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Preparation and magnetic properties of anisotropic MnBi powders
Physica B: Condensed Matter 530 (2018) 322–326
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Preparation and magnetic properties of anisotropic MnBi powders Bingbing Li a, b, Yilong Ma a, b, *, Bin Shao a, Chunhong Li a, b, Dengming Chen a, b, **, JianChun Sun a, Qiang Zheng a, b, Xueguo Yin a, b a b
College of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China Chongqing Key Laboratory of Nano/Micro Composite Material and Device, Chongqing 401331, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Anisotropic MnBi powder Heat treatment Ball milling Microstructure Magnetic properties
MnBi low-temperature phase (LTP) powders with composition of Mn50Bi50 were prepared using traditional casting followed by annealing and subsequent low energy ball milling with the assistance of heptane. The microstructure and magnetic performances were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and magnetometer measurements. The content of LTP MnBi increases with the extension of annealing time, and reaches 89.1 wt. % after 25 h annealing. The particle size decreases with increased milling time and powders with an average size of less than 1 μm can be effectively obtained by extending milling time to 5 h. The coercivity increases with milling time, but long-time ball milling greatly induces Ms. The magnetically aligned MnBi powders ball milled for 2 h exhibits a ratio Mr/Ms value of 96.2%, Ms of 55.1 emu/g and Hc of 14.1 kOe, respectively. The nanocrystal can be obtained during early brief time milling process, and more than 2 h ball milling has negligible effect on grain size.
1. Introduction Rare earth (RE) permanent magnetic materials such Nd-Fe-B magnets show excellent room temperature performance and are vastly used in various applications in almost every aspect of our lives, like personal computers, aerospace and electric motors etc. [1,2]. However, current RE supply crisis and environmental issue caused by over exploitation of RE resources have been headaches throughout the world, especially countries rich in RE resources, such as China, is faced with horrendous environment pollution and severe resource waste [3–5]. Moreover, RE permanent magnets usually feature some serious disadvantages such as poor corrosion resistance, low Curie temperature and poor hightemperature magnetic performances [6,7]. Thus, it's imperative to find an alternative to RE permanent magnet, and the research for new and substitutable RE free magnetic materials has received considerable attention globally in recent years [8–10]. Out of the most investigated materials, MnBi is an attractive candidate of permanent magnets contain RE elements, especially the ones for elevated applications. The LTP MnBi phase exhibits attractive magnetic properties with high magnetocrystalline anisotropy (1.6 106 J/m3) and positive temperature coefficient of coercivity [11–16]. It's attractive that Hc of LTP MnBi increases with temperature and becomes much larger
than that of Nd-Fe-B magnets at elevated temperature, thus making it a potential alternative for hard magnetic materials under certain circumstances [17,18]. MnBi exhibits a coercivity of more than 2.8 T at 530 K and a maximum theoretic energy product of 17.6 MGOe [19]. However, the maximum energy product of single MnBi bulk magnet prepared recently is 8.4 MGOe, which is still much lower than most of RE-rich magnets can furnish at room temperature due to its relatively low saturation magnetization [20]. To make the best of MnBi's high temperature magnetic performance, MnBi can be used as a hard phase in an exchange coupled hard/soft magnet, which is a research hotspot in recent years [15,21–25]. This requires the size of MnBi particles close to its single domain range (0.3–0.5 μm) [26,27]. Synthesis of high performance single phase MnBi compound is the principle step for fabrication of MnBi based exchange coupled magnets. It is a challenge because of the peritectic reaction of Mn and Bi and easy oxidation of Mn [28,29]. Various approaches have been developed to obtain MnBi single phase, including arc-melting, melt-spinning followed by ball milling, sintering and chemical synthetic route [13,16,22,30–32]. However, preparation of separated and anisotropic MnBi high performance powders is still a trouble in industrial production. A traditional method such as directly ball milling of heat treated MnBi alloy ingot can prevent Mn from secondary volatilization during melt-spinning and has
* Corresponding author. College of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China. ** Corresponding author. College of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China. E-mail addresses: [email protected] (Y. Ma), [email protected] (D. Chen). https://doi.org/10.1016/j.physb.2017.11.085 Received 9 August 2017; Received in revised form 6 November 2017; Accepted 30 November 2017 Available online 5 December 2017 0921-4526/© 2017 Elsevier B.V. All rights reserved.
