Supercond. Sci. Technol. 13 (2000) 846–849. Printed in the UK
PII: S0953-2048(00)11397-1
Pulse field magnetization of
melt-processed Sm–Ba–Cu–O
H Ikuta†, H Ishihara‡, T Hosokawa‡, Y Yanagi§, Y Itoh§,
M Yoshikawa§, T Oka§ and U Mizutani‡
† Center for Integrated Research in Science and Engineering, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8603, Japan
‡ Department of Crystalline Materials Science, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8603, Japan
§ IMRA Material Research and Development Co., Ltd, Hachiken-cho, Kariya,
Aichi 448-0021, Japan
Received 27 January 2000
Abstract. We studied the dynamical motion of flux lines in melt-processed Sm–Ba–Cu–O
(SmBCO) bulk-superconductors driven by a magnetic pulse at 77 K. The total amount of
trapped magnetic flux (T ) as a function of the magnitude of pulse exhibited a peak; the
decrease of T for large pulses is attributed to the increase in temperature of the sample due
to the resistive force exerted on moving flux lines. Compared to melt-processed Y–Ba–Cu–O,
it is found that the penetration of flux lines into the sample requires a larger pulse, reflecting
the strong pinning effect of SmBCO.
1. Introduction
2. Experimental details
It is well established that melt processing is a promising
technique for the preparation of bulk superconductors that
fit the criterion of practical applications. Yoo et al [1, 2] and
Murakami et al [3] have found that RE–Ba–Cu–O (RE = Nd,
Sm, Eu, Gd) superconductors melt processed in a reduced
oxygen atmosphere have higher critical temperatures than Y–
Ba–Cu–O (YBCO), and that the magnetic field dependence
of critical current density (Jc ) at 77 K exhibits a peak
at a few tesla, which proved the great potentialities of
RE–Ba–Cu–O. In practical applications of melt-processed
superconductors, however, it is often important that the
size of the sample is reasonably large and that the c-axis
is oriented perpendicular to the top surface. We have
recently succeeded in the preparation of c-axis aligned, meltprocessed Sm–Ba–Cu–O (SmBCO) superconductors up to
36 mm in diameter by adding Ag2 O in the starting materials
[4, 5]. The magnetic flux density trapped by these SmBCO
samples well exceeded that of YBCO, demonstrating their
high performance. To utilize this large trapped magnetic field
in practical applications, however, it is of great importance
to develop a compact and efficient means to magnetize the
materials. In our previous works on melt-processed YBCO,
we have shown that pulse field magnetization (PFM) would
be the most adequate method for this purpose [6, 7]. In the
present work, we applied the pulse field technique to SmBCO,
and studied the dynamical motion of vortices driven by the
pulse field.
SmBCO samples used in this study were synthesized from a
commercially purchased pre-mixture of the starting materials
(Dowa Mining Co, Ltd, Tokyo). The mixture consists of
sintered powders of SmBa2 Cu3 Oy (Sm123) and Sm2 BaCuO5
(Sm211) with a molar ratio of Sm123:Sm211 = 3:1, 0.5 wt%
of Pt, and 15 wt% of Ag2 O. The detail of sample preparation
was reported elsewhere [5].
The time-dependent evolution of the magnetic flux
density with the application of a magnetic pulse was
monitored using a pickup-coil technique, for which the
details are reported elsewhere [8]. Briefly, seven concentric
pickup coils were put in between a 2 mm gap of two meltprocessed SmBCO samples, as shown in figure 1. The
diameters of the pickup coil from outside to the concentric
centre were 43, 35.5, 30, 24, 18, 12 and 6 mm, and hereafter
the coils are referred to by E2, E1, five to one, respectively.
Note that coils E1 and E2 were located at the edge and
outside the sample, respectively. The pair, of the meltprocessed SmBCOs, was immersed into liquid nitrogen and
was magnetized by field pulses using a current source. The
rise time of the pulse was approximately 1 ms and the voltage
induced on the pickup coils was measured with a sampling
rate of 1 µs.
