INSTITUTE OF PHYSICS PUBLISHING

SUPERCONDUCTOR SCIENCE AND TECHNOLOGYSupercond. Sci. Technol. 17 (2004) 51–57

PII: S0953-2048(04)66338-X

Temperature rise in an Sm-based bulk
superconductor after applying iterative
pulse fields
Hiroyuki Fujishiro1, Kazuya Yokoyama2, Tetsuo Oka2
and Koshichi Noto112

Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
Iwate Industrial Promotion Center, 3-52-2, Iioka-shinden, Morioka 020-0852, Japan

E-mail: fujishiro@iwate-u.ac.jp

Received 8 July 2003
Published 19 November 2003
Online at stacks.iop.org/SUST/17/51 (DOI: 10.1088/0953-2048/17/1/009)
Abstract
The time evolutions and spatial distributions of temperature rise T (t) havebeen measured on the surface of a cryo-cooled SmBaCuO bulk
superconductor after iterative pulse field magnetizing with the same
amplitude. For the applied pulse field Bex  4.64 T, the maximum
temperature rise Tmax monotonically decreases and approaches a steadyT (t) behaviour with an increasing number of times of pulse applications.
On the other hand, for Bex = 5.42 T, Tmax keeps on increasing after thethird pulse application in a specific growth sector region and a clear
anisotropic T (t) behaviour has been found. The magnetic fluxes
preferentially enter and leave the bulk superconductor through this sector
region. The T (t) behaviours are closely correlated with the distribution of
the trapped field BT P and can be understood on the basis of the heatgeneration caused by the flux motion.

1. Introduction
Various kinds of practical applications using the meltprocessed REBaCuO superconductors (RE: rare earth ions)have been proposed and developed, e.g., magnetic bearings,
levitation systems, high-strength bulk magnets and so on [1].
For applicational purpose, the pulse-field magnetizing (PFM)
is a useful method as well as the static field cooled magnetizing(FCM). An improved PFM based on a series of iterative
magnetizing operations with sequentially reduced pulse field
amplitudes, called the IMRA method, has been proposed [2]and the YBaCuO bulk was reported to trap almost an equalamount of the magnetic flux as that of the FCM in liquid
nitrogen [3]. However, at lower temperatures, the maximum
trapped magnetic flux by PFM is generally lower than that by
FCM even using the IMRA method [4]. The cause has been
considered to be the temperature rise due to the dynamical
motion of the magnetic fluxes, resulting in the decrease of thepinning force Fp . Yanagi et al measured the temperature riseT on the surface of the SmBaCuO bulk after applying thepulse fields [5]. T increased with increasing amplitude of
the pulse field, which was as high as 15 K for the pulse field
0953-2048/04/010051+07$30.00

Bex = 4.7 T at 42 K. However, there have not been systematic
experimental studies on the temperature rise and the trappedfield.
We investigated the time evolution and spatial distributionof the temperature rise T (t) on the surface of the cryocooled YBaCuO bulk superconductor after applying the pulse
fields by the IMRA technique [6]. We obtained the following
conclusions: (i) the maximum temperature rise Tmax after
applying the pulse field increases with increasing the pulse
field strength and a larger temperature rise occurs just before
the magnetic flux is trapped in the vicinity of the centre of thebulk. (ii) The trapped field BT P by PFM cannot exceed
the trapped field BT FC by FCM operated at the maximum
temperature Tmax through the PFM operation. (iii) T (t)depends on the position on the bulk surface; the magnetic
fluxes move more easily through the growth sector regions(GSRs) than along the growth sector boundaries (GSBs). (iv)
T (t) firstly occurs at the circumference region of the bulk
disk and then heat propagates to the centre. These results could
well explain the distribution of BT P . The in situ temperaturemonitor on the bulk surface offered valuable information about
the magnetizing mechanism in the bulk superconductors.

© 2004 IOP Publishing Ltd Printed in the UK51

H Fujishiro et al

However, it is difficult to analyse the mechanism of
the flux trapping for a series of iterative magnetizing
operations such as the IMRA method because the magnitude
and the distribution of BT P is different at the stages before theapplication of each pulse field. It is desirable to investigate
BT P and T (t) under a simpler PFM operation. One possible
scheme is to use a single magnetic pulse application withvarious amplitudes, where we can obtain the results under an
identical initial condition of no already trapped flux. Anotheris to use the iterative pulse applications with fixed amplitude,
where we can obtain the effect of the pure superposition ofpulse fields. The experimental results for both cases may bevaluable to analyse the PFM mechanism.
In this paper, we investigate the time and position
dependence of the temperature rise T (t) after PFM for the
cryo-cooled SmBaCuO bulk superconductor. The five pulse
fields (nos 1–5) with the same amplitude (Bex = 2.32–5.42 T)
are applied and the relation between T (t) and BT P isinvestigated.

