IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017

6801105

Pulsed Field Magnetization of Bridge-Seeded Bulk
YBCO Using Solenoid and Split Coils
Mark D. Ainslie, Member, IEEE, Hiroyuki Fujishiro, Keita Takahashi, Yun-Hua Shi, and David A. Cardwell

Abstract—The multi-seeding process has the potential to enlarge
the sample size of (RE)BCO (where RE = rare earth or Y) singlegrain bulk superconductors with improved fabrication speed, andin previous studies on multi-seeding, a significant improvement wasmade in the alignment of the seeds in such samples using a novel
bridge-seeding technique. In this paper, we report the experimental
measurements of the pulsed field magnetization (PFM) of a 0°–0°bridge-seeded Y-Ba-Cu-O (YBCO) sample. The PFM is carried out
using a solenoid coil, as well as a split coil arrangement with an ironyoke, at temperatures of 65, 40, and 20 K, and the resultant trappedfields and magnetic flux dynamics for these two PFM techniques
are compared. It is shown that such bridge-seeded bulk YBCO canbe fabricated that performs as a bulk magnet with trapped fields
comparable to or better than standard, single-seeded high-Jc samples, with the potential of enlarging the sample size. Furthermore,
the split coil arrangement with an iron yoke is useful to enhance
the trapped field and has a positive effect on the maximum temperature rise in the sample, which increases at lower temperaturesand seriously impacts the achievable trapped field from PFM.
Index Terms—Bulk superconductors, high-temperature superconductors, magnetic flux penetration, magnetization processes,
multi-seeding, pulsed field magnetization, trapped field magnets.

I. INTRODUCTION
HE multi-seeding process has the potential to improve thefabrication speed and enlarge the sample size of (RE)BCO(where RE = rare earth or Y) single-grain, bulk superconductors, which can trap large magnetic fields in excess of 17 T attemperatures below 30 K [1], [2] and up to 3 T at 77 K [3]. In previous studies on multi-seeding, a significant improvement wasmade in the alignment of the seeds in such samples using a novel
bridge-seeding technique [4]–[6]. In our recent work, the pulsed
field magnetization (PFM) of multi-seed samples fabricated us-

T

Manuscript received September 1, 2016; accepted February 8, 2017. Date of
publication February 9, 2017; date of current version March 6, 2017. The workof M. D. Ainslie was supported in part by the Royal Academy of Engineering
Research Fellowship and in part by the Royal Society International Exchanges
Scheme under Grant IE131084. The work of H. Fujishiro was supported in partby the Open Partnership Joint Projects of the Japan Society for the Promotionof Science (JSPS) Bilateral Joint Research Projects, and in part by the JSPS
KAKENHI under Grants 23560002 and 15K04646.
M. D. Ainslie, Y.-H. Shi, and D. A. Cardwell are with the Bulk Superconductivity Group, Department of Engineering, University of Cambridge, Cambridge
CB2 1PZ, U.K. (e-mail: mark.ainslie@eng.cam.ac.uk; ys206@cam.ac.uk;
dc135@cam.ac.uk).
H. Fujishiro and K. Takahashi are with the Department of PhysicalScience and Materials Engineering, Faculty of Science and Engineering,
Iwate University, Morioka 020-8551, Japan (e-mail: fujishiro@iwate-u.ac.jp;t2216017@iwate-u.ac.jp).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TASC.2017.2667621

ing 45°-45° and 0°-0° bridge seeds was reported [7]; however,it was clear in this particular case that the seeds in the 0°-0°
configuration grew differently and sub-optimally, resulting indifferent sizes after polishing and a significantly asymmetrictrapped field and magnetic flux dynamics during the PFM process. In this paper, we report the experimental measurements of anew, well-grown 0°-0° bridge-seeded, bulk Y-Ba-Cu-O (YBCO)
sample. The PFM is carried out using a solenoid coil, as well
as a split coil arrangement with an iron yoke, and the resultant
trapped fields and magnetic flux dynamics for these two PFM
techniques are compared.

