Superconductor Science and Technology
Supercond. Sci. Technol. 28 (2015) 095009 (8pp)

doi:10.1088/0953-2048/28/9/095009

Ti-doping effects on magnetic properties ofdense MgB2 bulk superconductors
Tomoyuki Naito, Takafumi Yoshida and Hiroyuki Fujishiro
Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
E-mail: tnaito@iwate-u.ac.jp
Received 4 June 2015, revised 22 June 2015
Accepted for publication 25 June 2015
Published 27 July 2015
Abstract

We have studied the effects of Ti doping on the magnetic properties of MgB2
superconducting bulks. The trapped ﬁeld, which was obtained by ﬁeld-cooled magnetization,
BTFC, was about 3.6 T at 13 K at the surface of the single Ti5–20% doped MgB2 bulks, which
was about 1.3 times larger than that of the non-doped bulk. The BTFC of 4.6 T was achieved at
14.1 K in the centre of the doubly stacked Ti-doped MgB2 bulks. The remanent magnetic ﬂux
density, which corresponds to the trapped ﬁeld by zero-ﬁeld cooled magnetization, BTZFC , wascomparable with the absolute value of coercive force with a very small vortex creep rate ofabout 2% over 40 h. These results suggested that the MgB2 bulk was an excellent ‘quasipermanent’ magnet. The critical current density, Jc , under magnetic ﬁeld was also enhanced
by Ti doping; under 3 T at 20 K, the Jc of 4.0 × 103 A cm−2 for the pristine sample wasenhanced to that of 1.6–1.8 × 10 4 A cm−2 for the Ti-doped samples. The irreversibility ﬁeld
exceeded 5 T at 20 K for the Ti-doped samples. The existence of nanometric unreacted B andstrongly Mg-deﬁcient Mg-B particles and TiB2 layers at the periphery of Ti precipitates was
suggested in the Ti-doped bulks by microscopic observation. The improvement of the vortex
pinning properties in Ti-doped MgB2 originated from the creation of the nanometricnonsuperconducting particles and TiB2 layers acting as strong vortex pinning centres.
Keywords: MgB2, superconducting bulk magnet, Ti-doping
trapped ﬁeld was expected at lower temperatures, the quench
phenomenon occurred. A trapped ﬁeld over 2 T was successfully achieved at 6 K using an MgB2 bulk (28 mm in diameter)synthesized under a pressure of 2 GPa [10]. Recently, researchon MgB2 bulk magnets has come into the spotlight again [11–14], because it is rare-earth free and light weight, in addition to
weak-link-free, which makes them suitable for industrial
applications, such as magnetically levitated trains, wind power
generators, and so on [15]. We have reported that the trapped
ﬁeld was improved by densiﬁcation, from 1.5 T for the
MgB2 bulk (the ﬁlling factor, f, was about 50%) fabricated by
the capsule method under an ambient pressure [11], to 2.5 T forthe bulk (f was higher than 90%) fabricated by a hot isostaticpressing (HIP) method under a pressure of 98 MPa [12]; the
HIP method has been used to densify MgB2 in many works[6, 16, 17]. The densiﬁcation by pressure sintering results in an
increase in the number of grain boundaries, which are wellknown to act as vortex pinning centres, and at the same time, anincrease in the cross section of the supercurrent path. As a

1. Introduction
A quasi-permanent magnet using a superconducting bulk canbe realized by vortex pinning, which is one of the speciﬁcfeatures of type II superconductors. It has been intensively
studied using melt-processed, single-grain RE-Ba-Cu-O (RE:rare earth element) bulks [1, 2], because the bulk can trap amagnetic ﬁeld, BT , of several Tesla at 77 K [3] and over 17 T
below 30 K [4, 5]. Although the critical temperature, Tc , of
MgB2 [6] is much lower than that of RE-Ba-Cu-O bulks, it
has a nature of weak-link-free grain boundaries [7, 8].
Therefore, we can study ‘polycrystalline’ MgB2 bulk magnets, contrary to the RE-Ba-Cu-O melt-textured bulk.
In 2003, just after the discovery of MgB2 , Tesla-class
MgB2 bulk magnets were already reported by two groups[9, 10]. An MgB2 bulk with 30 mm diameter, which was
fabricated by a spark plasma sintering method under a pressureof 30 MPa, showed a critical current density of 105 A cm−2 andtrapped a magnetic ﬁeld of 1 T at 27.5 K [9]. Although a higher
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Supercond. Sci. Technol. 28 (2015) 095009