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Physica B: Condensed Matter 530 (2018) 322–326
Fig. 3. XRD patterns of MnBi powders with different milling times (a) 2 h, (b) 3 h, (c) 5 h; and (d) magnetically aligned powders after 2 h of balling milling.
Fig. 1. XRD patterns of (a) Mn50Bi50 ingot and the ingot annealed at 573 K for (b) 10 h, (c) 15 h, (d) 20 h, and (e) 25 h.
transition. The ingot heat treated at 573 K for 25 h was crushed into powders using mortar and pestle, and sieved using #180 and #200 mesh screens obtaining initial particles with average size of ~75 μm. Then low energy ball milling with the assistance of heptane for the initial powder was carried out for 2, 3, 5 h using home-made rotary milling equipment with a rotation speed of 150 rpm, the ball to powders weight ratio was 10:1. The as-milled powders were mixed with epoxy resin and then aligned in a mold under a magnetic field of 1.8 T at room temperature to obtain anisotropic powders. The structure properties were studied using X-ray diffraction (XRD) (Rigaku, SmartLab) with a Cu-Kα radiation and analyzed with Jade 9. Scanning electron microscopy (SEM) (JEOL, JSM-7800F) was used to examine the particle size and morphology of the powders. Microstructure characterization of the powders was further studied using transmission electron microscope (TEM) (JEOL, JEM-2100). Magnetic performances of powders before and after magnetic field alignment were measured using vibrating sample magnetometer (VSM) (Quantum Design, MPMS3 and JDAW-2000C&D).
Table 1 Phase content calculated by jade 9 for samples with different annealing time. Annealing time (h)
LTP MnBi content (wt. %)
Bi content (wt. %)
Mn content (wt. %)
0 10 15 20 25
54.8 79.1 84.1 85.5 89.1
39.4 16.1 15.8 14.0 10.4
5.7 4.8 0.1 0.4 0.5
been proved a feasible approach in industrial process [33]. In this work, the arc-melted MnBi ingot was heat treated and nanocrystal MnBi particles were obtained using low energy ball milling with the assistance of heptane to deduce agglomeration and oxidization. Then, the powders were aligned in a 1.8 T field, and the microstructure and magnetic properties were studied. 2. Experiment
3. Results and discussion
The MnBi powders with a nominal composition of Mn50Bi50 were prepared using arc-melting, annealing followed by ball milling. MnBi ingot was prepared from high purity elements (>99.9%) by arc-melting, the oxidation on the surface of high purity Mn chips was removed, and then Mn chips were crashed and mixed with Bi shots in approximately 1:1 atomic ratio. The mixture was pressed into a pellet before arcmelting, and the arc-melting process was repeated for three times for uniform composition. The obtained ingot was then annealed at 573 K for 10, 15, 20, 25 h to investigate the effect of heat treatment on phase
Fig. 1 shows the XRD patterns of Mn50Bi50 ingot sample before and after annealing at 573 K for different duration. As seen from Fig. 1, the peak intensities of MnBi increases after annealing, each pattern was analyzed by Whole Powder Pattern Fitting using jade 9 and the relative content fractions of LTP MnBi were calculated shown in Table 1. It shows that heat treatment leads to the formation of LTP MnBi phase, and the content of LTP MnBi increases with annealing time. The ingot annealed at 573 K for 25 h was selected for further
Fig. 2. Back scattered electron images of ingot (a) before and (b) after annealing at 573 K for 25 h. 323
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Physica B: Condensed Matter 530 (2018) 322–326
Fig. 4. SEM images of the MnBi powder after (a) 2 h and (b) 5 h of low energy ball milling.