II. SAMPLE INFORMATION
For this study, a 0°-0° bridge-seeded, bulk YBCO sample wasfabricated following the same fabrication process described in[7]. Firstly, single-grain Sm-Ba-Cu-O (SmBCO), which forms
a suitable seed for the melt-processing of YBCO due to its
higher melting temperature, was fabricated by top-seeded meltgrowth (TSMG) using a generic seed [8], [9]. The as-grownsingle grain was then cut into a slice of length 10 mm, parallelto the a direction of the crystallographic lattice, which was thenmachined into a bridge-shaped seed and used to multi-seed thebulk YBCO sample based on controlled alignment and relativeorientation of the two legs of the seed (0°-0°) [4]. Special care
was paid to the alignment of the seeds [10] by selecting a better
bridge seed by visual examination, as commonly carried out
when selecting single seeds for normal TSMG.
Precursor powders of 75wt% YBa2 Cu3 O7 (Y-123), 25wt%
Y2 BaCuO5 (Y-211) and 0.5wt% CeO2 (99.9% purity) weremixed thoroughly using a motorized mortar and pestle [6]. The
mixed powder was pressed uniaxially into a pellet and the carefully chosen bridge seed was placed on the top surface of thepellet. The TSMG technique was used to fabricate the sample,
which involves heating the pellet to 1045 °C, holding for 1 h,cooling at 100 °C/h to 1005 °C, and then cooling more slowly
at 0.4 °C/h to 970 °C. Finally, the sample was furnace-cooled
down to room temperature at 200 °C/h. The multi-seed sample, after machining and polishing, is 26.0 mm in diameter and13.2 mm in height.
A photograph of the top surface of the sample is shown in
Fig. 1(a) and the sample mounted in the copper sample holder for
PFM experiments using the split coil arrangement is shown in
Fig. 1(c), including a thin ring of indium to ensure a good thermal
contact. The 2D trapped field distributions presented throughout the paper correspond to the same seed alignment shown in

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6801105IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017

Fig. 2. Comparison of trapped fields at the centre of the top surface of the
0°-0° bridge-seeded, bulk YBCO sample after pulsed field magnetization withthe split coil with an iron yoke inserted (solid lines) and without (dashed lines)
at operating temperatures of 65, 40 and 20 K.

Fig. 1. (a) Photograph of the top surface of the 0°-0° bridge-seeded YBCO
bulk sample, including a thin ring of indium and copper ring around the peripheryof the sample to ensure a good thermal contact. (b) Location of the 0°-0°
bridge seed and growth sector boundaries (GSBs) shown by solid white lines.
(c) Sample mounted in the sample holder for PFM experiments using the split
coil arrangement.

Fig. 1(b). The peak trapped field of the sample, obtained by fieldcooling (FC) magnetization at 77 K in an external magnetic fieldof 1.5 T, was measured using a hand probe: BT (FC) = 0.79 T(top surface), BT (FC) = −0.59 T (bottom surface).

Fig. 3. Comparison of trapped flux in the 0°-0° bridge-seeded, bulk YBCO
sample after pulsed field magnetization with the split coil with an iron yoke
inserted (solid lines) and without (dashed lines) at operating temperatures of 65,
40 and 20 K.

III. PULSED FIELD MAGNETIZATION RESULTS
A. Split Coil Pulsed Field Magnetization
An overview of the split coil PFM experimental setup is described in [11], [12]. The sample was fastened in a copper holder
with thin indium foil (0.2 mm in thickness) and connected tothe cold stage of a Gifford–McMahon cycle, helium refrigerator
in the vacuum chamber. A Hall sensor (BHT 921; F W Bell)was adhered to the center of the bulk surface, and a Cernox
thermometer was connected to the copper holder. The split coilhas an inner diameter of 72 mm, an outer diameter of 124 mm
and a height of 35 mm, and is submerged in liquid nitrogen outside the vacuum chamber. A pair of soft iron, Ni-plated yokes(60 mm in diameter and 65 mm in height) can be inserted in
the central bores of the coil, which was shown to enhance thetrapped field in bulk high-temperature superconductors [11], as
well as bulk MgB2 [12]. The initial temperature of the bulk was
set to Ts = 65, 40 and 20 K, and magnetic pulses, Bex , up toaround 6 T, with a rise time of tr = 18 ms and duration of
approximately td = 200 ms were applied via a pulse current
in the coil.

