Structural and magnetic properties of bulk MnBi

JOURNAL OF APPLIED PHYSICS 109, 07A722 (2011)
Structural and magnetic properties of bulk MnBi permanent magnets
D. T. Zhang,1,2 S. Cao,1 M. Yue,1,a) W. Q. Liu,1 J. X. Zhang,1 and Y. Qiang2
1
College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
Department of Physics, University of Idaho, Moscow, Idaho 83844-0903, USA
2
(Presented 17 November 2010; received 24 September 2010; accepted 8 December 2010; published
online 4 April 2011)
Structural and magnetic properties of bulk nanostructural Mn100xBix (x ¼ 40, 45, 52) permanent
magnets prepared by spark plasma sintering technique were studied. The effect of the Mn/Bi ratio
on the MnBi low temperature phase (LTP) formation and its magnetic properties were investigated.
An increase of the bismuth amount in the magnets leads to better formation of LTP, resulting in the
improvement of both magnetization (Ms) and remanence (Mr), but decreasing the coercivity (Hc) of
the magnets. At room temperature, Ms increases from 27.87 emu/g for Mn60Bi40 to 45.31 emu/g for
Mn48Bi52, whereas Hc decreases from 12 to 7.9 kOe. The microstructure of Mn48Bi52 magnet is
composed of fine and uniform grains with an average size of 140 nm as shown in the TEM image.
The Mn48Bi52 magnet shows a high Hc of 19 kOe at 423 K, indicating a strong positive
C 2011 American Institute of Physics.
temperature coefficient of coercivity for the MnBi magnet. V
[doi:10.1063/1.3561784]
I. INTRODUCTION
MnBi is a ferromagnetic intermetallic compound with
hexagonal NiAs structure. Over the last decades, it has
attracted research interests mainly due to its high magnetocrystalline anisotropy of low-temperature phase (LTP) and
large Kerr rotation angle of quenched high temperature
phase.1–3 The LTP MnBi has a larger coercivity than that of
the Nd2Fe14B magnet4 at high temperature, so it has good
potential to be used in high temperature. It is difficult to
obtain single-phase MnBi using conventional methods, such
as arc melting and rapid solidification methods,5,6 because of
the formation of Mn precipitations and Bi matrix.7 Mn tends
to segregate from the MnBi liquid because of the peritectic
reaction, and the diffusion of Mn through MnBi is very
slow.8,9 A lot of work had been done to synthesize singlephase MnBi.5,10,11 Yang et al.12 obtained 90 wt. % LTP
MnBi magnets prepared by magnetic separation and fieldaligned resin binding. Guo et al.13 prepared the MnBi magnet with more than 95 wt. % LTP by rapid quenching, followed by a thermal treatment. It is reported that a
magnetocrystalline anisotropy field of over 6 T at room temperature and 9 T at 550 K with a coercivity of 1.8 T has been
measured for the MnBi melt-spun ribbons.4 The anisotropy
constant decreases with decreasing temperature, at which a
spin reorientation has been observed.14 The MnBi magnet
with an energy product of 4.3 MGOe has been reported,
which is much smaller than the theoretical value of 18
MG Oe.15 In the past, MnBi powders, ribbons, and bonded
magnets have been investigated, whereas nanostructural
dense MnBi sintered magnets have been studied very little.
Only Ko et al.16 reported that MnBi magnets fabricated by
spark plasma sintering have magnetic properties of
Mr ¼ 28.1 emu/g and Hc ¼ 5.7 kOe. However, no further
high temperature magnetic properties were measured.
a)
Electronic mail: yueming@bjut.edu.cn.
0021-8979/2011/109(7)/07A722/3/$30.00
In this paper, we report on the preparation of dense LTP
MnBi magnets using a new method that combines mechanical milling with spark plasma sintering (SPS).17 Also, we
report on the high coercivity and excellent coercivity coefficient b of the prepared bulk nanostructural MnBi magnets.
II. EXPERIMENT
Alloy ingots with nominal composition of Mn100 xBix
(x ¼ 40, 45, 52) were prepared by induction melting. The
ingots were annealed at 573 K for 10 h, and then crushed and
ball milled for 4 h in a high-energy ball mill. The as-milled
powders were put into a high-strength graphite die (with
outer and inner diameters of 50 and 20 mm, respectively,
and a height of 40 mm), and fast consolidated into cylindrical samples with diameters of 20 mm 5 mm by SPS technique. The samples were sintered at 573 K with a heating
rate of 50 K/min. The pressure during sintering was 30 MPa
and heat preservation time was 10 min.
The crystal structure of the magnet was examined by xray diffraction (XRD) with Cu Ka radiation. The microstructure of the magnet was studied by transmission electron microscopy (TEM). The magnetization hysteresis loops of the
magnet at room temperature and high temperature were
measured by a Lakeshore 7410 vibrating sample magnetometer with a high temperature oven, and no correction was
made for the demagnetization field effect. The density of the
magnet was measured by Archimedes method.
III. RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of Mn100 xBix
(x ¼ 40, 45, 52) sintered magnets prepared by SPS. After sintering at 573 K under 30 MPa, the obtained MnBi magnets
contained mainly MnBi LTP with a small amount of Mn and
Bi phase. Increasing the Bi amount leads to the better
109, 07A722-1
C 2011 American Institute of Physics
V
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07A722-2
Zhang et al.
