IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015

6800104

Recent Progress of MgB2 Bulk Magnets
Magnetized by Pulsed Field
Hiroyuki Fujishiro, Toru Ujiie, Hidehiko Mochizuki, Takafumi Yoshida, and Tomoyuki Naito

Abstract—A MgB2 superconducting bulk has a promising potential as a quasi-permanent magnet. In this paper, we have
performed pulsed-field magnetization (PFM) at Ts = 14–20 K
for MgB2 superconducting bulks with a higher critical current
density Jc (B) fabricated by hot isostatic pressing. During PFM,
a flux jump frequently took place, and trapped field Bz decreasedin the disk-shaped bulks for a higher magnetic field application.
However, in the cylindrical bulk with identical Jc (B) characteristics, the flux jump was avoided, and a maximum trapped fieldof Bz = 0.81 T was obtained at 14 K, which is a record-hightrapped field in a MgB2 bulk by PFM. The electromagnetic andthermal instability was discussed using conventional relations to
avoid the flux jump and to enhance the trapped field on the MgB2
bulks with higher Jc (B), higher thermal conductivity, and lower
specific heat.
Index Terms—Critical current density, flux jump, MgB2 bulk
magnet, pulsed-field magnetization (PFM), thermal and electromagnetic stability.

I. I NTRODUCTIONSUPERCONDUCTING bulk magnets using REBaCuO(RE: rare earth element or Y) are one of the typical candidates for practical applications such as sputtering cathodes,
magnetic separation and drug delivery systems (DDSs), which
can result in tesla-order quasi-permanent magnets [1]. However, it is difficult to fabricate a large single-domain bulk withgood, homogeneous superconducting properties over 100 mm
in diameter. MgB2 superconducting bulk has also a promisingpotential as a quasi-permanent magnet, which has a number of
attractive properties, such as low-cost, light-weight and homogeneous trapped field distribution, which are in clear contrast
with the REBaCuO bulk magnet. The problem of weak-links
at grain boundaries can be ignored in the MgB2 polycrystalline
bulk due to their long coherence length, ξ [2]. These characteristics enable us to realize better and larger polycrystalline MgB2
bulk magnets below the transition temperature Tc = 39 K,
which are expected to be applicable in magnetically levitatedtrains (MAGLEVs) and wind power generators using liquid
H2 or cryocooler system. Several groups have already reported
the trapped field for MgB2 bulks activated by the field-cooled
Manuscript received August 3, 2014; accepted September 15, 2014. Date of
publication September 19, 2014; date of current version January 13, 2015. This
work was supported in part by the Grant-in-Aid for Scientific Research underGrant 23560002 from the Japanese Ministry of Education, Culture, Sports,
Science, and Technology, and in part by the Japan Science and Technology
Agency under the Adaptable and Seamless Technology Transfer Program
through target-driven R&D for an Exploratory Research of FS stage underGrant AS242Z02673L.
The authors are with the Faculty of Engineering, Iwate University, Morioka
020-8551, Japan (e-mail: fujishiro@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.2014.2359544