from XRD patterns using Jade 9, the weight percentage of MnBi magnetic phase has decreased from 82 wt. % to 74 wt. % by extending ball milling time from 2 to 5 h. In Fig. 4(a), a SEM image from the sample obtained after 2 h of ball milling is presented. It shows particle size dramatically decreases from 75 μm to 0.5–5 μm, and most of particles show a size of approximately 2 μm. In the powder taken after 5 h of ball milling, most of the particles have reached an average size of less than 1 μm (Fig. 4(b)); there are also particles with different shape, forming agglomerates. Furthermore, MnBi powder after 2 h of ball milling shows more homogeneous particle size. To better understand the microstructure and the effect of low energy ball milling with the assistance of heptane on crystal structure of MnBi, the bright field and high resolution TEM studies on powders were performed. As shown in the left panels of Fig. 5(a) and (b), sample milled for 5 h were slightly smaller in grain size, the specimens consist of nanograins with an average size of 60 nm and 40 nm, respectively, this demonstrates that nano-crystalline has been obtained during early brief time ball milling, and more than 2 h of low energy ball milling has negligible effect on grain size. The size distribution in both the samples is uniform and some over-lapping grains are found according to bright field TEM images. The crystal planes of MnBi phase were marked out in high resolution TEM images in Fig. 5(a) and (b). The crystal lattice parameters in both of samples were confirmed using Fourier transform of the high resolution TEM images and shown in insets, the indexes of crystal planes for two specimens were determined and the zone axes in two regions corresponding to (220) and (020), respectively, which are consistent with the results of XRD patterns shown in Fig. 3. The demagnetization curves of annealed ingot and milled powders are shown in Fig. 6. It's
investigation because of its high content of LTP MnBi. The increase of Ms is closely related to phase transition after heat treatment, and the change can be also seen clearly in Fig. 2. Back scattered electron images of Mn50Bi50 ingot before and after annealing in Fig. 2 show a different phase contrast: black, dark gray and light gray, which responds to Mn, MnBi and Bi phase, respectively. It demonstrates that the content of LTP MnBi significantly increased after annealing. This can be attributed to the reaction of precipitated Mn and Bi during annealing, leading to the formation of LTP MnBi. Fig. 2(b) shows that LTP MnBi is uniformly distributed and forms a matrix for ingot after 25 h of annealing, and the Mn mental area is embedded in the LTP, suggesting that it is the product of HTP to LTP transformation. The Mn phase tends to segregate by peritectic reaction from MnBi high temperature phase (Mn1.08Bi) and causes nearby solution to be rich in Bi, which subsequently forms Bi areas near the Mn regions as Fig. 2 shown. The result indicates that high pure LTP MnBi can be obtained by 25 h of annealing. To improve the coercivity of MnBi powders, the effect of ball milling on particle size and magnetic properties were investigated [26,27,34,35]. XRD patterns of Mn50Bi50 powders prepared by 2, 3, and 5 h of low energy ball milling with the assistance of heptane are shown in Fig. 3. The Figure shows that the relative peak intensities of characteristic LTP MnBi decrease after 2 h of ball milling, indicates that LTP MnBi was decomposed during ball milling process, and the decomposition is increased with milling time. The change of MnBi phase was estimated
Fig. 5. TEM images of the specimens prepared from MnBi powders after (a) 2 h and (b) 5 h of ball milling. In both (a) and (b), the left panels show the TEM bright field images, the right panels show the high-resolution images and insets show the Fourier transform of the high resolution lattice fringes.
Fig. 6. Room temperature demagnetization curves of (a) MnBi ingot annealed at 573 K for 25 h and MnBi powders after (b) 2 h, (c) 3 h and (d) 5 h of ball milling. 324
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Physica B: Condensed Matter 530 (2018) 322–326
Fig. 7. (a) Room temperature demagnetization curves and (b) Ms and Hc versus ball milling time.
are increasing rapidly at the beginning, than increasing slower. The anisotropic powders milled for 2 h exhibit optimal magnetic performance with Ms of 55.1 emu/g, Hc of 14.1 kOe, and a ratio Mr/Ms value as high as 96.2% at room temperature, respectively. The decrease of Ms with an increase duration of milling time illuminates that LTP MnBi segregation occurred during ball milling process, a plummeting of Ms occurred when the ball milling duration were further extended from 2 to 5 h, which may be attributed to both phase decomposition and defects increase of powders.