result, the critical current density and the trapped ﬁeld wereenhanced. Recently, a trapped ﬁeld of 5.4 T at 12 K wasachieved in a uniaxially hot pressed MgB2 bulk using ballmilled nanometre-size powder [13], in which the concurrent
densiﬁcation and the grain-size reﬁning were quite effective toenhance the trapped ﬁeld.
Another approach to enhance the trapped ﬁeld is tointroduce the vortex pinning centres artiﬁcially. Chemical
doping, such as Ti [18, 19] and C [20], was reported to
improve the vortex pinning properties in MgB2 . In the
Mg1− x TixB2 (x = 0, 0.1, 0.2) samples fabricated underambient pressure, the x = 0.1 sample showed a criticalcurrent density of 106 A cm−2 in the self-ﬁeld at 20 K,
which was about three orders of magnitude larger than thatof the pristine (x = 0) sample and equivalent to that obtained
for the pressure-sintered undoped samples [10, 21]. However, the Jc of the x = 0.2 sample degraded to approximately
105 A cm−2 under the same conditions. Therefore, x = 0.1
was reported to be the optimal Ti-doping level, which was
also supported by other work [22]. Transmission electron
microscope (TEM) analysis suggested that Ti was notsubstituted for either the Mg- or B-sites but created a thin
TiB2 layer between MgB2 grain boundaries with nanometre-size, and the TiB2 layer prevented the growth of
MgB2 grains. Furthermore, TiB2 kept the connection
between the MgB2 grains, because it has the same crystal
structure as MgB2 and suppressed the creation of MgB4,
which usually deteriorates the connectivity. Therefore, Ti
doping offers high connectivity with ﬁne grains, resulting inhigh Jc values [18, 19]. In this paper, to achieve higher Jc
and BT values, Ti-doped MgB2 bulks with three different Ti
contents are prepared by the HIP method. We discuss
the effect of Ti doping on the vortex pinning properties
from the measurements of Jc and BT and microscopic
observations.

Disc-shaped MgB2 bulks of about 38 mm diameter and 6.9mm thickness were prepared by dry machining.
The MgB2 bulk samples were magnetized by ﬁeld
cooling (FC) in a magnetic ﬁeld of 5–8 T, applied parallelto the thickness direction using a 10 T cryogen-free superconducting magnet (JMTD-10T100, Japan Superconductor
Technology (JASTEC), Inc.), and the applied magnetic ﬁeld
was then decreased to 0 T at a rate of 0.022–0.22 T/min. The
bulks were placed on the cold stage of a Gifford–McMahon
type helium refrigerator by inserting a thin In sheet to obtain
good thermal contact. The trapped ﬁeld was measured by acryogenic Hall sensor (BHT-921, F.W. Bell, Inc.) mounted on
the centre of the bulk surface using GE7031 varnish. The
temperature of the bulk was monitored by a Cernox thermometer, which was adhered beside the Hall sensor on thebulk surface. After the FC magnetization, the superconducting properties, such as the critical temperature, Tc , andthe critical current density, Jc , were evaluated using a small
piece cut from the bulk. The magnetization, M, was measured
as functions of temperature and magnetic ﬁeld using a commercial SQUID magnetometer (MPMS-XL, Quantum
Design, Inc.). The Jc was estimated from the magnetic hysteresis using the extended Bean model [23, 24]:
Jc = 20ΔM a (1 − a 3b ), where ΔM is the width of thehysteresis, and a and b (a < b ) are the cross-sectional
dimensions of the sample perpendicular to the applied magnetic ﬁeld. The microstructure and composition were analyzed by an electron probe micro-analyzer with a ﬁeldemission electron gun (FE-EPMA).