The trapped magnetic flux density (BT ) was mapped
using an axial-type Hall sensor (F W Bell, model BHA
921), which was scanned 0.5 mm above the sample surface,
stepwise with a pitch of 2.0 mm. The active area of the device
is 0.5 mm in diameter and is located 0.64 mm away from the
bottom of the device.
0953-2048/00/060846+04$30.00
© 2000 IOP Publishing Ltd
Pulse field magnetization of Sm–Ba–Cu–O
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ΦT (10-4Wb)
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77 K
Sm FC
Sm PFM
Y FC
Y PFM
3
2
1
( (
/LT 1 Figure 1. A schematic diagram of the experimental set-up.
3. Results and discussion
Figure 2 is a plot of the total magnetic flux (T ) as a function
of applied field (µ0 Ha ). For PFM, µ0 Ha is defined by the
peak value of the current pulse multiplied by the coil constant
that was determined by an independent experiment. T was
calculated from the trapped-field distribution mapped by the
Hall sensor and by integrating BT over the region where BT
was positive. The distribution of BT in the pulse experiment
was taken on one sample of the pair, which was pulled apart
from the other sample after the application of the magnetic
pulse. Data of the YBCO sample used in our previous work
[8] is included in the figure. Obviously, there exists an
optimal value of µ0 Ha for PFM, and if the pulse is larger than
that value, T starts to decrease. The decrease in T at high
fields is attributable to the increase in sample temperature,
which is caused by the energy dissipation of moving flux
lines in the presence of a resistive force [6]. From figure 2, it
can be seen that the peak value of T by PFM is equal to the
amount of flux trapped by field cooling (FC) magnetization
in YBCO. On the other hand, SmBCO traps less magnetic
flux by PFM at low µ0 Ha compared to YBCO. For larger
pulses, T of SmBCO exceeds that of YBCO, but starts to
decrease before reaching the FC value, which corresponds to
the full capacity of the sample. To shed more light on this
difference in behaviour, we studied the motion of flux lines
using the pickup-coil technique.
The output signal of each pickup coil (V ) is proportional
to the time derivative of the total magnetic flux that is
encircled by that coil (); V = −d/dt. Therefore, the
magnetic flux density can be evaluated by integrating the
experimental data with respect to time. The evolution of the
magnetic flux density during the PFM process is depicted in
figure 3 for two magnetic pulses. The increasing field branch
is plotted on the left-half of each figure, while the right-half
is for the descending branch. In the pickup-coil experiments,
the external field can be evaluated by calculating the average
of magnetic flux density between coils E1 and E2, and the
peak value of thus determined external field is denoted as
µ0 Hm throughout this paper. The data in figure 3 implies that
only a small number of flux lines can reach the centre of the
sample during the ascending branch, even for µ0 Hm = 5.6 T.
This is in contrast to the results of YBCO, in which vortices
0
1
2
3
µ0Ha (T)
4
Figure 2. Total magnetic flux measured after magnetizing the
sample by FC or PFM.
penetrated into the sample very easily when µ0 Hm exceeded
about 2 T [8].
The relation that gives the pickup signal can be rewritten
as the following:
V = −d/dt = 2π rBv.
(1)
Here, v represents the velocity of flux lines moving across
the pickup coil, r is the radius of the coil and B is the
magnetic flux density. Figure 4 shows the velocity of vortices
at coil 5 for various pulse strengths evaluated according to
(1) and using data similar to those of figure 3. Obviously, v
exhibits a clear peak in the time dependence for relatively
large pulses. We observed a similar peak of v in the
previous study on YBCO, and had shown that this peak is
attributable to the temperature increase of the sample due to
the energy dissipation of moving vortices [8]. The increase
in temperature leads to the decrease in the pinning force (Fp )
and the viscous force (Fv ), which are exerted on vortices, and
results in the increase in v.
The force exerted on flux lines multiplied by their
velocity gives the energy loss. The dissipation of heat may
be neglected, because it is sufficiently slow compared to
the duration time of the field pulse. Therefore, we can
estimate the local temperature change (T ) of the sample by
integrating the loss with respect to time. Here, the viscous
force has to be evaluated according to the following equation
[9]:
B
µ0 Hc2
1+
(2)
Fv =
ρn
2µ0 Hc2
where µ0 denotes the magnetic permeability in vacuum, Hc2
is the upper critical field, φ0 is the flux quantum and ρn
is the resistivity in the normal state. The following values
were taken from the literature and used for the calculations:
µ0 Hc2 = 80.5 T [10] and ρn = 1.9 × 10−6 m [6].