J. Appl. Phys. 109, 07A722 (2011)
FIG. 1. XRD patterns of Mn100 xBix (x ¼ 40, 45, 52) sintered magnets prepared by SPS. (a) Mn48Bi52, (b) Mn55Bi45, and (c) Mn60Bi40.
FIG. 3. Hysteresis loops of Mn100 xBix (x ¼ 40, 45, 52) sintered magnets at
283 K. The maximum applied field is 2.2 T.
formation of LTP and is confirmed from the Mn48Bi52 magnet, which shows more than 90 wt. % of MnBi LTP as compared to other magnets. Our result is more reasonable than
others10,16 by this simple SPS method. An increase of the
Mn amount in MnBi increases the single phase of Mn as
shown in Fig. 1. The tendency of Mn to segregate from
MnBi liquid because of a peritectic reaction, and its slower
diffusion through MnBi makes it difficult to obtain singlephase MnBi, which turns out to be the reason why Mn
appears as a single phase with an increasing Mn amount.
The TEM image of the Mn48Bi52 magnet is shown in
Fig. 2. The bright field TEM image shows that the microstructure of the magnet is composed of fine grains with an
average grain size of about 140 nm. Due to rapid sintering
and short holding time, the grain boundary of the magnet is
not clear, but the density is rather high for the magnet. All
sintered magnets have a density of about 8.7 g/cm3, which is
over 93% of the theoretical density of the MnBi compound.
This makes the SPS method a good method for preparing
dense MnBi sintered magnets. The grain boundary would be
better if we apply a heat treatment to the magnet, which in
turn improves the magnetic properties.
The hysteresis loops measured up to a 2.2 T magnetic
field of the isotropic Mn100 xBix (x ¼ 40, 45, 52) magnets at
room temperature are shown in Fig. 3. With an increasing Bi
amount, Ms increases from 27.87 emu/g for Mn60Bi40 to
45.31 emu/g for Mn48Bi52. Ms would be improved under a
high magnetic field as magnets are not saturated completely
at a 2.2 T magnetic field. The Mn48Bi52 magnet has an Ms of
30 emu/g and an Hc of 7.9 kOe. At room temperature, the
Mn55Bi45 magnet shows a high coercivity of 12 kOe. The
presence of a minor Mn and Bi phase in the Mn60Bi40 magnet lowers the value of Ms and Hc as compared to other two
magnets. This calls for the necessity to improve the amount
of single-phase MnBi LTP, which is a challenge and will be
investigated in future.
The high temperature demagnetization curves (up to 2
T) of the Mn48Bi52 magnet are shown in Fig. 4. It shows that
the coercivity of the magnet increases from 7.6 kOe at 287 K
to 19 kOe at 423 K, and then decreases to 6.6 kOe at 473 K,
which indicates that the magnet has a large temperature
FIG. 2. TEM image of Mn48Bi52 sintered magnet. (Inset) The corresponding
selected area electron diffraction pattern of the magnet.
FIG. 4. Demagnetization curves of Mn48Bi52 sintered magnet at elevated
temperature (from 287 to 473 K), no correction was made for the demagnetization field effect. The maximum applied field is 2 T. (Inset) The dependence of coercivity on temperature.
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07A722-3
Zhang et al.
dependence of coercivity, as shown in the inset of Fig. 4.
The magnet exhibits a temperature coefficient b of 1.03%/K
in the temperature range of 287–423 K, which suggests that
the prepared MnBi sintered magnet has a positive temperature coefficient of coercivity for high temperature application. A lower coercivity at 473 K than that at room
temperature might be due to the anisotropy change. The
increase in coercivity for the MnBi magnet with increasing
temperature is due to the improvement in anisotropy
energy.18 This mechanism has been explained by different
models of domain nucleation and domain wall pinning, but
no single model can be fitted well. For melt-spinning ribbons
and bonded magnets, the coercive field might be primarily
controlled by domain wall pinning and nucleation hardening,
respectively.12 A hybrid domain wall pinning model,19
which combines Hilzinger and Kronmu¨ller’s scaling theory
for an anisotropic domain wall with Gaunt’s theory of thermal activation,20–22 gives an excellent fit to the temperature
dependence of Hc, and provides a good estimate for the
domain-wall energy and thickness over the temperature
range studied. Thus, a detailed study on the coercivity of
MnBi at an elevated temperature is needed, which will be
published at a later date.
IV. CONCLUSIONS
Isotropic Mn100 xBix (x ¼ 40, 45, 52) sintered magnets
were synthesized by the SPS technique. Mn48Bi52 magnet
contained more MnBi LTP with Mr ¼ 30 emu/g and Hc ¼ 7.9
kOe, and also showed a high temperature coercivity of 19
kOe at 423 K. The large positive coercivity coefficient b of
1.03%/K in the temperature range of 287 423 K indicates
that the MnBi sintered magnet has a good potential to
become a high temperature permanent magnet.
J. Appl. Phys. 109, 07A722 (2011)
ACKNOWLEDGMENTS
This work was supported by the National High Technology Research and Development Program of China
(2007AA03Z458). The work at the University of Idaho is
supported by DOE-BES (DE-FG02-07ER46386).
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