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

6800404

Trapped Field Enhancement of a Thin, High-Jc MgB2
Bulk Without Flux Jumps Using Pulsed Field
Magnetization With a Split-Type CoilWith a Soft Iron Yoke
H. Fujishiro, K. Takahashi, M. D. Ainslie, Member, IEEE, H. Mochizuki, and T. Naito

Abstract—We have investigated the suppression of flux jumps
and the enhancement of trapped field on a thin, high-Jc MgB2 bulk
(30 mm in diameter and 7 mm in thickness) for the pulsed field magnetization (PFM) using a split-type coil with a soft iron yoke, andcompared the results to those magnetized using the split-type coilwithout a yoke and a solenoid-type coil with a yoke. A maximumtrapped field, Bz , of 0.71 T at 14 K was achieved on the bulk surface
without flux jumps by using the split coil with yoke. On the other
hand, low Bz values with flux jumps were observed for the cases using the split-type coil without a soft iron yoke, and the solenoid-type
coil with a yoke. These results reproduce previous ones for a thick,high-Jc MgB2 bulk (22 mm in diameter and 15 mm in thickness),for which the trapped field was enhanced to a record high value of
Bz = 1.10 T at 13 K by PFM using the split-type coil with a yoke.
Index Terms—MgB2 bulk, pulsed field magnetization, split coil,flux jump, soft iron yoke.

I. INTRODUCTION
GB2 superconducting polycrystalline bulks as trappedfield magnets (TFMs) have been intensively investigated
due to their merits of being rare-earth-free, light-weight, presenting a symmetric trapped field distribution and their longcoherence length [1], [2], which are in clear contrast with REBaCuO bulk magnets (RE: rare earth elements and Y) [3]. MgB2
polycrystalline bulks with high critical current density, Jc , havebeen fabricated by various methods and activated by field-cooledmagnetization (FCM) [4]–[6]. A record-high trapped field ofM

Manuscript received August 31, 2016; accepted December 2, 2016. Date of
publication December 12, 2016; date of current version December 27, 2016.
This work was supported by the Royal Society International Exchanges Scheme
Grant IE131084. The work of H. Fujishiro was supported by the Open Partnership Joint Projects of Japan Society for the Promotion of Science (JSPS) Bilateral Joint Research Projects, and JSPS KAKENHI under Grant 23560002 and
15K04646. The work of M. D. Ainslie was supported by the Royal Academyof Engineering Research Fellowship.
H. Fujishiro, K. Takahashi, H. Mochizuki, and T. Naito are with the Department of Physical Science and Materials Engineering, Faculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan (e-mail: fujishiro@iwateu.ac.jp; madokureo@yahoo.co.jp; hide-m-wish@hotmail.co.jp; tnaito@
iwate-u.ac.jp).
M. D. Ainslie is with the Bulk Superconductivity Group, Departmentof Engineering, University of Cambridge, Cambridge CB2 1PZ, U.K.
(e-mail: mark.ainslie@eng.cam.ac.uk).
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.2016.2637363

Bz = 5.4 T has been attained at 12 K on a single MgB2 bulk
disk 20 mm in diameter [7].
To magnetize superconducting bulks, pulsed-field magnetization (PFM) has been also investigated intensively, usually using
a solenoid-type copper coil. However, the trapped field, Bz ,by PFM for REBaCuO bulks is generally lower than that by
FCM because of a large temperature rise caused by the dynamic
motion of the magnetic flux [8]. Multi-pulse techniques are effective to enhance Bz due to the reduction of this temperaturerise [9]–[11]. The PFM technique was also applied to MgB2
bulks using a solenoid coil [12], [13]. Bz (PFM) = 0.81 T at
14 K had been the highest value in a high-Jc MgB2 bulk pair[14], in which flux jumps took place frequently during PFM
due to a small specific heat, a large thermal conductivity, and anarrow temperature margin between the operating temperature,
Ts , and the superconducting transition temperature, Tc = 39 K.
The use of a split-type coil was effective to enhance the Bz value
for the REBaCuO bulks during PFM both experimentally andnumerically [15]–[17]. We have also analyzed the electromagnetic and thermal instability of a high-Jc MgB2 bulk using conventional parameters, such as the critical thickness, dc , and theminimum propagation zone (MPZ) length, lm [18]. Recently, we
have achieved a trapped field of Bz = 1.1 T on a thick, high-Jc
MgB2 bulk at 13 K without flux jumps using a split-type coilwith a soft iron yoke, which is a record high trapped field by
PFM for bulk MgB2 to date [19].
In this study, to confirm the superiority of the use of split coilwith soft iron yoke for other high-Jc MgB2 bulks of different
geometries, a thin, disk-shaped MgB2 bulk was magnetized, andwe confirmed an enhancement of Bz in addition to the avoidanceof the flux jumps.
II. EXPERIMENTAL PROCEDURE
A Ti-doped MgB2 superconducting disk was fabricated by
a HIP method [6]. The diameter and thickness are 30 mm and
7 mm, respectively, and the relative mass density was as highas 93% of the ideal value. The detailed experimental setup of
PFM using the split- and solenoid-coils is shown in the insets
of Figs. 1(a) and 1(b) and in ref. [19]. For the split-type coil,the MgB2 disk was fastened in a copper sample holder aroundthe bulk periphery and was connected to the cold stage of a1051-8223 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017

Fig. 2. The maximum temperature rise, ΔT m a x , as a function of applied field,
B e x , for the “split coil with yoke,” “split coil without yoke,” and “solenoid coilwith yoke.”