2. Experimental details
The SmBaCuO bulk superconductor of disk shape used inthis study was a highly c-axis oriented crystal fabricated by
Dowa Mining Co, Ltd, which consisted of four growth sectors(GSR1–GSR4). The crystal was composed of SmBa2 Cu3 Oy(Sm123) and Sm2 BaCuO5 (Sm211) with the molar ratioof Sm123:Sm211 = 1.0:0.3, 15.0wt% Ag2 O powder and
0.5wt% Pt powder. The detailed procedure of the melt growthhas been described elsewhere [7]. The initial size of the bulk
disk is 46 mm in diameter and 15 mm in thickness. The bulk
was uniformly impregnated by epoxy resin in vacuum. The
epoxy resin layer on both upper and lower sides of the bulk
was removed in order to measure the precise temperature on
the bulk surface and to reduce the thermal contact resistance
between the bulk and the cold stage. Figure 1 shows the
setup of the bulk to a Gifford McMahon (GM) cycle helium
refrigerator (AISIN, GR103). The bulk was tightly stuckon the sapphire plate on the cold stage by the insertionof an indium foil. The initial temperature Ts of the bulk
surface was 38 K. The temperature T0 at the centre of
the bulk (P0) and the temperatures, T1–T4, at P1–P4 in thefour GSRs were measured using a Teflon-coated chromelconstantan thermocouples (76 µm in diameter) adhered to thebulk surface using GE7031 varnish. P1, P2, P3 and P4 weresituated on the centre lines of each GSR with a 9 mm distancefrom P0. Each temperature was measured about seven times
per second just after applying the pulse field.
The bulk crystal was magnetized using a pulse coil dippedin liquid N2. The rise time of the pulse field Bex was about∼10 ms. The strength of Bex was calculated from the current
flowing through the coil, which ranged from 2.32 T to 5.42 T.
Five magnetic pulses with the same amplitude were applied
sequentially and T (t) and BT P were measured at eachstage. The Hall sensor (F W Bell, model BHT 921) was
adhered to the position PH with a 2.5 mm distance from P0
as shown in figure 1, which monitored the trapped magnetic
field BT P just on the bulk. The two-dimensional distributionof the trapped magnetic field BT 3 mm was monitored at eachstage using an axial-type Hall sensor, which scanned 3 mm
52

Growth sector region1 P1(T1)(GSR1)
P0(T0)

Epoxy resin
GSR2

GSR4

PH
P2(T2)

P4(T4)

Growth sector
boundary(GSB)

GSB
P3(T3)
GSR3

bulk

Pulse
Magnetizing
coil

Sapphire plate
In foil
Cold stage ofrefrigerator
Vacuum shroud

Figure 1. The experimental setup of the SmBaCuO bulk
superconductor for the temperature measurements. The setting of
the bulk in the Gifford McMahon (GM) cycle helium refrigerator isshown. The positions P0–P4 for the temperature measurement of
T0–T4 are indicated. The trapped fields BT 3 mm 0–BT 3 mm 4 are alsomeasured at 3 mm above the P0–P4. The trapped field BT P ismeasured using a Hall sensor fixed at the position PH.

above the bulk surface stepwise with a pitch 1.2 mm. The
BT 3 mm values at P0–P4 (BT 3 mm 0–BT 3mm 4) were extracted
from the distribution. The trapped field BT FC by FCM
was also measured at several temperatures using the cryocooled superconducting magnet. During FCM, the static
magnetic field of 5 T was decreased down to 0 T in 18 min(0.278 T min−1).