3. Results and discussion
Figure 1(a) shows the temperature dependence of the trappedmagnetic ﬁeld, BT (T ), of the MgB2 bulks with varying Ti
content. The BT value of the pristine bulk is about 2.5 T at thelowest temperature of 16 K, decreases with increasing temperature, and ﬁnally reaches zero at around 39 K. The BT
value is enhanced obviously by the Ti doping; the highest BT
value of the Ti5% bulk is about 3.6 T at 13.4 K, which is 1.3
times larger than the extrapolated value of 2.7 T for thepristine bulk. However, further doping does not provide
higher BT ; the BT (T ) curves of both Ti10 and Ti20% bulks
are almost the same as that of the Ti5% bulk. Figure 1(b)shows the BT value at 20 K as a function of Ti content. BT
increases from 2.2 T of the pristine bulk to a nearly constantvalue of 2.8 T for all the Ti-doped bulks. The temperature atwhich the BT value became negligible was about 39 K for all
the Ti-doped bulks, similar to the pristine bulk. The inset ofﬁgure 1(a) shows the temperature dependence of the reduced
magnetization, m(T)(= M(T)/M(5 K)), of the pristine and Tidoped samples under a magnetic ﬁeld of 4 mT after zero-ﬁeld
cooling (ZFC). All the m(T) curves are collapsed into theuniversal curve, and Tc is determined to be about 39 K for all
the samples. These results suggest that no Mg site substitution
by Ti took place, because this site substitution deteriorates Tc
in general.

2. Experimental procedure
MgB2 bulks were fabricated by an in-situ HIP method[6, 12, 16, 17]. The precursors were prepared as follows. Raw
powders of Mg (99% in purity, ⩽180 μm in grain size,
Kojundo Chemical Laboratory Co., Ltd.), Ti (99% in purity,⩽ 45 μm in grain size, Kojundo Chemical Laboratory Co.,
Ltd.), and amorphous B (99% in purity, ⩽46 μm in grain size,
Furuuchi Chemical Corp.) were weighted with Mg:Ti:B =(1 − x ) :x:2.0 (x = 0, 0.05, 0.1, and 0.2) in molar ratio andground well using an agate mortar and pestle. The mixed
powder was pressed into pellets of 40 mm in diameter and 20
mm in thickness under a uniaxial pressure of about 12 MPa,and subsequently the pellet was further densiﬁed by a cold
isostatic pressing method under the pressure of 196 MPa. The
obtained precursor pellet was sealed in a stainless steel container in a vacuum by electron beam welding. The
MgB2 precursor in the container was sintered at 900° C for
3 h under an Ar-gas pressure of 98 MPa in the HIP furnaceand cooled down to room temperature by furnace cooling.
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Supercond. Sci. Technol. 28 (2015) 095009

Figure 1. (a) Temperature dependence of the trapped magnetic ﬁeld

for the pristine and three Ti-doped MgB2 bulks. Inset shows
temperature dependence of the reduced magnetization at μ 0 H =4 mT after zero-ﬁeld cooling. (b) The trapped ﬁeld at 20 K as afunction of Ti contents.

We also examined the trapped ﬁeld properties of doubly
stacked bulks. Figure 2(a) shows the temperature dependenceof BT at the centre and the top surface of the stacked bulks,
which consist of the lower Ti5% and the upper Ti10% bulkswith a thin indium sheet between them, as depicted schematically in the inset of ﬁgure 2(a). The BT (T ) curve of thesingle Ti10% bulk shown in ﬁgure 1(a) is also depicted forreference. The highest BT values of 4.6 T at 14.1 K and 3.2 T
at 16.3 K are achieved, respectively, for the centre and surface
positions. The difference in the temperature between thepositions comes from the inﬂuence of radiation loss and/or thetemperature drop due to thermal contact resistance between
the bulks. Although the surface BT of the stacked bulks almost
corresponds to that of the single Ti10% bulk, the centre BT
value is about 1.3 times larger than BT at the surface bulk.
These results suggest that the thicker bulk is not so effective
for the enhancement of BT at the surface, which is supported
by our simulation results [25, 26], but a higher magnetic ﬁeld

Figure 2. (a) Temperature dependence of the trapped magnetic

ﬁeld at the centre and top surface positions for the doubly stacked
bulks consisting of Ti5% (lower) and Ti10% (upper) bulks. The
conﬁguration is schematically shown in the inset. (b) Magnetic
hysteresis loop of the stacked bulk at 19.6 K for the centre and at
21.0 K for the surface positions. (c) Time dependence of thetrapped ﬁeld at 20.0 K for the centre and at 21.6 K for the surface
positions.