Fig. 1. Applied pulsed field, B e x , dependence of the trapped field, B z , foreach case at (a) T s = 20 K and (b) 14 K using the “split coil with yoke,” “split
coil without yoke,” and “solenoid coil with yoke.”

Gifford–McMahon (GM) 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 CernoxTM thermometer
was connected to the sample holder. The split-type coil (72 mm
inner diameter (I.D.), 124 mm outer diameter (O.D.), 35 mm
height, wire cross-section of 1.3 × 3.0 mm2 and 117 turns foreach coil), which was submerged in liquid nitrogen, was placedoutside the vacuum chamber, in which a pair of soft iron yokes(60 mm in diameter and 65 mm in height) was inserted in thecentral bores of the coil. This coil setup is named “split coil withyoke (split w/ yoke)”. For comparison, the bulk was magnetizedusing the split coil without the soft iron yokes, which is named“split coil without yoke (split w/o yoke)”. The initial temperature, Ts , of the bulk was set to 20 and 14 K, and magnetic pulses,
Bex (t), with a peak up to 2.15 T, a rise time of 18 ms and a
duration of 180 ms were applied via a pulsed current in the coil.
During PFM, the time evolution of the temperature, T(t), thecentral field, Bz (t), and the subsequent trapped field, Bz , which
was defined as the final value of Bz (t) at 60 s, were measured at
the center of the bulk surface. After the removal of the split coil
15 minutes after the pulse application, two-dimensional trappedfield profiles of Bz were mapped at 2 mm above the bulk surface(on the outer surface of the vacuum chamber) by scanning a Hallsensor (BHA 921; F W Bell) using an x–y stage controller.

In the case of using thesolenoid coil, the same MgB2 disk
was mounted in a stainless steel (SS) ring using epoxy resin(Stycast 2850TM ) and was tightly anchored onto the SS holder
with a soft iron yoke (40 mm diameter and 20 mm height).
This was then connected to the cold stage of a refrigerator using indium foil [6], [11]. The soft iron yoke is fixed to themagnetizing fixture, so cannot be removed after the PFM is
carried out. This coil setup is named “solenoid coil with yoke(solenoid w/ yoke)”. The solenoid copper coil (99 mm I.D.,
121 mm O.D., 50 mm H, wire cross-section of 1.3 × 3.0 mm2
and 112 turns), which was submerged in liquid nitrogen, wasplaced outside the vacuum chamber. The initial temperature, Ts ,of the bulk was set to 20 and 14 K. Magnetic pulses, Bex , up
to 2.15 T, with a rise time of 13 ms and duration of 150 ms,
were applied via a pulsed current in the coil. The time evolution of the central field, Bz (t), and the subsequent trappedfield at 60 s, Bz , at the center of the bulk surface were monitored by a Hall sensor (BHA 921; F W Bell). Two-dimensional
trapped field profiles of Bz were mapped in a vacuum chamber
15 minutes after the pulse application, stepwise with a pitch of
1 mm, by scanning the same Hall sensor using an x–y stage
controller. The time evolution of the temperature, T(t), was alsomeasured separately using a thermometer adhered to the bulk
surface.
III. RESULTS AND DISCUSSION
Fig. 1 presents the trapped field, Bz , on the MgB2 disk by
PFM at Ts = 20 K and 14 K for the three magnetizing coilcases, as a function of the applied pulsed field, Bex . For all theused coils, Bz increases, takes a maximum and then decreases
with increasing Bex . At each temperature, the maximum Bz canbe achieved for the case of “split coil with yoke”; Bz = 0.71 T
at 14 K and Bz = 0.64 T at 20 K was achieved on the bulk
surface without flux jumps. On the other hand, flux jumps took
place for the case using the “split coil without yoke” at higher
Bex values than that which achieved the maximum Bz , and forthe case using the “solenoid coil with yoke” around Bex that
achieved the maximum Bz .

FUJISHIRO et al.: TRAPPED FIELD ENHANCEMENT OF A THIN, HIGH-Jc MGB2 BULK WITHOUT FLUX JUMPS USING PFM WITH A SPLIT-TYPE
COIL

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Fig. 3. The total trapped flux, Φ z (z = 2 mm), estimated 2 mm above thebulk surface, as a function of applied field, B e x , for the “split coil with yoke,”“split coil without yoke,” and “solenoid coil with yoke.”