magnetization (FCM). The trapped field Bz was reported to be
2.25 T at 15 K on the single MgB2 bulk disk [3] and 3.14 T at
17.4 K in the disk pair [4]. We have attained 3.6 T and 4.6 T
at 14 K in single and paired disks of Ti-doped MgB2 bulks
35 mm in diameter fabricated by a hot isostatic pressing (HIP)
method [5]. Recently, a record-high trapped field of 5.4 T has
been attained at 12 K on the MgB2 bulk 20 mm in diameter,
which was fabricated by hot-pressing of ball-milled Mg and B
powders using fine-grained boron powders [6].
Pulsed-field magnetization (PFM) has also been investigated to magnetize the bulk superconductors. However, for
REBaCuO bulks, the trapped field Bz achievable by PFM is
nonetheless lower than that achievable by FCM because ofa large temperature rise caused by the dynamic motion of
the magnetic flux. Several approaches using the multi-pulse
techniques have been reported to be effective to reduce thetemperature rise and then to enhance Bz [7], [8]. We have also
investigated the trapped field Bz and the temperature rise on thecryocooled REBaCuO bulks during PFM for various conditions[9]. We proposed a new PFM technique named a modified
multi-pulse technique with stepwise cooling (MMPSC) andsuccessfully realized the highest trapped field of Bz = 5.20 T
on a GdBaCuO bulk with 45 mm in diameter at 30 K [10],which is a record-high value magnetized by PFM to date. For
the MgB2 bulk, we have performed the PFM to the bulk fabricated by various methods. Bz = 0.41 T was achieved at 14 K
for the bulk (Bz (FCM) = 1.4 T at 20 K) fabricated by a reactive liquid Mg infiltration (Mg-RLI) method [11]. Bz = 0.71 T
was also obtained at 14 K for the bulk (Bz (FCM) = 1.5 T at
14 K) fabricated by a capsule method [12]. Considering the experimental results and theoretical estimation for the REBaCuO
system, the Bz (PFM) value for the MgB2 bulk is expected to
be enhanced by using the bulk with higher Bz (FCM) valueand higher critical current density Jc . Homogeneity is also very
important from studies on REBaCuO bulks and we should add
this as a factor for expected enhanced Bz (PFM) values.
In this study, we applied the PFM technique to the MgB2
bulks with higher Jc values fabricated by a hot isostatic pressing(HIP) method. For the MgB2 bulk during PFM, flux dynamics
and heat generation are supposed to be in clear contrast withthose in the REBaCuO bulk because of a small specific heat,large thermal conductivity, and narrow temperature margin
against Tc . The electromagnetic and thermal stability in the
MgB2 bulk during PFM was discussed.
II. E XPERIMENTAL P ROCEDURE
Three MgB2 superconducting bulks (hereafter, HIP#38, HIPTi20% and HIP#22) were fabricated by a HIP method [5]. The

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015

TABLE IS PECIFICATION OF THE M EASURED MgB2 B ULKS

specification of the bulks and the maximum of the trapped field
Bzmax by FCM are summarized in Table I. For the HIP#38 and
HIP#22 bulks, a precursor pellet was prepared by cold isostaticpressing at 196 MPa using mixed powders of Mg (99.5% in
purity,  180 μm in grain size) and B (99% in purity, 300 meshin grain size) with a molar ratio of 1.0: 2.0. For the HIP-Ti20%
bulk, Mg, B, and Ti (99% in purity, ≤ 45 μm in grain size) powders were mixed with a molar ratio of Mg:B:Ti = 0.8 : 2.0 : 0.2
and the precursor pellet was fabricated. These pellets weresealed under vacuum in a stainless steel container and were
heated at 900 ◦ C for 3 h under an isostatic pressure of 98 MPa.
For comparison, the CAP#30 bulk fabricated by a capsulemethod, was investigated [12]–[14].
Each MgB2 bulk was mounted in a stainless steel ring with aslightly larger inner diameter using epoxy resin (Stycast 2850).
The outer diameter of the ring was fixed to be 65 mm for eachbulk. The bulk was tightly anchored onto the cold stage of a
Gifford–McMahon (GM) cycle helium refrigerator. The detailsof the PFM procedure were described elsewhere [9], [12]. The
initial temperature Ts of the bulk was set at either 14 or 20 K.
The magnetizing solenoidal copper magnetizing coil (99 mm
I.D., 121 mm O.D., and 50 mm height), which was submersed
in liquid nitrogen, was placed outside the vacuum chamber. A
magnetic pulse Bex up to 2.2 T with a rise time of 0.013 s anda duration of 0.15 s was applied via a pulse current in the coil.
The time evolution of the local field BzC (t) and the subsequent
trapped field Bz at the bulk center were monitored by a Hallsensor (BHT 921; F W Bell), which was adhered to the centerof the bulk surface. Two-dimensional trapped field profiles ofBz (1 mm) were mapped on the bulk at 1 mm above the bulk
surface, scanned stepwise with a pitch of 1 mm by scanning anaxial-type Hall sensor (BHA 921; F W Bell) using an x−y stage
controller. During PFM, the time evolution of the temperature
T (t) was also measured on the side surface of the ring using athermometer (Cernox; Lakeshore).
III. R ESULTS AND D ISCUSSION
Fig. 1(a) and (b) show, respectively, the temperature dependence of the trapped field Bz for FCM (−0.222 T/min from
5 T), and the magnetic field dependence of the critical current
density Jc − B for each bulk. After the FCM experiment, themagnetization curves of several small pieces cut from the bulk
were measured, and the Jc − B characteristics were estimatedusing an extended Bean model [15]. The Bz values for the bulks
fabricated by HIP are higher than that for the CAP#30 bulk [13],which are closely correlated with the Jc − B characteristics
shown in Fig. 1(b). In particular, the Bz value of the HIP-Ti20%
bulk is the highest because of the highest Jc − B characteristics
due to the introduction of the strong pinning centers [5].

