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Physics Procedia 67 (2015) 958 – 962

25th International Cryogenic Engineering Conference and the International Cryogenic Materials
Conference in 2014, ICEC 25–ICMC 2014

Trapped magnetic ﬁeld properties of MgB2 bulks doped with Ti
Tomoyuki Naito∗, Takafumi Yoshida, Hiroyuki Fujishiro
Faculty of Engineering, Iwate University, Morioka 020-8551, Japan

Abstract
We have studied the trapped magnetic ﬁeld properties on doped MgB2 bulk with Ti (nominal content was Mg:Ti =0.9:0.1 and
0.8:0.2). The trapped magnetic ﬁeld, BT , was enhanced by approximately 1.3 times from 2.6 T of the pristine bulk to 3.5 T of
the Ti-doped bulks at 15 K. The extrapolated BT (T ) curve reached 5 T below 4.2 K, indicating that 5 T class bulk magnet can berealized using MgB2 with Ti-doping. The critical current density, Jc , under the magnetic ﬁeld was also enhanced by Ti-doping,
therefore, an irreversibility ﬁeld, Birr , of the Ti-doped bulks exceeded 5 T at 20 K. Ti and TiB2 impurities conﬁrmed by the powder X-ray diﬀraction acted as the pinning centers and resulted in the enhancement of the trapped ﬁeld and the critical current density.
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Publishedby Elsevier
B.V.B.V.
This is an open access article under the CC BY-NC-ND license
c 2015
2014The
TheAuthors.
Authors.
Publishedby Elsevier(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of ICEC 25-ICMC 2014.
Peer-review under responsibility of the organizing committee of ICEC 25-ICMC 2014

Keywords: MgB2 ; superconducting bulk magnet; Ti doping

1. Introduction
A large polycrystalline MgB2 superconducting bulk with centimeter size in diameter and in thickness has been
fabricated to realize a quasi-permanent magnet because of weak-link-free grain boundaries (GBs) (Kambara et al.
(2001)). However, since the bulk fabricated by an in-situ method under an ambient pressure has intrinsically a low
packing density of about 50% (Naito et al. (2012)), the high-pressure sintering is one of the routes to enhance thedensity and the critical current density, Jc , i.e., the trapped magnetic ﬁeld, BT . Just after the discovery of MgB2 ,MgB2 bulk sintered under a pressure of 2 GPa oﬀered the possibility of 2 T class bulk magnets at 6 K (Viznichenko
et al. (2003)). We also reported the 2.5 T MgB2 bulk magnet operated at 12.7 K using dense bulk, which was
prepared by a hot isostatic pressing (HIP) method under a pressure of 98 MPa (Sasaki et al. (2013)). Since the GBs
are well known to act as pinning centers in MgB2 , the eﬀect of the grain-size reﬁning on the ﬂux pinning has been
investigated. Recently, Fuchs et al. achieved a trapped ﬁeld of 5.4 T in hot pressed MgB2 using ball-milled powder,which demonstrates that simultaneous grain reﬁning and densiﬁcation are extremely eﬀective to enhance BT (Fuchs
et al. (2013)). Contrary to the mechanical milling, the impurity doping such as Ti was also reported to ﬁne down the
MgB2 grains. Zhao et al. argued that the critical current density was improved over three orders of magnitude from∼103 A/cm2 of the pristine sample to ∼106 A/cm2 of Ti10% doped sample at 20 K (Zhao et al. (2001)). However,∗ Corresponding author. Tel./fax: +81-19-621-6362.

E-mail address: tnaito@iwate-u.ac.jp

1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of ICEC 25-ICMC 2014
doi:10.1016/j.phpro.2015.06.162

Tomoyuki Naito et al. / Physics Procedia 67 (2015) 958 – 962

further Ti-doping suppressed the Jc value and the Jc of the Ti20% sample was about one order of magnitude smaller
than that of the Ti10% sample at 20 K. TiB2 with lamellar structure was created between nanometer-size grains andacted as the ﬂux pinning center, which was observed by TEM (Zhao et al. (2002)). Therefore, ﬁne grains, high packing
density and a novel ﬂux pinning center such as TiB2 are also possible to bring about a higher Jc and higher BT . In thispaper, we report on the fabrication of MgB2 bulks doped with two Ti contents by the HIP method in order to realize
the high density and the introduction of novel pinning center at the same time. We measured their trapped ﬁeld andcritical current density, and ﬁnally discuss the eﬀect of Ti-doping.