Furthermore, we adopted C = 1.00 × 102 J mol−1 K −1 [11]
for the heat capacity of the sample, which is used to calculate
the temperature change from the integrated loss. All these
values are those reported for YBCO, but we assume that the
difference between YBCO and SmBCO in these quantities is
small.
847
H Ikuta et al
6
4
4
3
3
2
2
1
2
2
1
1
1
0
0
-15 -10 -5 0
5 10 15
6
5
3
0
(b)
5
3
B (T)
B (T)
(a)
0
-15 -10 -5 0
Position (mm)
5 10 15
Position (mm)
Figure 3. Evolution of the magnetic flux density within the sample driven by a magnetic pulse of (a) µ0 Hm = 3.5 T and (b) µ0 Hm = 5.6 T.
The left- (right-) hand side of each figure is for the increasing (decreasing) field branch.
(a)
µ
+P
6P%&2
7
5
FRLO YBCO, coil 5
(µ0Hm=4.4 T)
4
∆T (r, t ) (K)
Y PV
7
7
7
7
7
3
SmBCO (µ0Hm=4.3 T)
coil 5
2
1
coil 4
coil 3
7
0
1
t (ms)
W PV
(b)
5
for various magnetic pulses. The broken vertical line plotted in the
figure corresponds to the time at which the applied pulse peaks.
4
Figure 5(a) plots the temperature change of SmBCO at
coils 3, 4 and 5 during a field pulse of µ0 Hm = 4.3 T. The
broken vertical line plotted in the figure corresponds to the
time at which the applied pulse peaks. The data of T at
coil 5 for YBCO with µ0 Hm = 4.4 T [8] is also included in
the figure for comparison. It can be seen that T increases
rapidly during the increasing field branch. However, when
compared to YBCO, T is smaller for SmBCO. Figure 5(b)
plots the increase in local temperature at coil 5 and t = 2 ms
as a function of µ0 Hm for both SmBCO and YBCO. T
increased rapidly when µ0 Hm exceeded about 2 T in YBCO,
but the temperature change is less significant in SmBCO.
This is because the strong pinning effect of SmBCO prevents
the penetration of flux lines into the sample. Consequently,
SmBCO traps a lesser amount of flux lines compared to
YBCO for small pulses, as shown in figure 2. Flux lines
848
∆T (K)
Figure 4. Time dependence of the velocity of flux lines at coil 5
2
Y-Ba-Cu-O
Sm-Ba-Cu-O
3
2
1
0
1
2
3
4
µ0Hm (T)
5
6
Figure 5. (a) Local temperature change of SmBCO during PFM
with µ0 Hm = 4.3 T. The data of YBCO at coil 5 and
µ0 Hm = 4.4 T [8] is included in the figure for comparison.
(b) Local temperature change at coil 5 and t = 2 ms. The curves
through the data points are guides for the eyes.
start to penetrate into the sample for larger pulses, and T
of SmBCO exceeds that of YBCO. However, at high µ0 Hm ,
Pulse field magnetization of Sm–Ba–Cu–O
the effect of heat generation becomes significant and flux
lines easily flow out of the sample after the duration of the
pulse. As a result, T plotted as a function of pulse strength
possesses a peak and the trapped magnetic flux by PFM is
smaller than that of the full capability of the sample.
4. Conclusion
In conclusion, we studied the dynamical motion of flux lines
in melt-processed SmBCO driven by magnetic pulses using a
pickup-coil technique. It was observed that the penetration of
flux lines into the sample requires a larger pulse for SmBCO
than YBCO, and heating due to the resistive motion of flux
lines is less significant for SmBCO. These behaviours reflect
the strong pinning force of SmBCO.
Acknowledgments
The work in Nagoya University was partially supported by
a Grant-In-Aid from the Ministry of Education, Science,
and Culture, and that in IMRA Material Research and
Development Co., Ltd by a fund of Chubu Bureau of
International Trade and Industry.
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