3. Results and discussion
Figures 2(a)–(e) summarize the results of the first (no 1) pulse
application. Figures 2(a)–(c) show the time evolutions of thetemperatures at the five positions after applying the pulse of
Bex = 2.32 T, 3.01 T and 3.87 T, respectively. We define therise time t(60%) to reach 60% of the maximum temperaturerise Tmax , which is presented in the insets of each figure.
Figure 2(d) shows the BT 3 mm after each pulse for respective
positions.
In figure 2(a) for Bex = 2.32 T, the maximum temperaturerise Tmax is only about 4 K and almost independent of themeasuring points. The t(60%) values of T1–T4 are about 2 s
and also nearly independent of the measuring positions. Thet(60%) value of T0, however, is almost 4 s and about two
times as long as those of T1–T4. The rise time of T(t) at ameasuring point is determined by the thermal diffusivity α(T )and the distance from the heat source [8]. The T(t) results
indicate that the heat is generated in the peripheral region of
the bulk and diffuses towards the central region. In figure 2(d),the trapped field BT 3 mm monitored 3 mm above each point isbelow 0.1 T for Bex = 2.32 T at all the measuring points, whichsuggests that the intrusion of the magnetic flux is inhibited by
the surface barriers. For Bex = 2.32 T, the heat generation
actually occurs just at the surface on the circumference of thebulk disk.

Temperature rise in an SmBaCuO after applying iterative pulse fields

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max

(K)

P0(T0)

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2
10

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0

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3
Bex(T)

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6

0

Figure 2. The time evolution of the temperatures T0(t)–T4(t) after applying the first pulse field of (a) Bex = 2.32 T, (b) 3.01 T and(c) 3.87 T, respectively. The circle in (b) indicates the shoulder-like anomaly in T3(t). The rise time t(60%) to reach 60% of the maximum
temperature Tmax is presented in the insets of each figure. (d) The BT 3 mm values at P0–P4 after each pulse. (e) T 0max and BT P as afunction of Bex after the first pulse.

In figure 2(b) for Bex = 3.01 T, T2(t) and T3(t) rise up
first with t(60%)  1 s, followed by T1(t) and T4(t) (t(60%) 2 s) and T0(t) rises up latest with t(60%)  3 s. Tmax is larger
for T2 and T3 (Tmax  12 K) than for other points (Tmax 
9 K). In figure 2(d), BT 3 mm for Bex = 3.01 T is larger for points
P2 (BT 3 mm 2  0.70 T) and P3 (BT 3 mm 3  0.75 T), while
BT 3 mm 1 and BT 3 mm 4 are quite small. It is to be noted thatBT 3 mm 0 (0.4 T) is pretty large. The behaviour of BT 3 mmsuggests that the surface barrier against the flux penetration

has been destroyed in GSR2 and GSR3 for Bex = 3.01 T,
while the barriers are kept intact in GSR1 and GSR4. The flux
penetrates into the bulk along the paths such as circumference
C–P2–P0 and C–P3–P0. The rapid and large rise of T2 and T3means that the powerful heat source is located near P2 and P3.
Especially, the characteristic T3(t) behaviour with a sharp peak
implies the heat generation in the very vicinity of P3. Since theflux motion necessarily generates heat inside the bulk throughboth the pinning power loss Pp and the viscous flow loss Pv ,53

H Fujishiro et al
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0

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Bex=4.64T
2
Position

3

4

Figure 3. The time evolution of (a) T0, (b) T1 and (c) T3 after applying five repeated pulses of 4.64 T. The ellipse in (c) indicates theshoulder-like anomalies. The insets of each figure present the pulse number dependences of t(60%) and BT 3 mm at respective positions.
(d) The position and pulse number dependences of BT 3 mm for Bex = 4.64 T pulses.