Table 2 The main magnetic properties for specimens after different time of ball milling. Sample
Hc (kOe)
Ms (emu/g)
Mr/Ms
Milled (2 h) Milled (3 h) Milled (5 h)
14.036 15.040 16.044
55.10 48.27 35.59
96.2 95.0 90.7
obvious that Ms reduces with increasing ball milling duration, indicating that phase segregation occurred during milling process. Combined with the XRD patterns shown in Fig. 1, these results support the hypothesis that the volume fraction of magnetic LTP MnBi deduces due to decomposition and thus causing the reduction in Ms [36]. Fig. 7(a) shows the easy-axis room temperature M-H curves of the magnetic-field aligned MnBi powders milled different time. Anisotropic magnetic behavior has been observed in powders with wide and nearly rectangular demagnetization curves, and XRD pattern in Fig. 3 (d) also demonstrates the strong c-axis orientation. It's obvious that magnetic field alignment has generated a large gain in both coercivity and Mr/Ms of MnBi powders, this change results from the fact that the powders are locally textured and can be easily rotated with their easy axis parallel with field direction. As shown in Fig. 7(b), the Hc tend to increase from 14.1 kOe to 16.1 kOe with extending milling from 2 to 5 h; the coercivity enhancement in powders is mainly attributed to the refinement of MnBi powders and a lightly decrease in grain size [36–39], and local isolation of MnBi grains by precipitated Bi may be also have a positive effect on Hc [37]. Moreover, Fig. 7(b) shows that Hc increases slowly when the duration of ball milling time is larger than 3 h. However, Ms decreases from 57.2 emu/g for ingot to 35.6 emu/g for 5 h milled sample, the 15.4% reduction of the quality fraction occupied by the magnetic MnBi phase is consistent with the decrease in magnetization by about 37.6% in powder specimen. The ratio Mr/Ms value and Ms decrease linearly after milling time extends to 2 h, and the plummeting of Ms and Mr/Ms value is mainly attributed to the decomposition of LTP MnBi. The magnetic properties for specimens after different time of ball milling are shown in Table 2. Furthermore, increase of defects on the particle's surface also plays significant role, which act as nucleation centers for reversed magnetization, further studies will be needed to identify the effect of surface defects on magnetization and coercivity [21,37].
Acknowledgment This work was supported by the Graduate Science and Technology Innovation Project of Chongqing University of Science & Technology (YKJCX1620202, YKJCX1620205 and YKJCX1620209), China, the National Natural Science Foundation of China (51201191), Chongqing Research Program of Basic Research and Frontier Technology (cstc2016jcyjA0232, cstc2017jcyjAX0378, cstc2014jcyjA50010 and cstc2015jcyjA50004), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1601321 and KJ1709202), the Program for Innovation Teams in University of Chongqing, China (Grant No. CXTDX201601032) and Research Foundation of Chongqing University of Science & Technology (CK2015Z12). References [1] O. Gutfleisch, M.A. Willard, E. Brück, et al., Magnetic materials and devices for the 21st century: stronger, lighter, and more energy efficient, Adv. Mater. (Deerfield Beach, Fla.) 23 (7) (2011) 821–842. [2] J.M.D. Coey, Hard magnetic materials: a perspective, IEEE Trans. Magn. 47 (12) (2011) 4671–4681. [3] T. Dutta, K.H. Kim, M. Uchimiya, et al., Global demand for rare earth resources and strategies for green mining, Environ. Res. 150 (2016) 182–190. [4] K.Y. Zhu, S.Y. Zhao, S.L. Yang, et al., Where is the way for rare earth industry of China: an analysis via ANP-SWOT approach, Resour. Policy 49 (2016) 349–357. [5] L. Wang, B.Q. Zhong, T. Liang, et al., Atmospheric thorium pollution and inhalation exposure in the largest rare earth mining and smelting area in China, Sci. Total Environ. 572 (2016) 1–8. [6] S. Cao, M. Yue, Y.X. Yang, et al., Magnetic properties and thermal stability of MnBi/ NdFeB hybrid bonded magnets, J. Appl. Phys. 109 (7) (2011), 07A740-07A740-3. [7] M. Sagawa, in: Proceedings of the 21st Workshop on REPM and Their Applications, Slovenia, 2010, p. 183. [8] M.J. Kramer, R.W. Mccallum, I.A. Anderson, et al., Prospects for non-rare earth permanent magnets for traction motors and generators, JOM 64 (7) (2012) 752–763. [9] V.V. Nguyen, N. Poudyal, X.B. Liu, et al., High-performance MnBi alloy prepared using profiled heat treatment, IEEE Trans. Magn. 50 (12) (2014) 1–6. [10] N.V. Rama Rao, A.M. Gabay, W.F. Li, et al., Nanostructured bulk MnBi magnets fabricated by hot compaction of cryomilled powders, J. Phys. D. Appl. Phys. 46 (26) (2013) 1–5. [11] T. Chen, W.E. Stutius, The phase transformation and physical properties of the MnBi and Mn1.08Bi compounds, IEEE Trans. Magn. 10 (3) (1974) 581–586. [12] D.T. Zhang, S. Cao, M. Yue, et al., Structural and magnetic properties of bulk MnBi permanent magnets, J. Appl. Phys. 109 (7) (2011), 07A722-07A722-3. [13] P. Kharel, V.R. Shah, R. Skomski, et al., Magnetism of MnBi-based nanomaterials, IEEE Trans. Magn. 49 (7) (2013) 3318–3321. [14] Y.B. Yang, X.G. Chen, S. Guo, et al., Temperature dependences of structure and coercivity for melt-spun MnBi compound, J. Magn. Magn. Mater. 330 (3) (2013) 106–110.
