Effect of a high magnetic field on the morphology and magnetic properties of the MnBi compound during the Mn1.08Bi-MnBi phase transformation process

Article abstract of DOI:10.1016/j.jmmm.2009.03.069

Effect of a 10 T high magnetic field on the morphology and magnetic properties of the MnBi compounds during the Mn_1.08Bi-MnBi phase transformation has been investigated. Results indicate that the field has split the MnBi crystal along the (0 0

Full text of DOI:10.1016/j.jmmm.2009.03.069

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Journal of Magnetism and Magnetic Materials 321 (2009) 2694–2700
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Effect of a high magnetic field on the morphology and magnetic properties of
the MnBi compound during the Mn1.08Bi–MnBi phase transformation process
Ã
Xi Li a,b, , Zhongming Ren a, Yves Fautrelle b, Kang Deng a
a Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Shanghai 200072, PR China
b EPM-SIMAP, ENSHMG BP 38402 St. Martin d’Heres Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Effect of a 10 T high magnetic field on the morphology and magnetic properties of the MnBi compounds
during the Mn1.08Bi–MnBi phase transformation has been investigated. Results indicate that the field
has split the MnBi crystal along the (0 01)-crystal plane during the Mn1.08Bi–MnBi phase transforma-
tion process and the split MnBi crystals align and aggregate along the magnetic field direction. Along
with the change of the MnBi phase morphology, the magnetic property changes greatly. Indeed, with
the alignment and aggregation of the spit MnBi phases, the saturation magnetization Ms and the
Received 27 September 2008
Received in revised form
23 February 2009
Available online 1 April 2009
Keywords:
High magnetic field
Bi–Mn alloys
magnetic susceptibility
w increase, and the coercive field Hc and the remnant magnetization
Mr decrease. This implies that a high magnetic field may have caused the magnetic property of the
MnBi phase to transform towards soft magnetism. Above results may be attributed to the enhancement
of the magnetization and the Mn1.08Bi–MnBi phase transformation in a high magnetic field.
& 2009 Elsevier B.V. All rights reserved.
Magnetization
Microstructure
Magnetic force
1. Introduction
phase transformation. It has been found that the field has
separated the MnBi crystal along the (0 01)-crystal plane and
the split MnBi crystals align and aggregate along the magnetic
field direction during the Mn1.08Bi–MnBi phase transformation
process. Along with the change of the MnBi phase morphology,
the magnetic property has changed greatly. Indeed, with the
alignment and aggregation of the spit MnBi phases, the saturation
Recently, owing to the development in the superconducting
magnets, a high magnetic field up to 12 T has been widely used to
improve material properties during material processing such as
the solidification, the electro-deposition and the phase transfor-
mation [1]. Owing to the unusual magnetic and magneto-optical
properties, the physical properties of the binary compound MnBi
magnetization Ms and the magnetic susceptibility
w increase, and
the coercive field Hc and the remnant magnetization Mr decrease.
This implies that the field may have caused the magnetic property
of the MnBi phase to transform towards soft magnetism.
have been investigated extensively [2–8]. Effect of
a high
magnetic field on the microstructure of the MnBi compounds
has been investigated. Savitisky et al. [9] and Asai et al. [10,11]
found that MnBi phase aligned regularly along the magnetic field
in Biꢀ0.9–10 wt%Mn alloy solidified in the 2.5–5 T magnetic field.
However, up to now, little work has been investigated the effect of
magnetic field on the morphology of MnBi phase and its magnetic
properties of the MnBi compounds. Especially, little work has
been investigated the effect of a high magnetic field on the
morphology and magnetic properties of the MnBi compounds
during the Mn1.08Bi–MnBi phase transformation process.
In Ref. [12], the effect of a high magnetic field on the
Mn1.08Bi–MnBi phase transformation has been investigated. This
work extends the work in Ref. [12] and investigates on the effect
of a high magnetic field on the morphology and magnetic
properties of the MnBi compounds around the Mn1.08Bi–MnBi
2. Description of the experiments
Bi–6 wt%Mn alloys were prepared using bismuth (99.999%
purity) and electrolytic manganese (99.5%). Raw materials were
melted in an induction furnace and cast to a graphite mold under
argon at a pressure of 50.6 kPa. The samples with 9.5 mm
diameter and 25 mm length were sealed in a graphite tube and
inserted into a resistance furnace placed into the magnet. The
intensity of the magnetic field (up to 14 T) could be adjusted and
the temperature in the furnace chamber could be controlled
automatically during the experiment as shown in Fig. 1. The
temperature in the furnace could reach 1000 1C, and was
measured with the precision of 71 K by
a
NiCr–NiSi
thermocouple which was in direct contact with the sample.
The samples were solidified under various solidification
conditions and the magnetic field was imposed at a certain stage
Ã
Corresponding author at: Department of Material Science and Engineering,
Shanghai University, Shanghai 200072, PR China.
E-mail address: xi@hmg.inpg.fr (X. Li).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2009.03.069