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Supercond. Sci. Technol. 28 (2015) 095009

can be realized between the bulks and in the hollow ofcylindrical bulk.
Figure 2(b) shows the magnetic hysteresis curves at
T = 19.6 K for the centre and at 21.0 K for the surface of thestacked bulks, respectively, which were taken after ZFC. The
shape of the hysteresis loop of the centre position is nearlysquare, but that of the surface is rather round-shaped, which is
caused by the demagnetization effect. The magnetization
process at the centre position is as follows. The ﬁrst branchfrom the applied magnetic ﬁeld of μ0 H = 0 T to 8 T (o →a )in the ﬁrst quadrant represents the ﬁrst magnetization. The
vortices start to reach the centre of bulk at about μ0 H = 3.8 T,
and then the ﬂux density increases with increasing magnetic
ﬁeld up to 8 T. Subsequently, the magnetic ﬁeld is swept from
8 to –8 T (a → b → c → d ). The remanent magnetic ﬂux
density of 3.6 T at μ0 H = 0 T (point b) is exactly the trappedmagnetic ﬁeld, and then the internal magnetic ﬁeld reaches
zero at μ0 H = −3.9 T (point c), which is the coercive force.
The magnetic ﬁeld begins to penetrate into the bulk centre
again, and the inverse magnetization increases with increasing
inverse magnetic ﬁeld (c → d ). The magnetization curve fromμ0 H = −8 T to 8 T (d → e →f →a ) is in line symmetry withthe d-a line. Finally, as the magnetic ﬁeld is ramped down to
0 T, the magnetization curve traces the initial a → b curve.
The fact that the remanent magnetic ﬂux density is nearlycomparable with the absolute value of the coercive force
demonstrates that the MgB2 bulk magnet is an excellent hardmagnet. Afterwards, we measured the time dependence of thetrapped ﬁeld to monitor the reduction in BT due to vortex
creep at the centre (T = 20.0 K) and at the surface positions(T = 21.6 K), respectively, as shown in ﬁgure 2(c). The initial
BT of 3.54 T (2.47 T) at t = 0 s, which is deﬁned by the end
time of the magnetising sweep, decreases to 3.48 T (2.43 T) at
t = 1.44 × 105 s (40 h) at the centre (surface) position. The
creep for both positions is estimated to be approximately 2%
for 40 h, which is small enough to use the bulk as a quasipermanent magnet.
Figure 3 shows the critical current density, Jc (μ0 H ), as afunction of the applied magnetic ﬁeld, μ0 H , for the pristineand the Ti-doped samples at 10 and 20 K, in which some data
points affected by ﬂux jumps are not plotted. Jc of the pristine
sample decreases strongly with increasing ﬁeld; in particular,a rapid decrease is observed under ﬁelds higher than 4 T at
20 K. The irreversibility ﬁeld, μ0 Hirr , is about 4.4 T for thepristine sample at 20 K, where μ0 Hirr is deﬁned as the magnetic ﬁeld, for which Jc = 10 A cm−2. Ti doping enhances theJc (μ0 H ) values signiﬁcantly in magnetic ﬁelds. The Jc of
4.0 × 103 A cm−2 for the pristine sample is enhanced to
1.6–1.8 × 10 4 A cm−2 for the Ti-doped samples at 20 K undera ﬁeld of 3 T. Furthermore, the irreversibility ﬁeld, μ0 Hirr , for
the Ti-doped samples clearly exceeds 5 T at 20 K. On theother hand, the Jc value in the self-ﬁeld, Jc (0), is not improved
so much by Ti doping. These results are summarized in theinset of ﬁgure 3, in which the normalized Jc value, Jc , x /Jc , x = 0 ,is plotted as a function of Ti content, x, at each indicatedmagnetic ﬁeld. Here, Jc , x andJc , x = 0 are the Jc of the Ti-doped
and the pristine samples, respectively. The magnitude of the

Figure 3. Magnetic-ﬁeld dependence of the critical current density,

Jc (μ 0 H ), at 10 K and 20 K for the pristine and three Ti-doped
MgB2 bulks. The inset shows the normalized Jc value, Jc , x /Jc , x = 0 , ofthe Ti-doped samples as functions of Ti content and magnetic ﬁeld.
Here, Jc , x and Jc , x = 0 are the Jc of the Ti-doped and the pristine
samples, respectively.

