Synthesis and characterization of rare-earth-free magnetic manganese bismuth nanocrystals

Article abstract of DOI:10.1039/c4ra12440e

Earth abundant manganese bismuth (MnBi) has long been of interest due to its large magnetocrystalline anisotropy and high energy density for advanced permanent magnet applications. However, solution synthesis of MnBi phase is challenging due to the reduction potential mismatch between Mn and Bi elements. In this study, we show a versatile MnBi synthesis method involving the metal co-reduction followed by thermal annealing. The magnetically hard MnBi crystalline phase is then exchange coupled with magnetically soft cobalt coating. Our processing approach offers a promising strategy for manufacturing rare-earth-free magnetic nanocrystals. This journal is

Full text of DOI:10.1039/c4ra12440e

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Synthesis and characterization of rare-earth-free
magnetic manganese bismuth nanocrystals†
Cite this: RSC Adv., 2015, 5, 5567
Jian Shen,‡a Huizhong Cui,‡a Xiaopeng Huang,a Maogang Gong,a Wei Qin,a
a
b
a
*
Alec Kirkeminde, Jun Cui and Shenqiang Ren
Received 15th October 2014
Accepted 15th December 2014
DOI: 10.1039/c4ra12440e
www.rsc.org/advances
Earth abundant manganese bismuth (MnBi) has long been of interest MnBi does not decrease while increasing the temperature.12
due to its large magnetocrystalline anisotropy and high energy density This unique temperature dependent behavior of MnBi phase is
for advanced permanent magnet applications. However, solution due to the variation of the crystal lattice ratio of c/a with
synthesis of MnBi phase is challenging due to the reduction potential temperature.
mismatch between Mn and Bi elements. In this study, we show a
In the past, solid-state synthetic methods of MnBi alloys have
versatile MnBi synthesis method involving the metal co-reduction been well studied, including arc melting, rapid solidication,
followed by thermal annealing. The magnetically hard MnBi crystal- and ball milling.13–15 However, it is challenging to obtain single-
line phase is then exchange coupled with magnetically soft cobalt phase MnBi nanoalloys using the standard air-free Schlenk line
coating. Our processing approach offers a promising strategy for technique due to the large negative reduction potentials of
manufacturing rare-earth-free magnetic nanocrystals.
soluble Mn salts (e.g., À1.185Eꢁ V). Therefore, a strong reducing
agent is needed to reduce Mn2+ to zerovalent Mn. Herein, we
report a chemical synthetic route by the combination of co-
reduction and thermal annealing to synthesize magnetic
MnBi nanocrystals. The reaction conditions (temperature, time,
solvent and precursor) dictate the formation of the MnBi
nanostructures. The X-ray diffraction (XRD) and differential
scanning calorimetry (DSC) conrm the formation of LTP-MnBi
phase. Though the LTP-MnBi shows high MCA energy, the
maximum energy product reported thus is far relatively low for
most practical applications due to its relatively low saturation
magnetization (Ms). Therefore, an improvement in the
magnetization of MnBi needs to be made to enhance its energy
product. This improvement could feasibly be achieved by
interacting hard and so nanocomposite phases through
magnetic exchange coupling.16–20 Resulting MnBi nanocrystals
are then further coated with a magnetically so cobalt (Co)
phase to improve the energy product through exchange
coupling.