Fig. 5. Time dependence of the local field, B z (t), at the center of the bulk
surface and the applied field, B e x (t), using the “split coil without yoke” at
T s = 14 K for (a) B e x = 0.77 T and (b) B e x = 2.21 T. The insets show thetrapped field profiles 2 mm above the bulk surface.

Fig. 4. Time dependence of the local field, B z (t), at the center of the bulk
surface and the applied field, B e x (t), using the “split coil with yoke” at T s =14 K for (a) B e x = 0.71 T and (b) B e x = 2.17 T. The insets show the trappedfield profiles 2 mm above the bulk surface.

Fig. 2 shows the applied pulse field dependence of the maximum temperature rise, ΔTm ax , using each magnetizing coil at
20 K and 14 K. ΔTm ax increases with increasing Bex at eachtemperature due to the increase in the viscous loss, Qv , of thevortex motion, and ΔTm ax at 14 K is larger and that at 20 due
to the small specific heat and the large pinning loss, Qp . ΔTm ax
is smallest for the “solenoid coil with yoke,” the “split coil withyoke” and the “split coil without yoke,” in this order.

Fig. 3 shows the total trapped flux, Φz (z = 2 mm), estimated
2 mm above the bulk surface, as a function of Bex for eachcase. Φz (z = 2 mm) was calculated by integrating the Bz (z =2 mm) value over the region where it was positive. All the Φz −Bex curves take a maximum around the Bex value at whichBz takes a maximum. A large Φz is necessary for practicalapplications such as rotating machines.
Fig. 4 shows the time dependence of the local field, Bz (t), at
the center of the bulk surface and the applied field, Bex (t), using
the “split coil with yoke” at Ts = 14 K for Bex = 0.71 T and
2.17 T. The trapped field profiles 2 mm above the bulk surface
are shown in each inset. The peak height of Bz (t) is nearly thesame as that of Bex (t), which is a common characteristic for the
split coil [19]. However, it should be noticed that the intrusionof the magnetic flux was delayed in the case with the yoke.
For Bex = 0.71 T, the local field, Bz (t), takes a maximum andthen becomes flat with increasing time and Bz = 0.71 T wasachieved. For Bex = 2.17 T, Bz decreases and a flux jump was
not observed up to Bex = 2.25 T. The trapped field profile isclose to a conical shape.
On the other hand, for the case using the “split coil withoutyoke” at 14 K, as shown in Fig. 5(a) and (b), the time delay inBz (t) is smaller at the peak, compared to that for the case using
the “split coil with yoke” shown in Fig. 4 due to the absence ofthe soft iron yoke. For Bex = 0.77 T, a flux jump can be seen
at t = 60 ms and the Bz value reduced. As a result, the trapped

6800404

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 27, NO. 4, JUNE 2017using the “split coil without yoke” and the “solenoid coil withyoke”. The avoidance of flux jumps was reproduced using the“split coil with yoke,” similarly to previous results for a thickbulk, which is an advantage to magnetize high-Jc MgB2 bulks.
However, the occurrence of the flux jump for the case of the“split coil without yoke” is in contrast with the thick bulk. The
cooling conditions, the shape of the MgB2 bulk and the Jc (B, T)
characteristics also influence the trapped field properties during
PFM. In any case, the use of the soft iron yoke with the split coilis likely to enhance the trapped field and avoid flux jumps in suchbulks.
REFERENCES

Fig. 6. Time dependence of the local field, B z (t), at the center of the bulk
surface and the applied field, B e x (t), using the “solenoid coil with yoke” at
T s = 20 K for (a) B e x = 0.81 T and (b) B e x = 2.22 T. The insets show thetrapped field profiles 2 mm above the bulk surface.

field profile was inhomogeneous due to the flux escaping thesample. The occurrence of the flux jump using the split coilwithout yoke is in clear contrast to the previous results for the
thick bulk [19], which may come from the difference of the
shape of the bulk and/or of the cooling ability.
Fig. 6 shows the time dependence of the local field, Bz (t), at
the center of the bulk surface and the applied field, Bex (t), using
the “solenoid coil with yoke” at Ts = 20 K for Bex = 0.81 T
and 2.22 T. The peak height of Bz (t) is smaller than that ofBex (t) with a very slight time delay around the peak of themagnetic pulse, which is a clear contrast with the case using
the split coil. For Bex = 0.81 T, a broad flux jump can be seen
at t = 70 ms and the Bz value reduces and an inhomogeneous
trapped field profile was observed. For Bex = 2.22 T, shown in
Fig. 6(b), a decrease of Bz with an inhomogeneous Bz profile
was observed.
IV. CONCLUSION
We have performed the pulsed field magnetization (PFM) ofa thin, high-Jc MgB2 disk (30 mm in diameter and 7 mm inthickness) using a split coil with a soft iron yoke, and compared
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coil with a yoke. A maximum trapped field, Bz , of 0.71 T
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