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during the solidification process. The samples obtained in the
above experiments were sectioned along the direction parallel
and perpendicular to the applied magnetic field Hf during the
solidification. The microstructures on both longitudinal and
transverse sections of the samples were polished mechanically
and characterized by the optical microscopy. Magnetic measure-
ments were conducted under the applied magnetic field Hm up to
50 kOe by a physical properties measurement system (PPMS/
Quantum Design). Samples were cut in the shape of cubes (to
avoid demagnetization factor consideration) with the edges
parallel and perpendicular to the Hf direction.
cooling rate of 10 K/min below 262 1C (eutectic point tempera-
ture) under a 10 T magnetic field. Fig. 2(a) and (b) show transverse
and longitudinal microstructures of the sample solidified from
350 1C, respectively. It can be observed that the elongated MnBi
phases (dark gray) are oriented with a longer axis along the
magnetic field direction in the Bi matrix (white). The hexagonal
profile of the crystals only appears in the section perpendicular to
the magnetic field. This should be attributed to the magnetic
crystalline anisotropy of the MnBi crystal and the alignment of the
MnBi phase under a high magnetic field [9–11]. Fig. 2(c) and (d)
show the transverse and longitudinal microstructures of sample
3. Results
Fig.
2 shows the microstructures heated to a certain
temperature and held for 30 min, and then solidified at
a
Fig. 1. Schematic diagram of the experimental device of metal solidification in
high magnetic field: (1) Sample frame, (2) water-cool cover, (3) heating furnace, (4)
superconductor magnet, (5) sample, (6) quenching water-pond and (7) controlling
temperature system.
Fig. 3. XRD patterns of the sample solidified from 360 1C in Fig. 2: (a) 0 T, (b)
section parallel to Hf and (c) section perpendicular to Hf. Peaks labeled by (h k l) are
for MnBi and others for Bi.
Fig. 2. Microstructures solidified from a certain temperature at a rate of 10 K/min after holding temperature for 30 min at this temperature under a 10 T magnetic field.

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solidified from 360 1C, respectively. Compared with the
microstructure solidified from 350 1C, it can be observed that
the morphology of the MnBi phase has been changed significantly.
Indeed, it can be observed that MnBi crystals are separated along
the section perpendicular to the Hf direction and oriented with a
shorter axis along the Hf direction. Furthermore, the XRD patterns
(Fig. 3) show that the /0 01S-crystal direction (i.e., the easy
magnetization axis) of the MnBi crystal is aligned along the
magnetic field direction. This means that the MnBi crystal in the
sample solidified from 360 1C is separated along the (0 01)-crystal
Fig. 4. Hysteresis loops of the same samples as shown in Fig. 2. (a) 350 1C and (b) 360 1C. ‘‘Parallel and perpendicular’’ indicate directions of the magnetic field imposed for
the magnetization measurement (Hm) with respect to the magnetic field imposed during the solidification (Hf).
Fig. 5. Microstructures and magnetization curves for the sample solidified from 400 1C at a rate of 10 K/min to 380 1C after being holding temperature for 30 min with a 10 T
magnetic field, and then quenched: (a) longitudinal section, (b) transverse section and (c) magnetization curves.

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plane. To investigate the magnetic property, the hysteresis loops
of samples have been measured. Because Bi is diamagnetic, its
effect has been ignored and only the MnBi compounds have been
considered in the magnetization measurement. Fig. 4 shows the
hysteresis loops parallel and perpendicular to the Hf direction for
the samples referred in Fig. 2. It can be learned that the coercive
field HC and the remnant magnetization Mr of the sample
solidified from 360 1C are much lower than the one of the
samples solidified from 350 1C. However, its saturation
magnetization Ms is larger than the one of the samples solidified
from 350 1C.
From the phase diagram of the Mn–Bi alloy [13], it can be
learned that the paramagnetic Mn1.08Bi compound is formed by
the peritectic reaction of Mn+L(Bi)-Mn1.08Bi at 446 1C; further-
more, the compound undergoes
a magnetic and structure
transformation to form ferromagnetic MnBi phase at 340 1C upon
cooling, and MnBi phase turns into the Mn1.08Bi at 355 1C upon
heating. The MnBi phase is called as low-temperature phase (LTP),
the Mn1.08Bi is called as high-temperature phase (HTP) and the
quenched state of HTP is called as the quenched high-temperature
phase (QHTP). Therefore, when the sample is solidified from
360 1C, the MnBi phase is formed during the Mn1.08Bi–MnBi phase
Fig. 6. Microstructures of the Bi–6 wt%Mn alloy cooled from 400 to 360 1C and held for 5 min (a, b), 10 min (c, d), 30 min (e, f) and 60 min (g, h) and then quenched in a 10 T
magnetic field, respectively.