2. Experimental
MgB2 bulk samples were fabricated by an in-situ HIP method. The precursors were prepared as follows; raw
powders of Mg (99% in purity), Ti (99% in purity), and amorphous B (99% in purity) were weighted with (1− x):x:2.0
in molar ratio and ground for 1 h. The mixture was pressed into a pellet 40 mm in diameter and 20 mm in thickness
under an uniaxial pressure of about 12 MPa. Subsequently the pellet was further densiﬁcated by a cold isostaticpressing method under a pressure of 196 MPa. The precursor pellet was sealed in a stainless steel container undervacuum by an electron beam welding method. The MgB2 precursor in the container was sintered at 900 ◦ C for 3 h
under Ar-gas pressure of 98 MPa in the HIP furnace and cooled down to room temperature by furnace cooling. The
disc-shaped MgB2 bulk with about 38 mm diameter and 6.9 mm thickness was prepared by the dry machining.
The MgB2 bulk was magnetized by ﬁeld cooling (FC) in a magnetic ﬁeld of 5 T parallel to the thickness direction
using a 10 T cryogen-free superconducting magnet (JMTD-10T100, Japan Superconductor Technology (JASTEC),
Inc.) and then the applied magnetic ﬁeld was decreased to 0 T at a rate of 0.022-0.22 T/min. The bulk was placed on
the cold-stage of a helium refrigerator by inserting a thin indium sheet to obtain good thermal contact. The trapped
ﬁeld was measured by a cryogenic Hall sensor (BHT-921, F.W. Bell Inc.) mounted on the center of the bulk surface.
The temperature of the bulk was monitored by a Cernox thermometer which was adhered beside the Hall sensor.
After the FC magnetization experiments, we evaluated the critical current density Jc , using a small piece cut fromthe bulk. Jc was estimated from the magnetic hysteresis using the extended Bean model (Bean (1962)) following
Jc = 20ΔM/a(1 − a/3b), where ΔM is the width of the hysteresis, and a and b (a < b) are the cross-sectional
dimensions of the sample perpendicular to the applied magnetic ﬁeld. The magnetic hysteresis was measured using acommercial SQUID magnetometer (MPMS-XL, Quantum Design Inc.).

3. Results and discussion
Fig. 1 shows the temperature dependence of the trapped magnetic ﬁeld BT (T )of the pristine and 10% and 20% Ti
doped bulk pieces. The BT value of the pristine bulk is about 2.46 T at the lowest temperature of 17 K, decreases
with increasing temperature, and ﬁnally reaches zero at around 39 K, which originates from the decrease of Jc withincreasing T . The highest BT of 3.45 T at 15 K was achieved for the Ti10% bulk; which was approximately 1.3 times
lager than the extrapolated value of 2.6 T for the pristine bulk at 15 K. The Ti20% bulk oﬀers almost the same BT
proﬁle as that of the Ti10% bulk. The BT (T ) curve extrapolated to lower temperature reaches 5 T around 4.2 K for
Ti10% bulk, which allowed us to realize a 5 T class superconducting bulk magnet. It is noteworthy that the criticaltemperature T c , deﬁned by the temperature at BT =0, is independent of the Ti content. The results suggest that Ti is
not a substitute for the Mg-site, because the site substitution usually deteriorates the superconducting properties.
Fig. 2 shows the magnetic ﬁeld dependence of the critical current density Jc (B)of the pristine and Ti-doped bulks
at 20 K. The Jc (B) curves of both Ti-doped bulks are nearly the same, which is consistent with the BT (T ) behavior.
The Jc value of the pristine bulk decreases monotonically with increasing magnetic ﬁeld, especially a strong decay
is observed under the applied ﬁeld higher than 4 T. Although the Jc in self-ﬁeld is not so enhanced by Ti-doping,but in magnetic ﬁeld it is rather improved. For instance, the Jc =4.1×103 A/cm2 of the pristine sample is increased to
2.0×104 A/cm2 under the applied ﬁeld of 3 T by 10% Ti-doping. An irreversibility ﬁeld Birr , deﬁned by the magnetic
ﬁeld at Jc =10 A/cm2 , obviously exceeds 5 T in the Ti10% sample. Assuming the uniform Jc distribution in the bulk,
the average macroscopic Jc , which creates the trapped ﬁeld, can be estimated by the following analytical equation

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Tomoyuki Naito et al. / Physics Procedia 67 (2015) 958 – 962

5

MgB2-Ti

4pristine
Ti 10%
Ti 20%BT (T)

3.45 T@15 K

3
2.6 T@15 K

2
1
0
0

10

20
T (K)

30

40

Fig. 1: Temperature dependence of the trapped magnetic ﬁeld of the pristine and Ti-doped MgB2 bulks. The dotted lines represent theextrapolation lines.