such a rapid rise of T2 and T3 suggests the existence of theeasy path for the flux motion along C–P2–P0 and C–P3–P0.
The formation of the easy paths may originate from relatively
weaker pinning force Fp of GSR2 and GSR3. Even in the caseof the flux penetration, T0(t) at the centre of the bulk rises up
latest because the main heat generation still takes place in the
peripheral region where the flux moves faster.
In figure 2(c) for Bex = 3.87 T, the position dependencesof Tmax and t(60%) become smaller. The t(60%) values
for T1–T4 are about 1 s and that of T0 is about 2 s. The
time profiles of T1–T4 suggest that the surface barriers against
the flux penetration have been destroyed in all the GSRs for
Bex = 3.87 T. The rapid rises of T1–T4 mean that theflux penetrates pretty uniformly through the surfaces of thecircumference of the four GSRs and heat is mainly generated in
the peripheral regions inside the bulk. In figure 2(d), BT 3 mm 1
and BT 3 mm 4 for Bex = 3.87 T are larger than BT 3 mm 2 andBT 3 mm 3, contrary to the case of Bex = 3.01 T. Especially
BT 3 mm 3 values are almost the same for Bex = 3.87 T and
Bex = 3.01 T. This fact indicates that a large amount of flux,
which has penetrated during the increasing cycle of the pulse
field, has also gone out of the bulk during the decreasing cycle
along P0–P2–C and P0–P3–C easy paths.
Figure 2(e) presents T 0max and the trapped magnetic
field BT P monitored by the Hall sensor fixed at PH as a functionof Bex (note that P0 is the nearest to PH). T 0max increases
54with increasing Bex almost linearly. BT P suddenly increases
for Bex = 3.87 T, takes a maximum for Bex = 4.64 T, anddecreases with further increase of Bex . The origin of the BT P
decrease will be discussed later.
Figures 3(a)–(c) show the time evolution of T0, T1 and T3
after applying five repeated pulses of 4.64 T, respectively. The
insets of each figure present the pulse number dependencesof t(60%) and BT 3 mm at respective positions. Figure 3(d)summarizes the position and pulse number dependences of
BT 3 mm for Bex = 4.64 T pulses. In figures 3(a)–(c), T are
the largest for the first (no 1) pulse and decrease with increasing
pulse number, approaching a fixed ultimate behaviour for thenos 4 and 5 pulses. This behaviour is quite reasonable because
the largest amount of the flux penetrates into the virgin statebulk during the no 1 pulse and the further flux penetration
for the no 2 and succeeding nos 3–5 pulses should be limited
because of the presence of the already trapped flux. Thenthe flux overall velocity is reduced, resulting in the reductionof the heat generation due to the flux motion. If BT P tends
to saturate, the reduction of T (t) is to equally saturate. A
clear position dependence of T(t) and the trapped field mustnot be overlooked. In the inset of figure 3(b), a saturation
behaviour of BT 3 mm 1 can be confirmed for the no 2 andfollowing pulses. The rise time t(60%) of T1 is ∼1.2 s forthe no 1 pulse, ∼1.6 s for no 2 and saturates at ∼2.4 s for theTemperature rise in an SmBaCuO after applying iterative pulse fields

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4

Figure 4. The time evolution of (a) T0(t), (b) T1(t) and (c) T3(t) after applying five repeated pulses of 5.42 T. The circle in (c) indicates theshoulder-like anomalies. The insets of each figure present the pulse number dependence of t(60%) and BT 3 mm at respective positions.
(d) The position and pulse number dependences BT 3 mm for Bex = 5.42 T pulses.

nos 3–5 pulses. These results indicate that the heat generation
occurs mostly at peripheral region in the C–P1–P0 path after
the no 3 and following pulses because the flux motion is small
in the vicinity of P1 owing to the saturated BT 3 mm 1. In
contrast, the BT 3 mm 3 values in the inset of figure 3(c) are
relatively small and continue to increase gradually for the nos
3–5 pulses without showing the saturation behaviour. It is
to be noted that BT 3 mm 3 (0.65 T) after the no 1 pulse of
Bex = 4.64 T is even smaller than the corresponding BT 3 mm
values for Bex = 3.01 T and 3.87 T. The t(60%) values of
T3 are almost independent of the pulse number and remains
at nearly a constant value (∼1.2 s). The small and constantvalues of t(60%) of T3 suggest that the heat generation always
takes place not only in the peripheral region but also in thevicinity of P3. This fact indicates that the flux motion along
the C–P3–P0 path is also intense even for the nos 2–5 pulses.
The unsaturating BT 3 mm 3 may be correlated with the intense
flux motion around P3 for every sequential pulse. In the inset
of figure 3(a), t(60%) of T0 measured at the centre of the bulk
is about ∼2.3 s for the no 1 pulse and then sharply increases
to about 4 s for the no 2 and succeeding pulses. This result
suggests that the flux motion near the central region occurs
only for the no 1 pulse. In figure 3(b), T1(t) shows a smoothvariation on time for each pulse, while the characteristic peakagain appears in T3(t) for the nos 2–4 pulses, supporting theviolent flux motion along the C–P3–P0 path. In figure 3(d),we can clearly see that the trapped field BT 3 mm is the largest
for the GSR1 and the smallest for the GSR3. This is consistent
with the hard C–P1–P0 path and the easy C–P3–P0 path for theflux motion. The GSR2 provides the second easy path and the
GSR4 provides the second hard path for the flux motion.
For higher pulse fields, the behaviour of T (t) andBT 3 mm remarkably changes. Figures 4(a)–(c) show T0(t),T1(t) and T3(t) after applying the five sequential pulses of
5.42 T. The insets of each figure present the pulse number
dependence of t(60%) and BT 3 mm at respective positions.
Figure 4(d) summarizes the position and pulse number
dependences BT 3 mm for Bex = 5.42 T pulses. In these figures,
somewhat anomalous time evolutions of T(t) and pulse number
dependences of t(60%) and BT 3 mm are observable at positions
P0 and P3. In figure 4(a), BT 3 mm 0 is the largest (∼1.3 T) for
the no 2 pulse and decreases for the following pulses. Thisis a clear sign that Bex = 5.42 T is too large to efficiently
magnetize the present Sm-based bulk. The temperature risedue to rampant flux motion weakens Fp and hinders the fieldtrapping at this high level of Bex . In the inset, t(60%) of T0
also takes a maximum for the no 2 pulse and decreases forthe following pulses. The short t(60%) of T0 suggests that theflux motion in the neighbourhood of the centre is enhanced forthe nos 3–5 pulses, possibly reflecting the escape of the flux
55