Fig. 4. Comparison of temperature rise in the 0°-0° bridge-seeded, bulk YBCO
sample during pulsed field magnetization with the split coil with an iron yoke
inserted (solid lines) and without (dashed lines) at operating temperatures of 65,
40 and 20 K.

Fig. 2 shows a comparison of the measured trapped fields
at approximately t = 300 ms after activation by PFM using
the split coil with and without the iron yoke. The split coilwith the iron yoke results in a significantly higher trapped field,
but a slightly higher activation field (the field required to fully
magnetize the sample [13]). It should be noted that these results

AINSLIE et al.: PULSED FIELD MAGNETIZATION OF BRIDGE-SEEDED BULK YBCO USING SOLENOID AND SPLIT COILS

Fig. 5. 2D trapped field profile measured 2 mm above the top surface of thesample after PFM at 40 K using the split coil with an iron yoke, for an appliedfield B e x = 3.68 T. This resulted in partial magnetization of the sample.

Fig. 6. 2D trapped field profile measured 2 mm above the top surface of thesample after PFM at 40 K using the split coil without an iron yoke, for an appliedfield B e x = 3.33 T. This resulted in partial magnetization of the sample.

include the presence of the iron yoke; two-dimensional (2D)trapped field distributions (see Figs. 5–8) were also measured
about t = 20 min after application of the pulse with the split coil(and iron yoke) removed, by scanning a Hall sensor using an x-y
stage controller on the outside surface of the vacuum chamber
at a distance approximately 2 mm above the bulk surface. Using
these 2D trapped field distributions, the total trapped flux in thesample was calculated, as shown in Fig. 3. It is clear that more
flux is trapped when the iron yoke is inserted into the split coilduring the PFM process.
Fig. 4 shows the temperature rise in the sample during the
PFM process with and without the iron yoke as measured by the
Cernox thermometer. For lower temperatures, in particular, thepresence of the iron yoke slightly reduces the temperature rise,
which is important for practical applications because the temperature rise limits the trapped field achievable with PFM [14]. The
much higher temperature rise when Ts = 20 K severely limits the trapped field performance and results in similar trappedfields/flux to that at Ts = 40 K because of this.
Fig. 5 shows the 2D trapped field distribution when the sample is partially magnetized, which helps understand the magnetic
flux dynamics during the PFM process and the Jc distribution

6801105

Fig. 7. 2D trapped field profile measured 2 mm above the top surface of thesample after PFM at 40 K using the split coil with an iron yoke, for an appliedfield B e x = 4.92 T. This resulted in full magnetization of the sample.

Fig. 8. 2D trapped field profile measured 2 mm above the top surface of thesample after PFM at 40 K using the split coil without an iron yoke, for an appliedfield B e x = 3.75 T. This resulted in full magnetization of the sample.

within the sample [13], for the case where the iron yoke is inserted into the split coil (Bex = 3.68 T, Ts = 40 K). Fig. 6
shows the same plot for the case where the iron yoke is notinserted (Bex = 3.33 T, Ts = 40 K). In the partially magnetized case, the flux appears to be trapped close to the growth
sector boundaries (GSBs) seen in Fig. 1 (left), consistent withthe results presented in [13] for a standard, single-seeded YBCO
sample, and the location of this trapped flux is opposite tothat observed in the partially magnetized cases for the 45°-45°
bridge-seeded sample presented in [7].
Figs. 7 and 8 show the 2D trapped field distributions when
the sample is fully magnetized, for the case where the ironyoke is inserted (Bex = 4.92 T, Ts = 40 K) and not inserted(Bex = 3.75 T, Ts = 40 K), respectively. The trapped field
exhibits a traditional conical profile (Bt ≈ 1.5 T (with yoke),
Bt ≈ 0.9 T (without yoke)), although twin peaks in the trappedfield profile due to the 0°-0° bridge seed could be expected if
measured much closer to the surface, depending on the depthof the grain boundary formed between the seeds. This profile
suggests this grain boundary is very shallow. The trapped fields
indicate that the sample performs as a bulk magnet comparably6801105IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017

Fig. 9. Comparison of trapped fields at the centre of the top surface of the
0°-0° bridge-seeded, bulk YBCO sample after pulsed field magnetization witha solenoid coil (dashed lines) and the split coil with an iron yoke (solid lines) at
operating temperatures of 65, 40 and 20 K.

