Magnetic-field-induced enhancement for synthesizing ferromagnetic MnBi phase by solid-state reaction sintering

Article abstract of DOI:10.1016/j.jallcom.2014.06.131

Ferromagnetic MnBi was synthesized by a solid-state reaction sintering method in high magnetic fields up to 15 T. The fraction of ferromagnetic MnBi phase in the sample sintered at 15 T was about 70% by weight, which was much larger than that sintered in a zero magnetic field. It was found that high magnetic fields dramatically enhanced the formation of MnBi phase from bismuth and manganese. In addition, the c-axis of a hexagonal structure of MnBi was oriented parallel to the magnetic field direction. These magnetic field effects on the synthesizing process were examined based on the gain of the Zeeman energy and the uniaxial magnetic anisotropy of MnBi phase.

Full text of DOI:10.1016/j.jallcom.2014.06.131

Journal of Alloys and Compounds 615 (2014) 131–134
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Magnetic-field-induced enhancement for synthesizing ferromagnetic
MnBi phase by solid-state reaction sintering
Yoshifuru Mitsui a, ,1, Rie Y. Umetsu a, Keiichi Koyama b, Kazuo Watanabe c
⇑
a Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
b Graduate School of Science and Engineering, Kagoshima University, Kagoshima 890-0065, Japan
c High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Ferromagnetic MnBi was synthesized by a solid-state reaction sintering method in high magnetic fields
up to 15 T. The fraction of ferromagnetic MnBi phase in the sample sintered at 15 T was about 70% by
weight, which was much larger than that sintered in a zero magnetic field. It was found that high
magnetic fields dramatically enhanced the formation of MnBi phase from bismuth and manganese. In
addition, the c-axis of a hexagonal structure of MnBi was oriented parallel to the magnetic field direction.
These magnetic field effects on the synthesizing process were examined based on the gain of the Zeeman
energy and the uniaxial magnetic anisotropy of MnBi phase.
Received 9 December 2013
Received in revised form 4 May 2014
Accepted 18 June 2014
Available online 26 June 2014
Keywords:
MnBi
High magnetic field
Reaction sintering
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
to investigate the high magnetic field effects on the synthesis of
MnBi, a solid-state reaction sintering in high magnetic fields up
Ferromagnetic MnBi with a hexagonal NiAs-type structure is
known to have a large uniaxial magnetic anisotropy along c-axis
and have large coercivity above room temperature [1,2]. Since
Adams produced MnBi having a potential use as a permanent mag-
net [3], many researchers have studied the magnetic and physical
properties of MnBi [4,5]. It is difficult to obtain highly pure MnBi
because the formation of MnBi is impeded by Mn segregation
during the peritectic reactions. To avoid the segregation of Mn
and Bi, a solid-state sintering process below eutectic temperature
is effective. However, it takes a quite long time to synthesize MnBi
by such a process because the reaction below eutectic temperature
is sluggish due to the low annealing temperature.
On the other hand, it is well known that magnetic fields influ-
ence phase equilibrium [6], diffusion coefficient [7], and segrega-
tion [8]. Additionally, in-field heat treatment (IFT) has been
found to lead the crystal orientation [9], alignment of the grains
[10], and enhancement of the coercivity [11]. The IFT effects on
Bi–Mn system have been studied for Bi-rich compositions.
Anisotropic microstructures and magnetic properties have been
obtained by synthesizing MnBi in a Bi-rich matrix in high magnetic
fields [12,13]. However, the IFT effects on MnBi in a stoichiometric
composition have not been studied. In the present study, in order
to 15 T was performed.
2. Experimental
The pellets with /10 mm and 8 mm thick in size were produced by pressing
under a pressure of 8 t/cm2 (ꢀ20 MPa) after mixing the powder elements (Bi:4N,
Mn:3N). The molar ration of Bi and Mn particles were 1:1. Bi particle within
20 lm and Mn particle within 75 lm were used for the sintering. The pellets were
sealed in a quartz tube with argon gas. After that, IFT was carried out at 523 K for
5 days at magnetic fields of 0, 10, and 15 T using cryogen-free superconducting
magnets. According to the report for the reaction sintering of MnBi in a zero field
[14], MnBi was synthesized by the heat treatment for 2–15 days at just below the
melting temperature of Bi (ꢀ544 K). In this study, in order to perform the solid-
state sintering, the annealing temperature was chosen just below the eutectic tem-
perature of Mn and Bi (ꢀ535 K). The magnetic field direction Bh was parallel to the
thickness of the pellet. The obtained sample was cut into pieces for measurement.
The pellet-shaped samples were used for X-ray diffraction (XRD) experiments and
the microstructural observation by scanning electron microscopy (SEM). Cubic-
shaped samples were used for the magnetization measurements by a vibrating
sample magnetometer (VSM) in magnetic fields up to 1.2 T.
3. Results and discussions
Fig. 1 shows the XRD patterns of the samples sintered at 523 K
for 5 days in magnetic fields of 0 T (a), 10 T (b), and 15 T (c). Here,
hkl denotes the Miller indices of MnBi. Other indices, Mn and Bi,
indicate that the diffraction peaks derived to the pure Mn and pure
Bi, respectively. At 0 T, the diffraction peaks of Mn and Bi were
observed, whereas those of MnBi were quite small. The ratio of
⇑
Corresponding author. Tel.: +81 99 285 8082.
E-mail address: mitsui@sci.kagoshima-u.ac.jp (Y. Mitsui).
Present Address: Graduate School of Science and Engineering, Kagoshima
1
University, 890-0065, Kagoshima, Japan.
http://dx.doi.org/10.1016/j.jallcom.2014.06.131
0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