4. Conclusion High purity MnBi ingot has LTP phase up to 89.1 wt. % was obtained through arc-melting and 25 h of annealing, then anisotropic particles with an average size of less than 1 μm can be effectively obtained through ball milling and magnetic field alignment. The process is compatible with current industrial production and easy to implement. Magnetically aligned MnBi powders display strong c-axis orientation, enhanced coercivity and ratio Mr/Ms value. With milling time extending, the coercivity 325
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[15] J. Cui, J.P. Choi, G. Li, et al., Thermal stability of MnBi magnetic materials, J. Phys. Condens. Mat. 26 (6) (2014), 064212. [16] S. Kavita, U.M.R. Seelam, D. Prabhu, et al., On the temperature dependent magnetic properties of as-spun Mn–Bi ribbons, J. Magn. Magn. Mater. 377 (377) (2015) 485–489. [17] X. Guo, X. Chen, Z. Altounian, et al., Magnetic properties of MnBi prepared by rapid solidification, Phys. Rev. B Condens. Matter 46 (22) (1992) 14578–14582. [18] N.V. Rama Rao, A.M. Gabay, G.C. Hadjipanayis, Anisotropic, hybrid magnets fabricated by hot compaction, IEEE Trans. Magn. 49 (7) (2013) 3255–3257. [19] N.V. Rama Rao, A.M. Gabay, G.C. Hadjipanayis, Anisotropic fully dense MnBi permanent magnet with high energy product and high coercivity at elevated temperatures, J. Phys. D. Appl. Phys. 46 (6) (2013) 1–4. [20] N. Poudyal, X.B. Liu, W. Wang, et al., Processing of MnBi bulk magnets with enhanced energy product, AIP. Adv. 6 (5) (2016) 1–7. [21] K. Kanari, C. Sarafidis, M. Gjoka, et al., Processing of magnetically anisotropic MnBi particles by surfactant assisted ball milling, J. Magn. Magn. Mater. 426 (15) (2016) 691–697. [22] K. Kanari, C. Sarafidis, M. Gjoka, et al., Processing of magnetically anisotropic MnBi particles by surfactant assisted ball milling, J. Magn. Magn. Mater. 426 (2016) 691–697. [23] J. Shen, H.Z. Cui, X.P. Huang, et al., Synthesis and characterization of rare-earthfree magnetic manganese bismuth nanocrystals, Rsc. Adv. 5 (8) (2014) 5567–5570. [24] X. Xu, Y.K. Hong, J. Park, et al., Magnetic self-assembly for the synthesis of magnetically exchange coupled MnBi/Fe–Co composites, J. Solid State Chem. 231 (2) (2015) 108–113. [25] B. Li, W. Liu, X.G. Zhao, et al., The structural and magnetic properties of MnBi and exchange coupled MnBi/Fe films, J. Magn. Magn. Mater. 372 (12) (2014) 12–15. [26] Y.L. Ma, X.B. Liu, K. Gandha, et al., Preparation and magnetic properties of MnBibased hard/soft composite magnets, J. Appl. Phys. 115 (17) (2014), 17A75517A755-3. [27] P.K. Nguyen, S. Jin, A.E. Berkowitz, Unexpected magnetic domain behavior in LTPMnBi, IEEE Trans. Magn. 49 (7) (2013) 3387–3390.