Introduction
High-performance rare earth magnets, such as Nd-Fe-B and
Sm–Co, have been developed as a result of the growing demand
for green energy technologies. The maximum energy product of
rare-earth based magnets (about ꢀ56 MGOe) has been achieved
in the past 10 years.1–4 However, due to the high cost and
considerably limited supplies of rare earth elements, global
efforts in material science and technology have been made on
potential substitutions.5,6 Recently, intermetallic compound
manganese bismuth (MnBi) has been shown to be a promising
permanent magnetic material that does not contain rare earth
elements and is attracting a lot of attention for its unique
structure and temperature-dependent magnetic properties.7–11
The MnBi phase exhibits large magnetocrystalline anisotropy
(MCA) in its low temperature phase (LTP), due to its hexagonal
NiAs crystal structure with two equal axes separated by 120ꢁ and
the third axis of different length perpendicular to both. In
addition, unlike other hard magnetic materials, the coercivity of
Results and discussion
In this study, two synthetic steps are involved, which includes
the Mn–Bi (shell–core) nanocrystal ink and thermally induced
crystalline MnBi alloys (Fig. 1). The Mn–Bi nanocrystal ink is
synthesized using a solution co-reduction approach, which is
aDepartment of Chemistry, University of Kansas, Lawrence, Kansas, 66045, USA.
E-mail: shenqiang@ku.edu
bPacic Northwest National Laboratory, Richland, WA 99352, USA
† Electronic supplementary information (ESI) available: Supplementary gures.
See DOI: 10.1039/c4ra12440e
ꢁ
followed by a high temperature (600 C) thermal annealing to
induce the elemental diffusion between Mn and Bi for the
‡ These two authors contributed equally to this work.
formation of MnBi alloys. The transmission electron
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Fig. 1 (a and b) Scheme, TEM and EDX mapping images (from left to
right) of MnBi nanocrystals through the Mn–Bi (shell–core) ink, and
thermal annealing formed MnBi alloy nanocrystals, respectively. The
red and green colored points represent the Mn and Bi elements,
respectively.
Fig. 2 (a) TEM image of synthesized Mn nanoparticles. (b) TEM images
of Mn–Bi (shell–core) ink after 20 minutes reaction. (c) TEM images of
Mn–Bi (shell–core) ink after 24 hours reaction. (d) The EDX elemental
mapping image of the sample (c). The red and green colored points
represent the Mn and Bi elements, respectively.
microscopy (TEM) image of Fig. 1a shows the representative
MnBi core–shell nanocrystals that have the Bi core with an
average diameter of ꢀ50 nm and the Mn shell with an average
thickness of ꢀ10 nm. The Bi core and Mn shell components can
be clearly distinguished in the bright-eld TEM image due to
their large difference in the electron penetration efficiency. The
MnBi alloy nanocrystals aer thermal annealing are shown in
the center TEM image of Fig. 1b, in which an irregular shape of
MnBi alloy nanocrystals are observed. The rightmost images of
Fig. 1a and b present the chemical composition of MnBi
nanostructures through the energy dispersive X-ray (EDX)
spectroscopy mapping, which further conrm the formation of
MnBi core–shell structure and MnBi alloy, respectively.
The X-ray diffraction (XRD) obtained from thermally
annealed samples conrms the formation of crystalline MnBi
alloy phase. Fig. 3a depicts the XRD pattern of the Mn–Bi (shell–
core) structure and thermally-annealed MnBi alloy nano-
crystals, which conrms the formation of LTP-MnBi phase. The
formation of the LTP-MnBi phase is also evidenced from the
differential scanning calorimetry (DSC) in Fig. 3b, which is
obtained from both Mn–Bi (shell–core) ink and thermally-
The difficulty of colloidal MnBi nanocrystal synthesis is
resulted from the large negative reduction potential of Mn ions
during the co-reduction with Bi precursors.21,22 In this study, our
hypothesis is to test n-butyllithium (BuLi) as the reducing agent
for the formation of zerovalent Mn in a diphenyl ether solution.