Fig. 1. (a) Temperature dependence of trapped field Bz on each bulk magnetized by FCM. (b) Magnetic field dependence of critical current density Jc foreach bulk at Ts = 20 K.

Fig. 2(a) shows the relation between the trapped field Bz by
PFM and the applied pulsed field Bex for the HIP#38 bulk at
Ts = 14 and 20 K. Similar to the previous experimental resultsfor PFM of REBaCuO and MgB2 bulks [9], [12], the Bz value
increases, takes a maximum and then decreases with further
C, at which theincreasing Bex . The critical applied field Bex
magnetic flux starts to trap at the bulk center, increases withdecreasing Ts due to the increase in Jc . However, for the bulk
with higher Jc value, flux jump took place frequently for higher
applied field.
Fig. 2(b) depicts an example of a flux jump; the time evolution of the local field BLC (t) at the center surface of the HIP#38
bulk and applied pulsed field Bex (t) at Bex = 1.97 T and at
Ts = 14 K. BLC (t) started to increase with a slight time delay,
took a maximum at 0.016 s and then suddenly dropped about
40% at t = 0.06 s by the flux jump. Then, BLC (t) approaches tothe final value Bz due to flux flow. In the figure, the estimated
BLC (t) curve was also shown, in which no flux jump appears
to take place during PFM. In Fig. 2(a), open circles and dottedlines show the estimated Bz values without a flux jump. If theflux jump can be avoided, the Bz value over 1 T can be realized
at Ts = 14 K. The flux jump was always observed for HIP#38
and HIP-Ti20% bulks for pulsed fields higher than 1.5 T. The
minimum applied pulsed field, at which the flux jump occurs,
did not depend on Ts .
Fig. 3(a) and (b) present the trapped field profiles Bz (1 mm)of the HIP#38 bulk after applying pulsed field of Bex = 1.42 T
and 1.79 T at 14 K, respectively. For Bex = 1.42 T, the profile

FUJISHIRO et al.: RECENT PROGRESS OF MgB2 BULK MAGNETS MAGNETIZED BY PULSED FIELD

6800104

Fig. 4. Trapped field Bz by PFM for each bulk at Ts = 14 K as a function of
the applied pulsed field Bex .

Fig. 2. (a) Applied pulsed-field dependence of trapped field Bz for the
HIP#38 bulk at Ts = 14 and 20 K. The dashed lines show the estimated Bzvalue if no flux jump takes place during PFM (see text). (b) Time evolution of
C (t) at the center of the HIP#38 bulk surface and pulsed field
the local field BL
Bex (t) at Bex = 1.97 T and at Ts = 14 K.

Fig. 3. Trapped field profiles Bz (1 mm) of the HIP#38 bulk after applying apulsed field of (a) Bex = 1.42 T and (b) 1.79 T at 14 K.

is concave and nearly concentric because of lower Bex . On theother hand, for Bex = 1.79 T as shown in Fig. 3(b), the profile
is asymmetric and the magnetic flux in upper regions might
escape due to flux jumps.
Fig. 4 shows the trapped field Bz for each bulk at Ts = 14 K
as a function of applied pulsed field Bex . In the figure, open
and closed symbols show the Bz values with and without flux
jumps, respectively. The magnetic flux easily penetrated into
the CAP#30 bulk and was trapped at the center of the bulk with
Bz = 0.71 T without flux jumps [12]. For the HIP#38 and HIPTi20% bulks, the magnetic flux starts to penetrate at higher Bex
than that for the CAP#30 bulk and was trapped in a similarmanner. However, the Bz value for the HIP-Ti20% bulk withthe highest Jc does not necessarily show the higher Bz value
because of the flux escaping.

Fig. 5. Maximum temperature Tmax during PFM for each bulk at Ts = 14 K
as a function of the applied pulsed field.