H Fujishiro et al

4Ts~38K

5
B

BTP (T)

5.42T
3

No5

PH

No5

4.64T
No5

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3.87T

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No1
No1
No1

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0

No5

40

3.10T
No1

50

60
T0

max

70
(K)

80

90

Figure 5. The summary of the trapped field BT P versus the
maximum temperature T 0max . The measured trapped field BT FC by
FCM corresponding to T 0max is also presented.

from this region of the bulk. The similar anomalous behaviour
can be seen for BT 3 mm 3 and t(60%) in the inset of figure 3(c).
On the other hand, T1(t) shows a smooth time evolution anda smooth pulse number dependence. BT 3 mm 1 and t(60%) of
T1 in figure 3(b) also exhibit the normal behaviour similar tothe case of Bex = 4.64 T pulse; BT 3 mm 1 saturates after the no
2 pulse and t(60%) gradually increases, showing a tendency
for saturation. In figure 4(d), we note that the BT 3 mm values
are pretty smaller at every measuring point in comparison to
Bex = 4.64 T pulses. We also note that the pulse number
dependences of BT 3 mm 0 and BT 3 mm 3 are much similar anddecrease with the pulse number. This result strongly supportsthat the magnetic fluxes leave the central region mainly throughthe easy P0–P3–C path in GSR3.
Figure 5 shows the summary of the trapped field BT P at PH
versus the maximum temperature T 0max at P0. The measured
trapped field BT FC by FCM corresponding to the temperature
T 0max is also presented in the figure. The temperature rise
T 0FC was monitored during the FCM operation, which
was 4.2 K at T 0max = 54 K and decreased with increasing
temperature. For Bex  4.64 T, the data set of (BT P , T 0max )is situated below the BT FC –T 0max line for the no 1 pulse.
For the succeeding pulses, T 0max monotonically decreases
and saturates. For Bex = 5.42 T, on the other hand, T 0max
touches the BT FC –T 0max line owing to the large temperaturerise (T 0max  70 K) for the no 1 pulse and BT P decreases
following the BT FC –T 0max line. As a result, BT P for the no 1
pulse of 5.42 T is smaller than that for 4.64 T. For the no 2 pulseof 5.42 T, BT P increases to the observed highest value of 3.22 T
also nearly following this line because of the decreased T 0max .
These analyses demonstrate that the flux trapping ability by
the PFM technique can be systematically explained as limitedby the BT FC –T 0max line. The slight decrease of BT P for thenos 4 and 5 pulses of 5.42 T may possibly be caused by the flux
leaking out through the easy path in the GSR3 as mentioned
above.
Finally, we comment on the possibility of the BT P
enhancement by the PFM method from the viewpoint of
the temperature rise. First of all, starting with the initial
temperature Ts = 38 K, the pulse fields smaller than ∼2.5 T are
56

