Trapped field and temperature rise in MgB2 bulks magnetized by pulsed field
Fujishiro H., Naito T., Sasaki T., Arayashiki T.
Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
The pulsed field magnetization (PFM) was performed for the MgB2 bulk 30 mm
in diameter by a capsule method, and the applied pulsed-field dependence of thetrapped field Bz, temperature rise ΔT and time dependence of the local field BLC(t)were measured on the bulk. Bz=0.71 T was achieved at 16 K after applying themagnetic pulsed field of Bex=1.55 T, where ΔTmax of about 10 K took place. Theresults were compared with those by field-cooled magnetization (FCM), and themagnetic flux intrusion and flux trap for the MgB2 bulk during PFM are discussed.

INTRODUCTION
The superconducting bulk magnet using a REBaCuO (RE: rare earth element or Y) is one of the
exemplary models for practical applications such as a sputtering cathode, magnetic separation and dragdelivery system [1], which produces tesla-order quasi-permanent magnets. However, the large singledomain bulk over 100 mm in diameter was difficult to fabricate. MgB2 bulk has also a promising potential
as quasi-permanent magnet, which has several attractive natures for bulk superconducting magnet, such as
low-cost and light-weight, which are clear contrast with the REBaCuO bulk magnet. The problem ofweak-links at the grain boundaries can be ignored in the MgB2 polycrystalline bulk due to their longcoherence length, ξ [2]. The characters enable us to realize the better and larger polycrystalline MgB2bulk magnets below the superconducting transition temperature Tc=39 K. Several groups have alreadyreported the trapped field on the MgB2 bulk by the field-cooled magnetization (FCM), and attained thetrapped field over 1.5 T at low temperatures [3-5].
A pulsed-field magnetization (PFM) has also been investigated to magnetize the bulk
superconductors because of an inexpensive and mobile experimental set-up with no need for a superconducting magnet. However, for the REBaCuO bulks, the trapped field Bz achievable by PFM is
nonetheless lower than that achievable by FCM because of the large temperature rise caused by thedynamical motion of the magnetic flux. Several approaches have been performed and succeeded inenhancing Bz using the multi-pulse techniques [6, 7]. We have experimentally examined the trapped fieldBz on the surface of cryocooled REBaCuO bulks during PFM for various starting temperatures Ts andapplied fields Bex [8]. To enhance Bz, the reduction in temperature rise and the lowering of Ts are effective.
Considering the obtained experimental results, we proposed a new PFM technique named a modified
multi-pulse technique with stepwise cooling (MMPSC) and successfully realized a highest field trap ofBz=5.20 T on a GdBaCuO bulk with 45 mm in diameter at 30 K [9], which is a record-high value by PFMto date.
In this paper, we applied the PFM technique to the large MgB2 bulk fabricated by a capsule method[10]. The PFM experiments for the MgB2 bulk were performed for the first time and the results werecompared with those by FCM.
EXPERIMENTAL PROCEDURE
MgB2 bulks were fabricated by the in-situ capsule method. The detailed procedure of the sample
preparation was described elsewhere [10]. Raw powders of Mg (99% in purity) and amorphous B (99% inpurity) were weighted with 1.1 : 2.0 in molar ratio, ground and was pressed into pellet 30 mm in diameterand 9 mm in thickness under uni-axial pressure in air. The precursor pellet sealed in the capsule under Ar
gas atmosphere was sintered at 800 ℃ for 6 h in a box furnace and cooled to room temperature.
The MgB2 bulk situated in the stainless steel flange of the capsule was tightly anchored onto thecold stage of a Gifford–McMahon (GM) cycle helium refrigerator. The initial temperature Ts of the bulk
was 14 K. The magnetizing solenoid coil (94 mm I.D., 153 mm O.D., and 67 mm height), which wasdipped in liquid nitrogen, was placed outside the vacuum chamber. A magnetic pulse Bex up to 1.85 Twith a rise time of 0.01 s and a duration of 0.15 s was applied to the bulk by flowing the pulsed current
from a condenser bank. The time evolutions of the local field BLC(t) and the subsequent trapped field Bz at
the center of the bulk surface were monitored by the Hall sensor (BHT 921; F W Bell) using a digital
oscilloscope, which was adhered the center of the bulk surface. Two-dimensional trapped field profiles ofBz(1 mm) were mapped on the bulk with 1 mm above the bulk surface, stepwise with a pitch of 1 mm byscanning the axial-type Hall sensor (BHA 921; F W Bell) using an x–y stage controller. During PFM, thetime dependence of temperature T(t) was also measured at the bulk surface of using the Cernox TM(Lakeshore Cryotronics, Inc.) thermometer.
FCM was also performed for the MgB2 bulk using a cryo-cooled superconducting solenoid magnet(JMTD-10T100, JASTEC. Inc.). Under the magnetic field of 5 T, the bulk was cooled to Ts between 16 K
to and 30 K, and then the applied field was reduced to zero with a speed of -3 mTs-1. The trapped field
BTC at the center of the bulk surface was measured by the Hall sensor (BHT 921; F W Bell) adhered at thecenter of the bulk surface. The trapped field profiles, BTFCM (4 mm) were measured at Ts=28 K on thevacuum sheath surface at a distance of z=4 mm from the bulk surface.
RESULTS AND DISCUSSIONFigure 1(a) shows the trapped field Bz=BTC at the center of the bulk surface, as a function of the strengthof the applied pulsed field Bex. BTC increases for Bex>1 T, takes a maximum at Bex=1.54 T and thendecreases with increasing Bex. The maximum BTC was 0.71 T. The BTC vs Bex curve is a typical one forPFM. Figure 1(b) shows the time dependences of the temperature change on the bulk surface for eachapplied field Bex. The temperature increases abruptly just after the pulse application and sharply decreases
with increasing time. The magnitude of the maximum temperature rise increases with increasing Bex. The
sharp temperature change results from the low specific heat and high thermal conductivity of the MgB2bulk, compared with those for the REBaCuO bulk at operating temperatures.
Figure 2 presents the trapped field profiles Bz on the MgB2 bulk 1 mm above the bulk surface forvarious applied pulsed fields Bex. For Bex=1.04 T as shown in Fig. 2(a), the magnetic flux was trapped at
the right lower region of the bulk, where the critical current density Jc seems to be relatively lower in theregion. For Bex=1.23 T as shown in Fig. 2(b), the magnetic flux was also trapped at the left upper region ofthe bulk. For Bex=1.42 T and 1.54 T as shown in Figs. 2(c) and 2(d), the trapped field profiles are nearly
the conical one. The trapped field profiles look like nearly homogeneous, compared with those for theREBaCuO bulks [8]. Because the MgB2 bulk was fabricated by the in-situ sintering method. On the otherhand, the REBaCuO bulks was fabricated by the melt-growth, in which the four-fold growth sector
boundaries (GSBs) existed and the inhomogeneous Jc distribution was shown in the bulk.
1(a)