On the other hand, the Bz -Bex relation for the HIP#22 bulk isin clear contrast to those for other HIP bulks; the magnetic fluxC(= 1.4 T), Bz increases withstarts to penetrate at higher Bex
increasing Bex and takes a maximum of 0.81 T without flux
jump at Bex = 1.97 T. Then, Bz suddenly decreased for higher
Bex than 2 T due to the flux jump. Bz = 0.81 T is a record-hightrapped field value by PFM for MgB2 bulk to date. The inset
shows the trapped field profiles Bz (1 mm) of the HIP#22 bulk
at Bex = 1.97 T. It should be noticed that the profile shows aconical profile, which is in clear contrast with other HIP bulks,even though the Jc (B) and Bz (FCM) of the HIP#22 bulk arenearly identical to those of the HIP#38 bulk. The significant
feature of the HIP#22 bulk is a small aspect ratio (d/tB ); thatis, the HIP#22 is cylindrical rather than disk-shaped. The result
is not supported by the numerical simulation for FCM [16],where the Bz value slightly increases with increasing diameter(d) of the bulk, but is almost independent of the thickness (tB ).
Increasing the tB and d values is not necessarily an effective
solution to enhance the Bz value. Since the travelling path ofthe magnetic flux along the radial direction in the HIP#22 bulk
is shorter than that in the HIP#38 bulk, the heat generation issuppressed and flux jumps might be avoided.
Fig. 5 presents the maximum temperature Tmax during
PFM for each bulk at Ts = 14 K, as a function of applied
pulsed field. The temperature rise for the HIP#22 bulk was thesmallest, which supports the above discussion, even though the6800104

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015

TABLE IIP HYSICAL PARAMETERS AND E STIMATED VALUES (lm AND dc ) FOR
MgB2 AND GdBaCuO B ULKS

temperature was measured indirectly on the side surface of thestainless steel ring. The results for the HIP#22 bulk may suggest
an enhanced Bz and an avoidance of flux jumps by PFM andalso by FCM.
We discuss the ease at which flux jumps occur and the magnetic instability during PFM using the conventional equations
for superconducting wires. The minimum propagation zone
(MPZ) lm and the critical thickness dc to avoid the flux jumpfrom stability parameter can be written as follows [17],12λ(Tc − Ts ) 2
(1)
lm =
ρJc2

3γC(Tc − Ts )dc = 2,(2)μ0 Jc2where Tc is the transition temperature and Ts is the operating
temperature. κ, Jc , C, and γ are thermal conductivity, criticalcurrent density, specific heat and mass density at Ts , respectively. ρ is the normal state resistivity just above Tc . Table IIshows these parameters and estimated lm and dc values for the
HIP#38, HIP-Ti20%, and CAP#30 bulks at Ts = 20 K and, forcomparison, those for the GdBaCuO bulk, which is assumed toperform PFM at Ts = 40 K.
The lm values, which indicate the thermal stability of superconductors, are varied for the MgB2 bulks, which arefrom twice to one order of magnitude larger than that of the
GdBaCuO bulk. In this sense, the thermal stability of the MgB2
bulk appears to be better than GdBaCuO bulks. However, fromthe comparison of the specific heat C, the MgB2 bulk is unstable because the transfer energy to the normal state for the MgB2
bulk is one twentieth smaller than that of the GdBaCuO bulk ateach Ts . The dc values, which indicate the magnetic stabilityof superconductors, decreases with increasing Jc values of thebulk and are also smaller than that of the GdBaCuO bulk. In
any case, the thermal and magnetic stability decreases for the
MgB2 bulk, compared with that for the GdBaCuO bulk at eachTs . The discussion using the lm and dc values is based on theheat flow in one dimension rather than the three dimensions in
the case of a bulk, which is therefore only a first approximation.
The detailed analysis remains to understand the thermal andmagnetic stability in the MgB2 bulk during PFM.

IV. C ONCLUSION
We have performed the pulsed-field magnetization (PFM)
for MgB2 superconducting bulks with higher critical current
density Jc (B). During PFM, a flux jump took place frequently
and the trapped field Bz decreased in the disk-shaped bulks forhigher applied magnetic field. However, in the cylindrical bulk
with identical Jc (B) characteristics, the flux jump was avoided.
The maximum trapped field of Bz = 0.81 T was obtained at
14 K, which is a record-high trapped field in an MgB2 bulk
by PFM. The key solution may include in an optional aspectratio (d/tB ) of the bulk to avoid the flux jumps during PFM.
The electromagnetic and thermal stability was discussed using
the parameters of minimum propagation zone (MPZ) lm andcritical thickness dc , suggesting that MgB2 bulks with higher
Jc (B) are unstable, compared with the GdBaCuO bulk.
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