610

T=20 K

5

10

4

Jc (A/cm2)

10

3

10

MgB2-Ti

210pristine

1

10

Ti10%

0

Ti20%

10

Birr

-1

10

0

1

2

B (T)

3

4

5

Fig. 2: Magnetic ﬁeld dependence of the critical current density at 20 K for the pristine and Ti-doped MgB2 bulks. An irreversibility ﬁeld, Birr , isdeﬁned by a criterion of Jc =10 A/cm2 .

based on the Bio-Savart law,⎛⎞(D/2) + (D/2)2 + (z + t)2(D/2) + (D/2)2 + z2 ⎟⎟⎟⎟
μ0 Jc ⎜⎜⎜⎜
− z lnBT (z) =⎜(z + t) ln
⎟⎠ ,
2 ⎝
z+t
z

(1)

where, D and t are the diameter and thickness of the disc and z is the distance from the center of the top surface of thebulk. The trapped ﬁeld of 2.17 T for the pristine bulk is enhanced to 2.79 T by the Ti10% doping at 20 K, as shownin Fig. 1. Using Eq. (1) and the measured BT values, the average macroscopic Jc is estimated to be 2.9×104 A/cm2
for the pristine and 3.7×104 A/cm2 for the Ti10% bulk, which correspond to the measured Jc under the applied ﬁeldof 2.0 T and 2.6 T, respectively. These results suggest that the experimentally obtained BT values can be roughly
reproduced by the Bio-Savart law.
Fig. 3 shows the powder X-ray diﬀraction (XRD) pattern taken at room temperature for the pristine and Ti-doped
bulks. The XRD peaks of the MgB2 phase are labeled by “MgB2 ”, and an impurity phase is not found for the pristine

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Ti20%

Ti10%

20

30

40

50
60
2θ(deg.)

70

80

2

MgB

2

MgB

2

2

pristine
MgB

MgB
2
MgB
2
MgB

2

MgB

MgB

2

MgB

2

MgB

2

Ti

TiB

2

Ti

TiB

2

Intensity (arb. unit)

Tomoyuki Naito et al. / Physics Procedia 67 (2015) 958 – 962

90Fig. 3: Powder X-ray diﬀraction patterns taken at room temperature for the pristine and Ti-doped MgB2 bulks.

bulk. On the other hand, the impurity phases such as Ti and TiB2 appear for the Ti-doped bulks, especially the peakintensity due to TiB2 increases obviously with the increase in the Ti contents. Ti contained impurities act as the pinning
center, and enhance both the Jc and BT . If the grain of MgB2 is ﬁned down to nanometer size by Ti-doping, as pointed
out by Zhao et al. (Zhao et al. (2001, 2002)), the trapped ﬁeld is expected to be over 5 T at around 12 K as observedin the hot-pressed bulk using ball-milled powder Fuchs et al. (2013). However, the highest BT of 3.45 T at 15 K
in the Ti-doped bulks does not reach the expected value. Although the Ti-doping successfully certainly introduced
the ﬂux pinning centers, the reﬁning eﬀect of the grain size was not be conﬁrmed at the present stage. Microscopic
observation to clarify the grain size is now in progress.
4. Summary
We have studied the eﬀects of Ti-doping on the trapped magnetic ﬁeld and critical current density of the MgB2
bulks. Two bulk samples with the nominal composition of Mg:Ti=0.9:0.1 and 0.8:0.2 have been measured. The
trapped magnetic ﬁeld was considerably increased by Ti-doping; the highest value of 3.45 T achieved for both Tidoped bulks at 15 K was about 1.3 times larger than that of the pristine bulk. The trapped ﬁeld line extrapolated
to lower temperature reached 5 T below 4.2 K. The critical current density was also enhanced by Ti-doping and theirreversibility ﬁeld, which was deﬁned by the criterion of Jc =10 A/cm2 , exceeded 5 T at 20 K. Ti and TiB2 phases
conﬁrmed by the X-ray diﬀraction acted as the ﬂux pinning centers. It is noteworthy to point out that both trapped
ﬁeld and critical current density obtained for the Ti-doped bulk pieces hardly depended on the Ti contents. Further
studies such as microstructure analysis and further Ti-doping must be considered.
Acknowledgments
We would like to acknowledge Dr. H. Teshima of Nippon Steel & Sumitomo Metal Corporation for his assistance in the production of the MgB2 bulks using the HIP furnace. This work was partly supported by JST A-STEP(AS242Z02673L), Hitachi Metals · Materials Science Foundation, and JSPS KAKENHI Grant Number 23560002.
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