132
Y. Mitsui et al. / Journal of Alloys and Compounds 615 (2014) 131–134
c-axis of the hexagonal structure was parallel to the direction of
the applied magnetic field Bh.
Fig. 2 shows the SEM images of the samples sintered at 0 T (a)
and at 15 T (b). The white areas indicate pure Bi and the gray areas
indicate MnBi or Mn. As shown in Fig.2 (a), clusters of residual Bi
powders were observed, whereas clusters of Bi particles were not
observed for the sample sintered at 15 T. These results suggested
that the solid state reaction of Mn + Bi ? MnBi was actually
enhanced by application of a magnetic field.
Fig. 3 shows the magnetization curves for the sample sintered
in 0 T (a) and 15 T(b). B\ and B// indicate the magnetic field
direction perpendicular and parallel to Bh, respectively. The isotro-
pic magnetic properties were obtained for the sample sintered in a
zero fields. The magnetization at 1.2 T was obtained to be 3.7 A m2/
kg, which did not saturated. The coercivity field Hc was obtained to
be 0.22 T. On the other hand, anisotropic magnetic properties were
clearly obtained for the sample sintered in 15 T. The direction B//
was found to be the easy axis of the magnetization and the direc-
tion B\ was the hard axis. Since the easy axis of MnBi is c-axis of
the hexagonal structure, the results are consistent with those of
XRD experiments. The saturation magnetization Ms and Hc were
54 A m2/kg and 0.08 T, respectively. Since the theoretical value of
MnBi is 80 A m2/kg, the fraction of MnBi in the sample sintered
in a zero field and in 15 T was evaluated to be 4.6 wt.% and
~68 wt.%, which was consistent with the results obtained by
XRD. As mentioned above, although the Hc was comparatively
low, the magnetic properties indicated that highly dense and
oriented MnBi was synthesized by IFT at 15 T.
The magnetic field influences the activation energy of the reac-
tion and the formation enthalpy of synthesizing MnBi from Mn and
Bi. The Zeeman energy is considered to be the one of the reasons
for the enhancement of MnBi synthesis. With the application of a
magnetic field, the Gibbs free energy of MnBi, Mn, and Bi decreases
due to the Zeeman energy. In this case, the magnetization of
Fig. 1. X-ray diffraction pattern of the sample sintered in magnetic field of 0 T (a),
10 T (b), and 15 T (c). hkl denotes the Miller indices of MnBi.
the largest intensity of the peak for Bi (2hꢀ27°) and MnBi (2hꢀ28°)
was 35:1. The residual Mn was same molar fraction as residual Bi.
As mentioned above, the fraction of MnBi in the sample was
estimated to be less than 5 wt.% by weight.
In the case of the samples sintered in magnetic fields, the
diffraction peaks derived from MnBi were much higher than those
of samples sintered in a zero field. The diffraction peaks derived
from MnBi were clearly observed in the sample sintered at 10 T
and at 15 T. At 15 T, the fraction of MnBi to Bi by weight was esti-
mated to be 8:2 from powder XRD measurement by the ratio of the
largest intensity of the peak for Bi and MnBi. The residual Mn is
same molar fraction as residual Bi. In other words, the fraction of
residual Mn was ꢀ1/4 as that of Bi by weight. Thus, synthesized
MnBi phase in the sample was ꢀ70 wt.%. Meanwhile, the fraction
of MnBi in the sample sintered in 10 T was evaluated to be
ꢀ60 wt.% by the powder XRD measurement. These results show
that the fraction of MnBi in the sample increased drastically with
increasing magnetic field intensity.
These results suggested that the reaction sintering process of
MnðsolidÞ þ BiðsolidÞ ! MnBiðsolidÞ
was enhanced by the application of a magnetic field. Furthermore, it
should be noted that the intensity of 00l diffraction peaks were
especially enhanced, whereas those of hk0 diffraction peaks were
suppressed for the sample sintered in magnetic fields. This result
indicated that the formed MnBi crystals were oriented and the
Fig. 2. Scanning electron microscopy images of the samples sintered at 0 T (a) and
15 T (b). The white area indicates Bi. The gray and black areas indicate MnBi or Mn.