useless because of the presence of the surface barrier against
the flux intrusion. The main strategy to realize the largest BT P
is how to approach BT FC –T 0max line without touching it. In
figure 5, a series of data points for the nos 1 and 5 pulses
are connected by the dotted and solid lines, respectively. The
region left side of the solid line corresponds to the inevitabletemperature rise region, which we cannot utilize by the present
PFM apparatus. The hatched region surrounded by the dotted,
BT FC –T 0max , and solid lines is the stage, on which the actual
PFM process takes place. As for the no 1 pulse, Bex = 5.42 T
is too large because of too large T . Bex = 4.64 T for the no 1
pulse seems to be nearly the best choice and succeeding pulses
with slightly larger Bex may enhance the BT P . But it should
be noted that the no 2 pulse of Bex = 5.42 T have realized
almost the possible extreme value of BT P . Although final
pulse smaller than Bex  4.64 T may be effective to locallysupplement the flux flowing out through the weaker pinning
path (e.g., P0–P3–C), there seems to be only a little room for
the BT P enhancement with Ts = 38 K.
One possible scheme to overcome this difficulty is tomake Ts lower. The maximum temperature of the bulk is
expected to decrease with decreasing Ts for the same input heatgeneration. However, the effect is supposed to be small, even if
Ts decreases down to 10 K, because the specific heat of the bulk
becomes smaller and the cooling power of the GM refrigerator
becomes lower in the lower temperature region. The second
possible scheme may be to enhance the heat exhaust from thebulk. The dispersion of the Ag metal particles in the bulk
to enhance the thermal conductivity and the specific heat is
a possible method. The metal ring fitting tightly to the bulk
disk may be effective in the efficient heat exhaust to the cold
stage. If the temperature rise can be eliminated by increasing
the heat exhaust against the same heat generation, BT P may
be improved even with the same initial temperature Ts .

4. Summary
The time evolution and position dependence of the temperaturerise T (t) after the five iterative pulse fields (nos 1–5) withthe same amplitude have been measured on the surface of thefour growth sector regions (GSR1–4) and of the centre (P0) of
a cryo-cooled SmBaCuO bulk superconductor. The strengthof the pulse field Bex was varied between 2.32 T and 5.42 T
and the initial temperature was set at Ts = 38 K. Important
experimental results and conclusions obtained in this study aresummarized as follows:(1) For the no 1 pulse application, the trapped magnetic field
BT 3 mm monitored at 3 mm above the bulk surface is verysmall for Bex = 2.32 T and the time constant of thetemperature rise t(60%) is long (2 s) for all GSRs. The
results indicate the presence of the surface barrier against
the flux intrusion and the heat generation due to the pulse
application takes place just on the circumference of thebulk sample.
(2) The surface barrier is destroyed by Bex = 3.01 T pulse
for GSR2 and GSR3 and by Bex = 3.87 T pulse for GSR1
and GSR4. The t(60%) values for the no 1 pulse become
short (∼1 s) in GSRs with destroyed barrier.

Temperature rise in an SmBaCuO after applying iterative pulse fields(3) In GSR3, there is a special path along which the magnetic
flux moves through preferentially. The flux pinning force
Fp may be relatively small in GSR3.
(4) For Bex  4.64 T, the temperature rise T (t) is the largest
for the no 1 pulse and decreases for the succeeding pulses.
The increase of the trapped field BT P is also the largest
for the no 1 pulse, followed by a gradual increase for thenos 2 and 3 pulses. BT P saturates for the nos 4 and 5
pulses. The behaviours of T (t) and BT P are stable forBex  4.64 T. The rise time t(60%) is the shortest for the
no 1 pulse and gradually increases with increasing pulse
number, which suggests that the heat generation due toflux motion becomes limited in the peripheral regions forthe succeeding pulses.
(5) For Bex = 5.42 T, BT P , which is smaller than that for
Bex = 4.64 T after the no 1 pulse, takes a maximum forthe no 2 or 3 pulse and decreases for the nos 4 and 5 pulses.
t(60%) at the centre of the bulk sample also shows an
unstable behaviour for the sequential pulse applications,taking a maximum for the no 2 pulse.
(6) The behaviour of BT P can be understood on the basis of
the diagram of the trapped field BT versus the maximum
temperature T 0max at the centre. For Bex  4.64 T, thedata set (BT P , T 0max ) does not touch the BT FC –T 0max line
for the flux trapping by the field cooled magnetizing. On
the other hand, (BT P , T 0max ) touches the BT FC –T 0max
line for Bex = 5.42 T due to too large T after the
no 1 pulse, resulting in the smaller BT P than that for
Bex = 4.64 T. The touch to the BT FC –T 0max line is theorigin of the unstable behaviours of BTP and t(60%) in theiterative pulse field magnetizing process.

Acknowledgments
The valuable suggestions by professor M Ikebe of Iwate
University are greatly acknowledged. This work is partially
supported by Japan Science and Technology Corporation
under the Joint-research Project for Regional Intensive in the
Iwate Prefecture on Development of practical applications ofmagnetic field technology for use in the region and in everyday
living.

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