This has been conrmed by the formation of the Mn nano-
particles using BuLi reducing agent (Fig. 2a). As increasing the
reaction time from 20 minutes to 24 hours aer the injection of
n-BuLi, an increase of the Bi core particle size (from ꢀ20 nm to
ꢀ50 nm) is observed and the crystallized core–shell MnBi
formation becomes pronounced (Fig. 2b and c). As shown in
Fig. S1,† the crystalline lattice structure is clearly presented
between the Mn shell and the Bi core phases. The EDX
elemental mapping conrms the Mn–Bi (shell–core) nano-
structures, shown in Fig. 2d. In addition, the different manga-
nese precursor effects on the MnBi alloy formation have been
investigated under different reaction conditions, which are
shown in Fig. S2.†
Fig. 3 (a) The XRD patterns of the MnBi before and after annealing
(from top to bottom). (b) The DSC scan of Mn–Bi (shell–core) ink and
MnBi alloy nanocrystals under the heating rate of 20 ꢁC minÀ1. (c) The
FTIR spectrum of the Co2(CO)8 and MnBi mixture (black) and Co
coated MnBi (red), which was obtained before and after annealing
step.
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annealed MnBi alloys under a moderate heating rate of 20 C MnBi phase is encapsulated by the Co nanowire (the SEM
minÀ1. For the Mn–Bi (shell–core) ink, there is only one endo- images of cobalt nanowire and MnBi are shown in Fig. S4†).
thermic peak at 276.7 ꢁC observed, which indicates the melting Fig. 4d exhibit room temperature magnetic hysteresis loops of
temperature of Bi phase. However, for the MnBi alloy, except the cobalt-coated MnBi, where no kink is observed in both
endothermic peak at 272.1 ꢁC, an exothermic peak at 277.3 ꢁC is approaches, indicating potential exchange coupling between
shown as the evidence of formation of the LTP-MnBi phase. To cobalt and MnBi phases. Meanwhile, the cobalt coating
enhance the energy product of MnBi nanomagnet, we select dramatically increases the magnetization of the synthesized
magnetically so Co coating to investigate exchange coupling MnBi phase. The magnetization of MnBi nanoalloys can be
between magnetically hard MnBi core and so Co coating. The increased by 13.5% aer in situ cobalt coating (Fig. 4b). As
in situ Co shell coating is formed on the surface of MnBi comparison, the magnetization of cobalt nanowire coated
nanocrystals via thermal decomposition of Co2(CO)8 precursor. MnBi is depended on the weight ratios between the cobalt
Fourier transform infrared spectroscopy (FTIR, Fig. 3c) is nanowires and MnBi. By increasing the cobalt nanowire ratio,
utilized to investigate the thermal decomposition of Co2(CO)8 the Co–MnBi magnetization is also increased. In addition,
phase before and aer thermal annealing. Aer the thermal due to magnetic shape anisotropy of cobalt nanowires, the
annealing, the full decomposition of Co2(CO)8 and the forma- nanowire coated MnBi does not show a signicant decrease of
tion of magnetically so Co shell are conrmed by the disap- coercivity at a high loading of cobalt nanowires,23 where the
pearance of the following bands at 1857, 2024 and 2074 cmÀ1
,
magnetic characteristics are shown in Fig. S5 and S6.†
corresponding to [Co(CO)4]À1 and the terminal CO ligands,
respectively.
Conclusions
The thickness of magnetically so Co shell (Fig. 4a) can be
controlled by changing the reaction conditions (detailed shell
thickness control of the cobalt coating is shown in Fig. S3†).
To investigate the weight ratio effects on the exchange
coupling between MnBi and Co phases, magnetic character-
istics are shown in the Fig. 4b. The magnetically so Co shell
coating is characteristic of high saturation magnetization
(Ms) and low coercivity (Hc). In other words, the Co coated
MnBi sample would exhibit a high Ms as increasing the Co
shell thickness or weight ratio contribution, while the Hc of
Co coated MnBi sample would decrease extensively. To
investigate the potential shape anisotropy effect from Co
nanowires, we have synthesized the magnetically so Co
nanowires which are mixed with MnBi at different ratios.
From the scanning electron microscopy (SEM, Fig. 4c) image
of cobalt nanowire coated MnBi, it should be noted that the
In this study, rare-earth-free MnBi nanoalloys are synthesized
by using the combination of chemical synthesis and thermal
annealing. The reaction conditions play an important role in
the formation of nal MnBi nanostructures. A magnetically so
cobalt phase is employed to exchange couple with the MnBi
alloy to further enhance its magnetization. The synthesized
rare-earth-free MnBi magnetic composites could nd their
unique green energy applications, such as low energy
consumption power electronics, wind generators and electric
motors.