(b)

MgB bulk (φ 30mm, t 9mm)

302ss

0.6T (K)CB2

T =14KT(T)0.8

MgB bulk (φ 30mm, t 9mm)
T =14K

0.4

Bex=0.42T
1.23T
1.42T
1.55T
1.84T

25

20
0.2
000.5

1B

ex

1.5(T)

21501020
Time (s)3040Figure 1 (a) The trapped field BTC at the center of the bulk surface as a function of applied pulsed field Bex onthe MgB2 bulk at Ts=14 K. (b) The time dependences of the temperature change on the bulk surface for eachapplied field Bex

Figure 2 The trapped field profiles Bz on the MgB2 bulk 1 mm above the bulk surface at (a) Bex=1.04 T, (b)1.23 T, (c) 1.42 T and (d) 1.54 T.
22(b)(a) Ts=14K, Bex=1.54TBex

BLC

10.5

0Bex

1.5BLC

B (T)

B (T)

1.5

Ts=14K, Bex=1.98T

10.500.05

0.1Time (s)

0.15

0.2

000.050.1Time (s)0.150.2

Figure 3 Time dependences of the local field BLC(t) and the applied pulsed field Bex(t) for (a) Bex=1.54 T and (b)1.98 T. The waving in BLC comes from the smoothing process.

Figure 3 shows the time dependences of the local field BLC(t) after applying the pulsed field ofBex=1.54 T and 1.98 T. Time dependence of pulsed field Bex(t) is also shown. For each Bex, BLC(t) starts toincrease with a slight time delay, takes a maximum at 0.015 s and then decreases to a final value due to theflux flow. The maximum BLC(t) increases with increasing Bex and is smaller than that of Bex(t) because of
the shielding effect in the superconductor.
Figure 4(a) depicts the trapped field BTC by FCM at the center of the bulk surface as a function oftemperature. BTC increases with decreasing temperature, which originates from the enhancement of Jc at
low temperatures. The highest BTC value was 1.50 T at 16 K, which was smaller than that for the reported
MgB2 bulk with 28 mm in diameter fabricated under high pressure [5]. This might come from the fact thatthe density (approximately 1.3–1.4 g/cm3) of the present bulk is lower than that of the bulk in Ref. 5.
Figure 4(b) presents the trapped field profile BTFCM (4 mm) on the bulk magnetized by FCM at
Ts=28 K. The maximum BTFCM (4 mm) was 0.32 T. The trapped field profile is conical, which suggeststhat the Jc distribution in the bulk is nearly homogeneous from the view point of the profile by FCM. For
practical applications, the magnetic field over 0.3 T can be used on the vacuum chamber at present, which
is lower than that of the Nd-Fe-B permanent magnet. The MgB2 bulk magnet over 3 T on the bulk surface
and over 1 T on the vacuum sheath can be realized by the enhancements of Jc, the mass density and theconnectivity between the grains in the polycristalline MgB2 bulk.

2(a) FCM

BTC(4 mm)

1B

T(T)

1.5C(b)

BTC(0 mm)0.50102030

40T (K)

Figure 4 (a) The trapped field BTC by FCM at the center of the bulk surface as a function of temperature Ts.
(b) The trapped field profile BTFCM (4 mm) on the bulk magnetized by FCM at Ts=28 K. The maximum BTFCM (4mm) was 0.32 T.

SUMMARY
The pulsed-field magnetization (PFM) and the field-cooled magnetization (FCM) were performed for theMgB2 bulk 30 mm in diameter fabricated by a capsule method, and the applied pulsed field dependence ofthe trapped field Bz, temperature rise ΔT and time dependence of the local field BLC(t) were measured
during PFM on the MgB2 bulk. Bz=0.71 T was trapped by PFM at 14 K after applying the magnetic pulseof Bex=1.55 T, where ΔTmax of about 10 K took place. BT=1.5 T was trapped by FCM at 16 K at the centerof the bulk surface with a conical trapped field distribution. PFM is a promising technique to magnetize
the MgB2 bulk besides the FCM.
ACKNOWLEDGMENT
This work was partially supported by Japan Science and Technology Agency under a Research for
Promoting Technological Seeds 2009 (02-051) and an Adaptable and Seamless Technology Transfer
Program through target-driven R&D for an Exploratory Research of FS stage (AS232Z02579B).
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