Jc enhancement evidently increases with increasing magnetic
ﬁeld, and a slight peak is observed for the Ti content of 10%,except for 4 T, which indicates that 10% is the optimal doping
level. In a previous report [18], Jc (0) at 20 K was increased
from ∼2 × 103 A cm−2 in the non-doped sample to∼1 × 106 A cm−2 in the Ti10% doped sample; however, theJc (μ0 H ) of the latter sample started to decay steeply above 3
T and reached 10 A cm−2 at about 4 T. As a result, thereported μ0 Hirr value is lower than that of the present nondoped bulk. At 10 K, the Jc in the magnetic ﬁeld is alsoenhanced by Ti doping, and the relationship between themagnitude of the Jc increase and the Ti contents is similar tothat at 20 K. These results suggest that the Ti doping in thepresent study offers effective pinning centres in high ﬁelds.
Figure 4(a) shows the magnetic ﬁeld dependence of thepinning force density, Fp (μ0 H ), for the pristine and the Ti10%
doped samples at 10 and 20 K. The magnitude of the Fp value
is clearly enhanced by Ti doping at each temperature, whichmeans that the pinning centres are created by Ti doping. The
peak value, Fpmax , of the Ti10% bulk is about twice that of thepristine bulk at both temperatures, and the magnetic ﬁeld at
the peak of 0.76 T (0.98 T) for the pristine sample is shifted to
1.1 T (1.5 T) for the Ti10% sample at 20 K (10 K). Although
ﬂux jumps wiped out the peak of Fp for both the Ti5% andTi20% bulks, they also showed similar Fp (μ0 H ) behaviour(not shown here), and the highest Fp value is obtained for the
Ti10% bulk. To determine the dominant pinning mechanism,
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Supercond. Sci. Technol. 28 (2015) 095009

magnetic ﬁeld, and μ0 Hn is deﬁned as the magnetic ﬁeld at
which Fp drops to half of its maximum, Fpmax , at magnetic
ﬁelds above the peak magnetic ﬁeld. Furthermore, the peak
position almost independent of the anisotropy, γ, and thepercolation threshold, psh , where psh is deﬁned as the minimum fraction of the superconducting region for a continuous
supercurrent path. The peak ﬁeld h n,peak , was calculated forthe ﬁve typical pinning mechanisms with γ = 4 and
psh = 0.25 [29]: (1) h n,peak = 0.34 for normal surface pinning(p = 1/2, q = 2), (2) h n,peak = 0.47 for normal point pinning(p = 1, q = 2), (3) h n,peak = 0.56 for superconducting volume
pinning (p = 1, q = 1), (4) h n,peak = 0.62 for superconducting
surface pinning (p = 3/2, q = 1), and (5) h n,peak = 0.65 forsuperconducting point pinning (p = 2, q = 1). Figure 4(b)shows the fp (h n ) curve at 20 K for both pristine and Ti10%
bulks. The peak ﬁeld, h n,peak , shifts from 0.41 for the pristine
to 0.49 for the Ti10% bulk. The main pinning source in
MgB2 is well known to be the surface (grain boundaries). The
h n,peak value of 0.41 for the pristine bulk indicates that thegrain boundaries are the dominant pinning centres, although it
is somewhat larger than the calculated value of 0.34. On theother hand, the dominant pinning sources for the Ti10% bulk
should be the point-like normal cores, because the h n,peakvalue of 0.47 is almost the same as 0.49 calculated for thenormal core pinning, and Ti doping does not introduce a weaksuperconducting region in the MgB2 matrix, as discussed in
the inset of ﬁgure 1(a). Consequently, the observed peak shiftstrongly suggests that the pinning mechanism is changed by
Ti doping. Remembering that the scaling analysis for thepinning force density can determine only the single pinningmechanism, we can conclude that the grain boundary pinningand additional point pinning related to Ti doping are effective
for the Ti doped bulk. The details of the pinning centres
introduced by Ti doping are discussed below.
Figure 5(a) shows the x-ray diffraction (XRD) patterns
for the pristine and Ti-doped samples used in this study [30].
The dominant phase is MgB2 , and impurity phases such as
Ti and TiB2 begin to appear with Ti doping even for the Ti5%
sample, and the intensity of their peaks increases withincreasing Ti content. Figure 5(b) shows the ratio of the peakintensity of impurities to that of the MgB2 (101) plane. The
impurity intensity ratio of Ti gradually increases from about
7% for the Ti5% bulk to 10% for the Ti20% bulk. On theother hand, the amount of the TiB2 phase monotonicallyincreases with increasing Ti content, and the impurity intensity ratio reaches 10% for the Ti20% bulk. Both normal
conducting Ti and TiB2 phases are expected to act as thevortex pinning centres.
We observed the microstructure and composition of thebulk surface by FE-EPMA. Figure 6 shows the secondary
electron images for the three Ti-doped bulks. The dominant
gray region is the MgB2 matrix phase, which demonstrates
that the dense and homogeneous structure is obtained by the
HIP process, similar to the literature [17]. The bright parts,
which are Ti, obviously increase with Ti content, and the dark
parts, which represent cracks and voids, also increase simultaneously. We then analyzed the chemical composition of the