Y. Mitsui et al. / Journal of Alloys and Compounds 615 (2014) 131–134
133
Bi in the liquid phase enhanced by the application of magnetic
fields. Thus, in order to understand the magnetic field induced
enhancement of the reaction, further investigation of such factors,
as diffusion coefficients and activation energy in high magnetic field
were required.
The large uniaxial magnetic anisotropy of MnBi caused the
crystal orientation in the process of reaction sintering. MnBi has
the positive thermal coefficient of uniaxial magnetic anisotropy
for T ꢀ 550 K along the c-axis. At 523 K, the uniaxial magnetic
anisotropy energy of MnBi was reported to be 2 ꢂ 106 J/m3 [1]. At
the early stage of the reaction sintering, the boundary between
Mn and Bi particles partially melt because of their reaction heat.
After that, MnBi was crystallized from the liquid. Because of solid-
state process, it is hard for MnBi grain to rotate in the magnetic field.
Hence, the oriented MnBi was crystallized at the boundary of the
particles, and MnBi grains were grown in the intraparticles.
In this study, it was found that the phase formation and crystal
orientation was enhanced by magnetic field. The results of XRD indi-
cated that the fraction of MnBi increased with increasing magnetic
field. However, the crystal orientation depends on not only the mag-
netic field intensitybut also the anisotropyfield Ba because magnetic
anisotropy energy is saturated over Ba. When the IFT is performed in
higher magnetic field than Ba of MnBi, it is predicted that magnetic
field effect for the crystal orientation will be saturated.
According to the previous study of IFT for Bi-rich composition,
for instance, Li et al. described that the magnetic field effect on
the crystal orientation are the magnetic dipole interaction between
the ferromagnetic MnBi grains, and magnetic alignment in the Bi-
rich liquid, and so on [13]. On the other hand, in the case of solid
state sintering process, it is hard to move for the ferromagnetic
grain in magnetic field because solid Mn and Bi particles surround
MnBi grains. In addition, the enhancement of the phase formation
of MnBi phase is clearly observed. As mentioned above, magnetic
field effects for solid-state sintering were supposed to have differ-
ent mechanism from the previous study for Bi-rich compositions.
The advantage of in-field solid state sintering over the conven-
tional methods such as arc-melting, induction melting, crystalliza-
tion from melt-spun ribbons, and sintering, is to achieve the crystal
orientation during the formation of MnBi phase. For obtaining the
anisotropic magnet, the magnetic alignment was necessary after
alloying by the conventional method. Meanwhile, in-field solid-
state sintering leads the crystal orientation and phase formation
at the same time. Therefore, in-field solid state sintering is expected
to be a suitable method for producing the anisotropic magnet.
Fig. 3. Magnetization curves for the samples which were sintered in 0 T (a) and in
15 T (b). Bh indicates the magnetic field direction of in-field heat treatment. B\ and
B// indicate the magnetic field direction of VSM measurements, which are
perpendicular and parallel to Bh, respectively.
ferromagnetic MnBi is much larger than that of paramagnetic Mn
and diamagnetic Bi. Gibbs free energies of Mn and Bi are almost
completely independent of the magnetic field because the gain of
Zeeman energies of Mn and Bi are negligibly small. Then, the
formation enthalpy of MnBi at 15 T is much lower than that in a
zero magnetic field, resulting in the enhancement of the reaction
sintering by application of magnetic field. However, according to
Bi–Mn phase diagram calculated on the basis of CALPHAD
(computer coupling of phase diagram) method, Gibbs free energy
of MnBi GMnBi (J/mol) is expressed as
4. Conclusions
In conclusion, the magnetic field effects on the solid-state reac-
tion sintering process of MnBi were investigated. The solid-state
reaction from Bi and Mn into MnBi was enhanced clearly. Further-
more, although the phase formation of MnBi is a solid-state pro-
cess, the c-axis of MnBi was oriented parallel to the direction of
the magnetic field. The results indicated that highly dense and
highly oriented MnBi could be synthesized by the in-field solid
state sintering process. These magnetic fields effects for reaction
sintering realize the anisotropic magnetic properties. Conse-
quently, in-field solid state sintering is expected to be a suitable
method for producing the anisotropic magnet.
GMnBi ¼ ꢁ13771 þ 11:879 T þ GBi þ GMn þ Gmag
ð1Þ
where T, GBi and GMn and Gmag are the temperature (K), Gibbs
energy of Bi and Mn (J/mol) and magnetic contribution in a zero
field, respectively [15]. The formation enthalpy of MnBi from Mn
and Bi Gf is ꢁ13,771 + 11.879 T (J/mol). Gf at 523 K is evaluated to
be 7.5 kJ/mol. Meanwhile, the Zeeman energy was calculated by
obtaining magnetization of MnBi on the basis of mean field theory.
The Zeeman energy of MnBi at 523 K and 15 T was calculated to be
0.25 kJ/mol, which is only 3.3% to Gf. As above, it is difficult to
explain the magnetic fields effects completely by the contribution
of the Zeeman energy to the formation enthalpy. On the other hand,
magnetic field influences the activation energy. For instance,
undercooling of solidification for pure Bi were suppressed by
magnetic fields [16]. Li et al. explained that the activation energy
of solid–liquid transformation decreased and nucleation of solid
Acknowledgements
The in-field heat treatments were performed at High Field
Laboratory for Superconducting Materials, Institute for Materials
Research, Tohoku University. The magnetization measurements
were carried at the Cooperative Research and Development Center

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Y. Mitsui et al. / Journal of Alloys and Compounds 615 (2014) 131–134
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