326
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Preparation and magnetic properties of anisotropic MnBi powders Bingbing Li a, b, Yilong Ma a, b, *, Bin Shao a, Chunhong Li a, b, Dengming Chen a, b, **, JianChun Sun a, Qiang Zheng a, b, Xueguo Yin a, b a b
College of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China Chongqing Key Laboratory of Nano/Micro Composite Material and Device, Chongqing 401331, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Anisotropic MnBi powder Heat treatment Ball milling Microstructure Magnetic properties
MnBi low-temperature phase (LTP) powders with composition of Mn50Bi50 were prepared using traditional casting followed by annealing and subsequent low energy ball milling with the assistance of heptane. The microstructure and magnetic performances were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and magnetometer measurements. The content of LTP MnBi increases with the extension of annealing time, and reaches 89.1 wt. % after 25 h annealing. The particle size decreases with increased milling time and powders with an average size of less than 1 μm can be effectively obtained by extending milling time to 5 h. The coercivity increases with milling time, but long-time ball milling greatly induces Ms. The magnetically aligned MnBi powders ball milled for 2 h exhibits a ratio Mr/Ms value of 96.2%, Ms of 55.1 emu/g and Hc of 14.1 kOe, respectively. The nanocrystal can be obtained during early brief time milling process, and more than 2 h ball milling has negligible effect on grain size.
1. Introduction Rare earth (RE) permanent magnetic materials such Nd-Fe-B magnets show excellent room temperature performance and are vastly used in various applications in almost every aspect of our lives, like personal computers, aerospace and electric motors etc. [1,2]. However, current RE supply crisis and environmental issue caused by over exploitation of RE resources have been headaches throughout the world, especially countries rich in RE resources, such as China, is faced with horrendous environment pollution and severe resource waste [3–5]. Moreover, RE permanent magnets usually feature some serious disadvantages such as poor corrosion resistance, low Curie temperature and poor hightemperature magnetic performances [6,7]. Thus, it's imperative to find an alternative to RE permanent magnet, and the research for new and substitutable RE free magnetic materials has received considerable attention globally in recent years [8–10]. Out of the most investigated materials, MnBi is an attractive candidate of permanent magnets contain RE elements, especially the ones for elevated applications. The LTP MnBi phase exhibits attractive magnetic properties with high magnetocrystalline anisotropy (1.6 106 J/m3) and positive temperature coefficient of coercivity [11–16]. It's attractive that Hc of LTP MnBi increases with temperature and becomes much larger
than that of Nd-Fe-B magnets at elevated temperature, thus making it a potential alternative for hard magnetic materials under certain circumstances [17,18]. MnBi exhibits a coercivity of more than 2.8 T at 530 K and a maximum theoretic energy product of 17.6 MGOe [19]. However, the maximum energy product of single MnBi bulk magnet prepared recently is 8.4 MGOe, which is still much lower than most of RE-rich magnets can furnish at room temperature due to its relatively low saturation magnetization [20]. To make the best of MnBi's high temperature magnetic performance, MnBi can be used as a hard phase in an exchange coupled hard/soft magnet, which is a research hotspot in recent years [15,21–25]. This requires the size of MnBi particles close to its single domain range (0.3–0.5 μm) [26,27]. Synthesis of high performance single phase MnBi compound is the principle step for fabrication of MnBi based exchange coupled magnets. It is a challenge because of the peritectic reaction of Mn and Bi and easy oxidation of Mn [28,29]. Various approaches have been developed to obtain MnBi single phase, including arc-melting, melt-spinning followed by ball milling, sintering and chemical synthetic route [13,16,22,30–32]. However, preparation of separated and anisotropic MnBi high performance powders is still a trouble in industrial production. A traditional method such as directly ball milling of heat treated MnBi alloy ingot can prevent Mn from secondary volatilization during melt-spinning and has
* Corresponding author. College of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China. ** Corresponding author. College of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China. E-mail addresses: [email protected] (Y. Ma), [email protected] (D. Chen). https://doi.org/10.1016/j.physb.2017.11.085 Received 9 August 2017; Received in revised form 6 November 2017; Accepted 30 November 2017 Available online 5 December 2017 0921-4526/© 2017 Elsevier B.V. All rights reserved.
B. Li et al.
Physica B: Condensed Matter 530 (2018) 322–326
Fig. 3. XRD patterns of MnBi powders with different milling times (a) 2 h, (b) 3 h, (c) 5 h; and (d) magnetically aligned powders after 2 h of balling milling.