Figure 4. (a) Magnetic-ﬁeld dependence of the pinning force density,
Fp (μ 0 H ), at 10 K and 20 K for the pristine and the Ti10%-doped
MgB2 bulk. (b) The reduced pinning force density fp versus the

reduced ﬁeld h n at 20 K proposed by Eisterer [29] (see text).

the Fp (μ0 H ) curve is usually analyzed by the scaling formula,Fp ∝ h i p (1 − h i )q , where h i is the reduced ﬁeld, h i = μ0 H /μ0 Hirr , and the exponents p and q are characteristics for thedominant pinning mechanism [27]. For instance, p = 1/2 andq = 2 are predicted to be for grain boundary pinning and p = 1
and q = 2 for normal core point pinning. This procedurerequires the μ0 Hirr value; however, for the present Ti-doped
samples, this is higher than 5 T and cannot be determined
experimentally, as shown in ﬁgure 3. In such a case, the
μ0 Hirr value is often estimated by the theoretical Kramer plot[28]. However, Eisterer pointed out that this procedure was
not suitable for anisotropic materials like MgB2, because theanisotropy strongly affects the Kramer plot, which results inunderestimating the μ0 Hirr value [29]. He proposed a modiﬁed scaling procedure, allowing the identiﬁcation of thedominant pinning mechanism from the peak position in the fp(h n ) curve without the μ0 Hirr value. Here, fp (= Fp /Fpmax ) is the
reduced pinning force density, h n (= μ0 H / μ0 Hn ) is the reduced
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Supercond. Sci. Technol. 28 (2015) 095009

force density. The microstructural analysis gives us evidence
that the Ti-contained impurities, such as Ti and TiB2,
increase with increasing Ti content, which is consistent withthe results of the XRD. It is noteworthy that the TiB2 layer
observed here is quite different from the lamellar TiB2, which
existed between the nanometre-size MgB2 grains [19], whichbrings about the distinct pinning mechanism between thepresent and previous works. The typical size of Ti is estimated to be of the order of 10 μm. This is three orders ofmagnitude larger than 2ξ ∼ 10 nm [31] (ξ is the coherence
length); therefore, the contribution of Ti on the vortex pinning
is not so efﬁcient.