Experimental section
Materials
Manganese acetate (98+%), bismuth chloride (98+%), n-buty-
lithium (1.6 M solution in hexanes) and diphenyl ether (99%)
were purchased from Acros Organics. Octacarbonyl dicobalt
(stab with 1–5% hexane, crystalline) was purchased from Alfa
Aesar. Hexane (99.9%) was purchased from Fisher Scientic.
Oleylamine (>50%) were purchased from Tokyo Chemical
Industry CO. LTD.
Synthesis of MnBi
All synthesis is done under argon atmosphere using standard
Schlenk line procedures. 0.4 mmol of Mn(CH3COO)2 and 0.4
mmol of BiCl3 was placed in a 50 mL 3-neck ask 1. Next, 15 mL
of diphenyl ether and 0.8 mmol of oleic acid were added to the
3-neck ask. Also, 5 mL of diphenyl ether and 4.8 mmol of 1.6 M
n-BuLi (in hexane) was added to a 25 mL 3-neck ask 2. The 3-
neck ask 1 was sonicated for ꢀ30 min, until all of the
Mn(CH3COO)2 and BiCl3 was dissolved. At the same time, the
ask 2 was connected to a Schlenk line under ultra-high pres-
sure (UHP) Ar, and the hexanes were removed from the n-BuLi
solution by vacuum for 30 min in order to prevent dangerous
overpressure during injection. Next, the ask 1 was attached to
the Schlenk line, ushed, and evacuated with Ar three times to
Fig. 4 (a) The TEM image of the Co coated MnBi core. (b) The weight
ratio between Co and MnBi dependent magnetic properties. (c) The
SEM image of Co nanowire coated MnBi. (d) The magnetic hysteresis
(M À H) loops of Co nanowire coated MnBi (the weight ratio depen-
dent) and MnBi alloy nanocrystals (the inset image).
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remove any air that may have leaked into the ask during
sonication. The ask 1 was then lled with Ar and heated to 200
ꢁC, at which point the n-BuLi solution was lled with Ar and was
injected rapidly into the ask 1. The nal mixture continued to
heat and stir for an additional 20 min at 200 ꢁC, then the
heating source was removed and the reaction was allowed to
cool to room temperature. The samples were separated by
centrifugation and puried by being re-dissolved and crashed
by hexane. The product aer purication was collected and
dried under Ar ow to avoid overpressure during annealing. In
order to obtain MnBi alloy, the resulted MnBi powders were
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Characterizations
XRD was done at room temperature using monochromated
Cu-Ka radiation on a Bruker proteum diffraction system
equipped with Helios multilayer optics, and APEX II CCD
detector and a Bruker MicroStart microfocus rotation anode
X-ray source operating at 45 kV and 60 mA. Powders were
suspended in Paratone N oil and placed into a nylon loop and
mounted on a goniometer head. TEM images and electron
diffraction patterns of the MnBi nanoparticles were recorded
using a eld emission FEI Tecnai F20 Xt at 200 kV. Samples
were dispersed in hexane, and one drop of the solution was
placed on a 200-mesh carbon-coated copper TEM grid.
Structural transformations of the MnBi nanoparticles were
independently monitored by a TA Instruments DSC-Q100
differential scanning calorimeter (DSC). The magnetic
hysteresis (M À H) loops were taken on a Microsense EZ27
vibrating sample magnetometer.
Acknowledgements
S. R. thanks the nancial support from the National Science
Foundation under Award no. NSF-CMMI-1332658 for material
synthesis and assembly. The U.S. Department of Energy's
Advanced Research Projects Agency-Energy (ARPA-E) under
contract no. 11/CJ000/09/03 is acknowledged (J. C at Pacic
Northwest National Laboratory, and S. R. at the University of
Kansas).
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