Fig. 1. XRD patterns of (a) Mn50Bi50 ingot and the ingot annealed at 573 K for (b) 10 h, (c) 15 h, (d) 20 h, and (e) 25 h.
transition. The ingot heat treated at 573 K for 25 h was crushed into powders using mortar and pestle, and sieved using #180 and #200 mesh screens obtaining initial particles with average size of ~75 μm. Then low energy ball milling with the assistance of heptane for the initial powder was carried out for 2, 3, 5 h using home-made rotary milling equipment with a rotation speed of 150 rpm, the ball to powders weight ratio was 10:1. The as-milled powders were mixed with epoxy resin and then aligned in a mold under a magnetic field of 1.8 T at room temperature to obtain anisotropic powders. The structure properties were studied using X-ray diffraction (XRD) (Rigaku, SmartLab) with a Cu-Kα radiation and analyzed with Jade 9. Scanning electron microscopy (SEM) (JEOL, JSM-7800F) was used to examine the particle size and morphology of the powders. Microstructure characterization of the powders was further studied using transmission electron microscope (TEM) (JEOL, JEM-2100). Magnetic performances of powders before and after magnetic field alignment were measured using vibrating sample magnetometer (VSM) (Quantum Design, MPMS3 and JDAW-2000C&D).
Table 1 Phase content calculated by jade 9 for samples with different annealing time. Annealing time (h)
LTP MnBi content (wt. %)
Bi content (wt. %)
Mn content (wt. %)
0 10 15 20 25
54.8 79.1 84.1 85.5 89.1
39.4 16.1 15.8 14.0 10.4
5.7 4.8 0.1 0.4 0.5
been proved a feasible approach in industrial process [33]. In this work, the arc-melted MnBi ingot was heat treated and nanocrystal MnBi particles were obtained using low energy ball milling with the assistance of heptane to deduce agglomeration and oxidization. Then, the powders were aligned in a 1.8 T field, and the microstructure and magnetic properties were studied. 2. Experiment
3. Results and discussion
The MnBi powders with a nominal composition of Mn50Bi50 were prepared using arc-melting, annealing followed by ball milling. MnBi ingot was prepared from high purity elements (>99.9%) by arc-melting, the oxidation on the surface of high purity Mn chips was removed, and then Mn chips were crashed and mixed with Bi shots in approximately 1:1 atomic ratio. The mixture was pressed into a pellet before arcmelting, and the arc-melting process was repeated for three times for uniform composition. The obtained ingot was then annealed at 573 K for 10, 15, 20, 25 h to investigate the effect of heat treatment on phase
Fig. 1 shows the XRD patterns of Mn50Bi50 ingot sample before and after annealing at 573 K for different duration. As seen from Fig. 1, the peak intensities of MnBi increases after annealing, each pattern was analyzed by Whole Powder Pattern Fitting using jade 9 and the relative content fractions of LTP MnBi were calculated shown in Table 1. It shows that heat treatment leads to the formation of LTP MnBi phase, and the content of LTP MnBi increases with annealing time. The ingot annealed at 573 K for 25 h was selected for further
Fig. 2. Back scattered electron images of ingot (a) before and (b) after annealing at 573 K for 25 h. 323
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Fig. 4. SEM images of the MnBi powder after (a) 2 h and (b) 5 h of low energy ball milling.
from XRD patterns using Jade 9, the weight percentage of MnBi magnetic phase has decreased from 82 wt. % to 74 wt. % by extending ball milling time from 2 to 5 h. In Fig. 4(a), a SEM image from the sample obtained after 2 h of ball milling is presented. It shows particle size dramatically decreases from 75 μm to 0.5–5 μm, and most of particles show a size of approximately 2 μm. In the powder taken after 5 h of ball milling, most of the particles have reached an average size of less than 1 μm (Fig. 4(b)); there are also particles with different shape, forming agglomerates. Furthermore, MnBi powder after 2 h of ball milling shows more homogeneous particle size. To better understand the microstructure and the effect of low energy ball milling with the assistance of heptane on crystal structure of MnBi, the bright field and high resolution TEM studies on powders were performed. As shown in the left panels of Fig. 5(a) and (b), sample milled for 5 h were slightly smaller in grain size, the specimens consist of nanograins with an average size of 60 nm and 40 nm, respectively, this demonstrates that nano-crystalline has been obtained during early brief time ball milling, and more than 2 h of low energy ball milling has negligible effect on grain size. The size distribution in both the samples is uniform and some over-lapping grains are found according to bright field TEM images. The crystal planes of MnBi phase were marked out in high resolution TEM images in Fig. 5(a) and (b). The crystal lattice parameters in both of samples were confirmed using Fourier transform of the high resolution TEM images and shown in insets, the indexes of crystal planes for two specimens were determined and the zone axes in two regions corresponding to (220) and (020), respectively, which are consistent with the results of XRD patterns shown in Fig. 3. The demagnetization curves of annealed ingot and milled powders are shown in Fig. 6. It's
