 40 K. However,(40 K) to 0.88 T (50 K). This result indicates that the surface
barrier against the flux penetration is destroyed at 50 K also inandincrease, resultantly. In the inset
GSR4 and thatmm values decrease at P1, P2, and P4, andof Fig. 2(d), themm distribution is more broadened compared
the V-shaped
at 40 K.
with those forto reach 60% of the maximum
We define the rise time
temperature rise
. Figs. 3(a) and 3(b) show
as aand 4.64 T. In Fig. 3(a),function of forforandis smaller at
K because of the destruction
forand
become short atof the surface barrier.
K and 60 K, respectively, for the same reason. All
the surface barriers in the circumference region against the flux
K for
. Thesepenetration are destroyed atwith the increase in .
results are caused by the decrease ofThevalue is determined by the thermal diffusivityof the bulk and the distance between the measuring point anddecreases with increasing (the heat source.
and 8.2 mm
at 40 K and 70 K, respectively) [7]. Accordingly,
the decrease of
at P4 and P1 with increasing does not, but originates from the approach of the heatcome from,source to P4 and P1. In Fig. 3(b) forfor P1 P4 is 0.9 1.2 s and independent of and the position.
These results are consistent with the uniform flux intrusion andat P0 shows a large
heat generation. On the other hand,value (2.4 3.5 s) and monotonically increases with increasing, which is attributable to the decrease of.
Fig. 4 showsat P0 for eachas a function of. The inset presents thedependence ofmonitoredby the Hall sensor fixed at PH shown in Fig. 1. (note that P0

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 14, NO. 2, JUNE 2004

T0(t)  T4(t)

T = 40Fig. 2. The time evolution of temperatures
for P0 P4 at
K after applying the pulse field of (a) 3.01 T and (b) 4.64 T, respectively. (c)mm values at
at
K after applying the same pulse fields as (a) and (b), respectively. The insets of figures present theand (d) showeach position.

T0(t)  T4(t) T = 50

B

1T0

T

Fig. 3. The
stage temperaturet(60%) value at the positions (P0P4) as a function of the initial
T for (a) B = 3:01 and (b) 4.64 T.

Fig. 4. The maximum temperature riseat P0 for eachas a
. The inset presents thedependence ofmonitored byfunction of
the Hall sensor fixed at PH shown in Fig. 1.

is the nearest position to PH). For each ,increases. The
and then shows a saturation tendency with increasing
values are unexpectedly almost identical for eachat
, but increases with decreasing , especiallyfor higher. These results can be explained by the decrease
and by the increase of the specific heat at higher temof
K,increases
perature. In the inset of Fig. 4, atin comparison to, takes a maxfor
imum, and decreases with further increase of.
at 3.01 T
increases with increasing up to 60 K and then decreases. On
at 5.42 T monotonically decreases with inthe other hand,creasing .

Fig. 5 shows the summary ofvs.
in this study. For) are plotted foreach , four data sets of ( ,, 3.84, 4.64 and 5.42 T. The measured trapped fieldFC
by FCM corresponding to the temperatureis also presented. In the case ofK, the two data sets of ( ,) are situated below theFCline for. For,
touches the line owing to thedecreases following the line.
large temperature rise and thenIn the case ofK, only the data set forisFC
line, and for,situated below thetouches the line and thedecreases. On the other hand,and 70 K, all the data sets touch the line already at
for

B

B

B

FUJISHIRO et al.: FLUX MOTION STUDIES BY MEANS OF TEMPERATURE MEASUREMENT

1057

ability of the bulk necessarily decreases. In this sense, must
be decreased as low as possible in order to obtain the higher
values. The
behavior at the various
and for various
can be clearly understood on the basis of theFCline, which corresponds to the maximum-trapped field of thebulk.
is caused by both the pinning power
The temperature riseloss
and the viscous flow power loss . Which power loss
in the case of each
andis open tois dominant forquestion. In order to clarify this problem and to improve theenhancement, it is necessary to anPFM technique for thealyze the data more closely based on the generated heat forand.
variousFig. 5. The summary of the trapped field B at PH vs. the maximum
at P0. For each T , four data sets of (B , T 0) aretemperature T 0plotted for B = 3:01, 3.84, 4.64 and 5.42 T. The trapped field B (FC) by
FCM is also shown.

andmonotonically decreases with increasing. These results demonstrate that the flux trapping ability by
the PFM technique can be systematically explained as limitedFCline [6].
by theof aIn summary, we measured the temperature rises
cryo-cooled SmBaCuO bulk superconductor after applying thepulse magnetic fields ( 10 ms of the rise time) fromto 5.42 T for various initial stage temperatures
K . The maximum temperature rise
, the time evo, and the trapped fieldlution of temperature
mm value for each position changes depending on theandvalues. When increases,
decreases due to theand the specific heat as shown
reduction of the pinning force
in Fig. 4. As a result, the magnetic fluxes easily penetrate into
the bulk and can be trapped at higher even for relatively lower
value;
increases with increasing for,
as shown in the inset of Fig. 4. However, at higher , there
is only small temperature margin between and the superconducting transition temperature
K and the flux trapping

ACKNOWLEDGMENT
The authors greatly thank Prof. M. Ikebe of Iwate University
for valuable suggestions.
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