Fig. 10. 2D trapped field profile measured 2 mm above the top surface ofthe sample after PFM at 40 K using the solenoid coil, for an applied fieldB e x = 3.60 T. This resulted in partial magnetization of the sample.

to or better than standard, single-seeded high-Jc samples [11],[15], with the potential of larger sample sizes.
B. Solenoid Coil Pulsed Field Magnetization
An overview of the solenoid coil PFM experimental setup isdescribed in [13]. The bulk sample was mounted tightly usinga sample holder fabricated from 316 stainless steel, of inner diameter slightly larger than the diameter of the samples and outer
diameter 56 mm, to match the dimensions of the available cold
stage of the pulse system. Stycast was used to mount the samples
in the holder, which was then mounted on the cold stage of a
Gifford-McMahon (GM), closed cycle helium refrigerator, anda copper magnetizing solenoid pulse coil, cooled using liquid
nitrogen, was placed outside the vacuum chamber. The directionof external applied field is parallel to the c-axis of the bulk samples. The solenoid magnetizing coil can provide pulsed external
fields up to Bex = 5.4 T, with a rise time of tr = 12 ms andduration of approximately td = 120 ms. In this setup, an ironyoke is embedded in the magnetizing fixture, as shown in [11],which is present for all of the following measurements.
The trapped field close to the centre of the top surface of thesample was measured dynamically during each pulse using a
Hall sensor, and 2D trapped field distributions were measured
inside the vacuum chamber using an x-y stage controller and thesame Hall sensor, approximately 2 mm above the top surface.
Fig. 9 shows a comparison of the measured trapped fields atapproximately t = 300 ms after activation by PFM using thesolenoid coil and the split coil with the iron yoke. The solenoid
coil results in a significantly lower trapped field than the split
coil with the iron yoke, as well as a higher activation field, whichis particularly noticeable when Ts = 40 and 20 K.
Fig. 10 shows the 2D trapped field distribution when thesample is partially magnetized using the solenoid coil (Bex =3.60 T, Ts = 40 K). As found for the split coil in the previous
section, the flux appears to be trapped close to the GSBs andthe location of this trapped flux is opposite to that observedin the partially magnetized cases for the 45°-45° bridge-seeded
sample presented in [7].

Fig. 11. 2D trapped field profile measured 2 mm above the top surface ofthe sample after PFM at 40 K using the solenoid coil, for an applied fieldB e x = 5.20 T. This resulted in full magnetization of the sample.

Fig. 11 shows the 2D trapped field distribution when the sample is fully magnetized using the solenoid coil (Bex = 5.20 T,
Ts = 40 K). Again, the trapped field exhibits a traditional conical profile (Bt ≈ 1.3 T), but the magnitude of the trapped field
is lower than that obtained using the split coil with an iron yoke.
As found for the PFM results for the split coil with an iron yoke,
the trapped field is comparable to or better than that achieved
with the same setup for standard, single-seeded high-Jc samples[11], [15].
IV. CONCLUSION
In this paper, the experimental measurements of the PFM of
a 0°-0° bridge-seeded, bulk Y-Ba-Cu-O (YBCO) sample werecarried out using a solenoid coil, as well as a split coil arrangement with and without an iron yoke. The resultant trapped fields
and magnetic flux dynamics for these two PFM techniques arecompared, with a number of important findings:
1) 0°-0° bridge-seeded bulk YBCO can be fabricated, by
carefully selecting an appropriate bridge seed to control
the alignment of the seeds, that performs as a bulk magnet
with trapped fields and flux comparable to or better than