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transformation process. The above results imply that a high
magnetic field has changed the microstructure and magnetic
properties of the MnBi phase during the Mn1.08Bi–MnBi phase
transformation process.
Moreover, the effect of
a high magnetic field on the
morphology and magnetic properties of the paramagnetic
Mn1.08Bi phase has also been investigated. Fig. 5 shows the
microstructures of the samples solidified from 400 1C at a cooling
rate 10 K/min and quenched at 380 1C. It can be observed that
there are no clear influences of the field on the morphology of the
primary Mn1.08Bi phase. This means that a 10 T magnetic field
cannot align the paramagnetic Mn1.08Bi phase. Further, the
magnetization of the sample was measured and Fig. 6 presents
the magnetizations parallel and perpendicular to the Hf direction
for samples in Fig. 5. It can be observed that there is no obvious
magnetic anisotropy. This is consistent with the alignment
behaviours of the paramagnetic Mn1.08Bi phases under the
magnetic field. This shows that an application of a 10 T high
magnetic field has few influences on the paramagnetic Mn1.08Bi
phase. Therefore, the change of the morphology and magnetic
property of the sample under a high magnetic field should be
attributed to the effect of the magnetic field on the MnBi phase.
Further, the effect of the holding temperature times at 360 1C
under a 10 T magnetic field on the morphology and magnetic
property of the MnBi compounds has been investigated. Fig. 6
shows the microstructures solidified from 400 1C and held various
times at 360 1C before quenching. It can be observed that with the
increase of the holding temperature time under the magnetic
field, the phase is separated gradually along the plane perpendi-
cular to the magnetic field, and the split phases align and
aggregate along the magnetic field direction. Fig. 7 shows the
magnetizations (Fig. 7(a)) and hysteresis loops (Fig. 7(b)) for
the samples as referred in Fig. 6. It can be observed that with the
increase of the holding temperature time under the magnetic
field, the saturation magnetization Ms and the magnetic
susceptibility
w increase. The remnant magnetization Mr and the
ratio of Mr/Ms (Fig. 7(c)) decrease. The above results indicate that
the morphology and the magnetic properties (Mr and Mr/Ms) of
the MnBi compounds have changed significantly with the increase
of the holding temperature time under a high magnetic field.
4. Discussion
Above experimental results have shown that the morphology
and the magnetic property of the MnBi compounds have been
changed significantly during the Mn1.08Bi–MnBi phase transfor-
mation process under a high magnetic field. This should be
attributed to the enhancement of a high magnetic field on the
magnetization of the MnBi phase and the Mn1.08Bi–MnBi phase
transformation.
As shown in Fig. 8(a), the ferromagnetic MnBi phase (LTP) will
nucleate on the surface of the paramagnetic Mn1.08Bi phase (HTP)
during the Mn1.08Bi–MnBi phase transformation process. Along
with the formation of the MnBi phase, the magnetic domains will
produce. When the magnetic field is applied, the MnBi phase
will be magnetized; as a consequence, the magnetic domains will
rotate and the magnetic domain walls will move. Thus, their
same-name magnetic poles will align along the magnetic field
direction (Fig. 8(b)) and then repulsive forces will produce among
them. The repulsive force can be expressed as [14]:
Fig. 7. The magnetic properties of the same sample as shown in Fig. 6. (a)
Magnetizations, (b) hysteresis loops, (c) remnant magnetization Mr and the Mr/Ms.
The directions of the magnetic field imposed for the magnetization measurement
(Hm) is parallel to the magnetic field imposed during the solidification (Hf).
m0
ð
w
4
þ 1Þ Qm1Qm2
p
r2
FM
¼
(1)
phase, their volumes increase and the inter-grains distances
decrease. Thus, according to the formula (1), the repulsive forces
increase significantly. Consequently, the force will separate the
where Qm1, Qm2 are the strength of two magnetic poles, r the
distance between two magnetic poles. With the growth of the LTP