4. Summary
We have studied the effects of Ti doping on the magnetic
properties of MgB2 bulks. The trapped magnetic ﬁeld, BT ,critical current density, Jc , and microstructure were measured
and observed using four Mg1− x TixB2 (x = 0, 0.05, 0.1, and0.2) bulks prepared by an in-situ hot isostatic pressing
method. The important results are described below.
(i) The trapped magnetic ﬁeld was enhanced by Ti doping.
The BT value of 3.6 T at 13 K was obtained for the Tidoped bulk, which was 1.3 times larger than that of thepristine bulk. The BT (T ) curves are almost independentof the Ti contents from x = 0.05–0.2.
(ii) The highest trapped ﬁeld of 4.6 T was achieved at thecentre of the doubly stacked bulks at 14.1 K, which waslarger than that at the surface of the stacked bulks BT= 3.6 T. A square-shaped magnetic hysteresis curve, inwhich the residual magnetism (trapped ﬁeld) of 3.6 T
and the absolute value of the coercive force of −3.9 T
were comparable at 20 K, revealed that the MgB2 bulk
magnet was an excellent quasi-permanent magnet.
(iii) The trapped magnetic ﬁeld at 20 K decayed only 2%
over 40 h, which indicated high stability of the
MgB2 bulk magnet.
(iv) The pinning properties under magnetic ﬁeld were
improved by Ti doping. The Jc of 4.0 × 103 A
cm−2 for the pristine sample was enhanced to that of
1.6–1.8×10 4 A cm−2 for the Ti doped samples at 20 K
under a ﬁeld of 3 T. The irreversibility ﬁeld exceeded
5 T at 20 K for the Ti doped samples. The pinning force
density, Fp , was enhanced by Ti doping and was
maximized for Ti10%, which indicated that the pinningcentres were introduced by Ti doping. The analysis of
the Fp curve by Eisterer’s scaling theory showed that thepeak of the reduced Fp was shifted to a higher magnetic
ﬁeld by Ti-doping, which suggested that the pinningmechanism was changed; the point-like pinning became
important, in addition to the grain boundary pinning.
(v) The unreacted B and strongly Mg-deﬁcient Mg-B
particles having nanometric dimensions was found.
The Ti-contained impurities, such as Ti and TiB2,
increased with increasing Ti content. The nanometric B

Figure 5. (a) Powder x-ray diffraction patterns taken at room

temperature for the pristine and three Ti-doped MgB2 bulks [30]. (b)Peak intensity ratio of impurity peak of Ti(100) and TiB2(101) to themain peak of the MgB2 (101).

bulks from the characteristic x-ray image for each element ofMg, Ti, and B, as shown in ﬁgures 7(b)–(d); the SE image ofthe same area is also shown in ﬁgure 7(a). The bright part
represents each element. Mg and B disperse uniformly in the
MgB2 matrix phase. However, we can ﬁnd some dark grey
spots having a micrometric or nanometric dimension in the
SE image; some of them are indicated by red arrows. They
also appear as the dark and bright spots in the Mg and B
maps, respectively, as indicated by red arrows, but not in theTi map. Therefore, they are unreacted B or strongly Mgdeﬁcient Mg-B particles; especially their nanometric particles
could act as the point pinning centres. On the other hand, Ti isfound as large precipitates, which correspond to brightregions in the SE image. Comparing chemical maps of Ti andB, B is found to be concentrated in the periphery of Ti; that is,
B (thick bright line) surrounds Ti (large dark lump), as seen inﬁgure 7(c). This thick region with micrometric dimension
must be the polycrystalline TiB2. The interfaces between
nonsuperconducting TiB2 and superconducting MgB2 grains
also possibly act as the point-like pinning centres. Therefore,
these point-like pinning centres enhance both the Jc and μ0 Hirr
in high ﬁelds; this is supported by the analysis of the pinning
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Supercond. Sci. Technol. 28 (2015) 095009

Figure 6. Secondary electron images for the Ti-doped bulks; (a) 5%, (b) 10%, and (c) 20%.

Figure 7. (a) Secondary electron image and characteristic x-ray images of (b) Mg, (c) B, and (d) Ti for the Ti10% bulk. The red arrows

indicate the B or Mg-deﬁcient Mg-B particles (see text).

and Mg-B particles and the interfaces between the
TiB2 and MgB2 grains acted as a vortex pinning centre,
and enhanced both Jc and BT . On the other hand, thesize of the Ti was about three orders of magnitudelarger than the coherence length; thus the pinning effectof Ti was considered to be negligibly small.

Science Foundation, and JSPS KAKENHI Grant Numbers
23560002, 15K04718, and 15K04646.

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Acknowledgments
We would like to acknowledge Dr H Teshima of Nippon
Steel and Sumitomo Metal Corporation for his assistance inthe production of the MgB2 bulks using the HIP furnace andMiss M Sasaki and Mr K Nonaka of the Faculty of Engineering, Iwate University, for their help in the microstructure
analysis using FE-EPMA. The authors also would like tothank Dr M D Ainslie of the University of Cambridge for his
critical reading of this paper. This work was partly supported
by JST A-STEP (AS242Z02673L), Hitachi Metals · Materials
7

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