AINSLIE et al.: PULSED FIELD MAGNETIZATION OF BRIDGE-SEEDED BULK YBCO USING SOLENOID AND SPLIT COILS

standard, single-seeded high-Jc samples [11], [15], withthe potential of enlarging the sample size.
2) When the sample is partially magnetized, magnetic flux
appears to be trapped close to the GSBs and the locationof this trapped flux is opposite to that observed in thepartially magnetized cases for the 45°-45° bridge-seeded
sample presented previously [7].
3) There is a much higher temperature rise for lower operating temperatures, which severely limits the trapped field
performance. When Ts = 20 K, the magnitude of thetrapped field/flux is similar to that observed at Ts = 40 K
because of this.
4) A split coil arrangement with an iron yoke can enhance
the trapped field in bulk superconductors in comparison toa conventional split coil (without an iron yoke) or solenoid
coil, and the use of the iron yoke appears to have a positiveeffect on the maximum temperature rise in the sample,
which increases at lower temperatures.
REFERENCES[1] M. Tomita and M. Murakami, “High-temperature superconductor bulk
magnets that can trap magnetic fields of over 17 tesla at 29 K,” Nature,
vol. 421, pp. 517–520, Jan. 2003.
[2] J. H. Durrell et al., “A trapped field of 17.6 T in melt-processed, bulk
Gd-Ba-Cu-O reinforced with shrink-fit steel,” Supercond. Sci. Technol.,vol. 27, no. 8, Aug. 2014, Art. no. 082001.
[3] S. Nariki, N. Sakai, and M. Murakami, “Melt-processed Gd-Ba-Cu-O
superconductor with trapped field of 3 T at 77 K,” Supercond. Sci. Technol.,vol. 18, no. 2, pp. S126–S130, Feb. 2005.
[4] Y-H. Shi, J. H. Durrell, A. R. Dennis, and D. A. Cardwell, “Bulk YBCO
seeded with 45°-45° bridge-seeds of different lengths,” Supercond. Sci.
Technol., vol. 26, no. 1, Jan. 2013, Art. no. 015012.

6801105[5] Y-H. Shi et al., “Properties of grain boundaries in bulk, melt processed YBa-Cu-O fabricated using bridge-shaped seeds,” Supercond. Sci. Technol.,vol. 25, no. 4, Apr. 2012, Art. no. 045006.
[6] Y-H. Shi et al., “A Comparison of 0°-0° and 45-45 Bridge-Seeded,
YBCO Single Grains,” J. Amer. Ceram. Soc., vol. 96, no. 6, pp. 1757–62,
Jun. 2013.
[7] M. D. Ainslie et al., “Pulsed field magnetization of 0°-0° and 45°-45°
bridge-seeded Y-Ba-Cu-O bulk superconductors,” Supercond. Sci. Technol., vol. 28, no. 12, Dec. 2015, Art. no. 125002.
[8] Y. Shi, N. Hari Babu, and D. A. Cardwell, “Development of a genericseed crystal for the fabrication of large grain (RE)-Ba-Cu-O bulk superconductors,” Supercond. Sci. Technol., vol. 18, no. 4, pp. L13–L16,
Apr. 2005.
[9] N. Hari Babu, Y. Shi, K. Iida, and D. A. Cardwell, “A practical route forthe fabrication of large single-crystal (RE)-Ba-Cu-O superconductors,”
Nature Mater., vol. 4, pp. 476–480, Jun. 2005.
[10] Y. Shi et al., “Factors affecting the growth of multi-seeded superconducting single grains,” Cryst. Growth Design, vol. 16, pp. 5110–5117,
Jul. 2016.
[11] M. D. Ainslie et al., “Enhanced trapped field performance of bulk hightemperature superconductors using split coil, pulsed field magnetization
with an iron yoke,” Supercond. Sci. Technol., vol. 29, no. 7, Jul. 2016,
Art. no. 074003.
[12] H. Fujishiro, H. Mochizuki, M. D. Ainslie, and T. Naito, “Trapped fieldof 1.1 T without flux jumps in an MgB2 bulk during pulsed field magnetization using a split coil with a soft iron yoke,” Supercond. Sci. Technol.,vol. 29, no. 8, Aug. 2016, Art. no. 084001.
[13] M. D. Ainslie et al., “Modelling and comparison of trapped fieldsin (RE)BCO bulk superconductors for activation using pulsed fieldmagnetization,” Supercond. Sci. Technol., vol. 27, no. 6, Jun. 2014,
Art. no. 065008.
[14] M. D. Ainslie and H. Fujishiro, “Modelling of bulk superconductor
magnetization,” Supercond. Sci. Technol., vol. 28, no. 5, May 2015,
Art. no. 053002.
[15] M. D. Ainslie et al., “Pulsed field magnetization of singlegrain bulk YBCO processed from graded precursor powders,”
IEEE Trans. Appl. Supercond., vol. 26, no. 4, Jun. 2016,
Art. no. 6800104.