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MnBi grains. At the same time, the split MnBi crystals will align
and aggregate along the magnetic field direction as shown in
Fig. 8(c). Thus, the microstructures in Figs. 2 and 6 will form.
Moreover, from the magnetism point of view, the magnetic
domains in the substance rotate their easy magnetic axes towards
the magnetic field direction by moving their magnetic domain
walls during the magnetization process. The grain boundary stress
will retard the rotation and movement of the domains. Thus, the
large coercive field HC and the remnant magnetization Mr will
form. It is well known that the LTP (MnBi phase) has the large
coercive field HC and the remnant magnetization Mr. During the
Mn1.08B–MnBi phase transformation under a high magnetic field,
the magnetic domain walls may be destroyed. Thus, the stresses in
the grains are released and the magnetostriction is reduced; which
will lead to the reduction of the coercive field HC and the remnant
magnetization Mr in the MnBi phase. Furthermore, the reduction of
the magnetic domain walls and magnetostriction causes the
motion of the magnetic domain to be easier. As a result, its
magnetic susceptibility
w will increase. It has been reported that
the cracks occurred frequently during the directional solidification
of the TbDyFe alloy under a high magnetic field [15]. This may
be attributed to the same mechanism presented here. As they
both endure the peritectic transformation during the solidifica-
tion process. From this mechanism, it may be concluded that
the application of a high magnetic field during the peritectic
phase transformation involving of a ferromagnetic phase will
result in the increase of the magnetic susceptibility
the decrease of the coercive field Hc and the remnant magnetiza-
w and
tion Mr.
As the MnBi crystal has owned a larger saturation magnetiza-
tion Ms, the increase of the saturation magnetization Ms during
the Mn1.08Bi–MnBi phase transformation process in
a high
magnetic filed may be attributed to the increase of the MnBi
phase. There are several reasons; first, owing to the decrease of
the MnBi free energy under the magnetic field, the Mn1.08Bi–MnBi
phase transformation course will be enhanced. Consequently, the
content of the MnBi phase increases. Second, owing to the
increase of the TC under the magnetic field [12] the equilibrium
liquidus near MnBi may be elevated. If ignoring the influence of
the field on the melting point of Bi (because it is diamagnetic) and
liquidus near it, the application of the magnetic field will cause an
increase of eutectic temperature and a shift leftward of the
eutectic composition as shown in Fig. 9(b). Thus, the content of
the MnBi phase in the alloy will increase. Moreover, the
separation of the MnBi phase also enhances the Mn1.08Bi–MnBi
phase transformation process and leads to the increases of the
MnBi phase. Moreover, the above experimental results have
shown that with the increase of the holding temperature time,
the saturation magnetization Ms will increase. This should be
attributed that with the increase of the holding temperature time,
the Mn1.08Bi–MnBi phase transformation has been enhanced.
Thus, the content of the MnBi phase increases. Since the MnBi
crystal has owned a large saturation magnetization Ms, the
saturation magnetization Ms of the sample will increase.
Fig. 8. Schematic illustration of the Mn1.08Bi–MnBi phase transformation process
in the magnetic field: (a) generation of magnetizing forces between the
ferromagnetic MnBi phases (LTP) in the magnetic field, (b) the increase of the
magnetizing forces along with the growing up of the MnBi phase and (c)
aggregation of the MnBi phases.
Fig. 9. Effect of the magnetic field on the equilibrium phase diagram of Mn–Bi system: (a) equilibrium phase diagram of Mn–Bi system near Bi, red circle showing the
Mn1.08Bi–MnBi phase transformation and (b) schematic illustration of the modification of the phase diagram by the magnetic field.

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5. Conclusions
(No. 50801045) and the Changjiang Scholars and Innovative
Research Team in University (No: IRT0739).
Effect of a high magnetic field on the morphology and
magnetic properties of the MnBi compounds during the
Mn1.08Bi–MnBi phase transformation process has been investi-
gated. Results indicate that the field has separated the MnBi
crystal along the (0 01)-plane during the Mn1.08Bi–MnBi phase
transformation process and the split MnBi crystals align and
aggregate along the magnetic field direction. Along with the
change of the morphology of the MnBi compounds, the magnetic
properties of the MnBi phase has been changed significantly; as a
result, the saturation magnetization Ms and the magnetic
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susceptibility
w increase, and the coercive field Hc and the
remnant magnetization Mr decrease. This should be attributed
to the transformation of the magnetic property of the MnBi phase
towards soft magnetism during the Mn1.08Bi–MnBi phase trans-
formation under a high magnetic field and the enhancement of a
high magnetic field on the magnetization of the MnBi phase and
the Mn1.08Bi–MnBi phase transformation.
Acknowledgments
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This work is supported partly by the European Space Agency
through the IMPRESS project, Natural Science Foundation of China