investigation because of its high content of LTP MnBi. The increase of Ms is closely related to phase transition after heat treatment, and the change can be also seen clearly in Fig. 2. Back scattered electron images of Mn50Bi50 ingot before and after annealing in Fig. 2 show a different phase contrast: black, dark gray and light gray, which responds to Mn, MnBi and Bi phase, respectively. It demonstrates that the content of LTP MnBi significantly increased after annealing. This can be attributed to the reaction of precipitated Mn and Bi during annealing, leading to the formation of LTP MnBi. Fig. 2(b) shows that LTP MnBi is uniformly distributed and forms a matrix for ingot after 25 h of annealing, and the Mn mental area is embedded in the LTP, suggesting that it is the product of HTP to LTP transformation. The Mn phase tends to segregate by peritectic reaction from MnBi high temperature phase (Mn1.08Bi) and causes nearby solution to be rich in Bi, which subsequently forms Bi areas near the Mn regions as Fig. 2 shown. The result indicates that high pure LTP MnBi can be obtained by 25 h of annealing. To improve the coercivity of MnBi powders, the effect of ball milling on particle size and magnetic properties were investigated [26,27,34,35]. XRD patterns of Mn50Bi50 powders prepared by 2, 3, and 5 h of low energy ball milling with the assistance of heptane are shown in Fig. 3. The Figure shows that the relative peak intensities of characteristic LTP MnBi decrease after 2 h of ball milling, indicates that LTP MnBi was decomposed during ball milling process, and the decomposition is increased with milling time. The change of MnBi phase was estimated
Fig. 5. TEM images of the specimens prepared from MnBi powders after (a) 2 h and (b) 5 h of ball milling. In both (a) and (b), the left panels show the TEM bright field images, the right panels show the high-resolution images and insets show the Fourier transform of the high resolution lattice fringes.
Fig. 6. Room temperature demagnetization curves of (a) MnBi ingot annealed at 573 K for 25 h and MnBi powders after (b) 2 h, (c) 3 h and (d) 5 h of ball milling. 324
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Fig. 7. (a) Room temperature demagnetization curves and (b) Ms and Hc versus ball milling time.
are increasing rapidly at the beginning, than increasing slower. The anisotropic powders milled for 2 h exhibit optimal magnetic performance with Ms of 55.1 emu/g, Hc of 14.1 kOe, and a ratio Mr/Ms value as high as 96.2% at room temperature, respectively. The decrease of Ms with an increase duration of milling time illuminates that LTP MnBi segregation occurred during ball milling process, a plummeting of Ms occurred when the ball milling duration were further extended from 2 to 5 h, which may be attributed to both phase decomposition and defects increase of powders.
Table 2 The main magnetic properties for specimens after different time of ball milling. Sample
Hc (kOe)
Ms (emu/g)
Mr/Ms
Milled (2 h) Milled (3 h) Milled (5 h)
14.036 15.040 16.044
55.10 48.27 35.59
96.2 95.0 90.7
obvious that Ms reduces with increasing ball milling duration, indicating that phase segregation occurred during milling process. Combined with the XRD patterns shown in Fig. 1, these results support the hypothesis that the volume fraction of magnetic LTP MnBi deduces due to decomposition and thus causing the reduction in Ms [36]. Fig. 7(a) shows the easy-axis room temperature M-H curves of the magnetic-field aligned MnBi powders milled different time. Anisotropic magnetic behavior has been observed in powders with wide and nearly rectangular demagnetization curves, and XRD pattern in Fig. 3 (d) also demonstrates the strong c-axis orientation. It's obvious that magnetic field alignment has generated a large gain in both coercivity and Mr/Ms of MnBi powders, this change results from the fact that the powders are locally textured and can be easily rotated with their easy axis parallel with field direction. As shown in Fig. 7(b), the Hc tend to increase from 14.1 kOe to 16.1 kOe with extending milling from 2 to 5 h; the coercivity enhancement in powders is mainly attributed to the refinement of MnBi powders and a lightly decrease in grain size [36–39], and local isolation of MnBi grains by precipitated Bi may be also have a positive effect on Hc [37]. Moreover, Fig. 7(b) shows that Hc increases slowly when the duration of ball milling time is larger than 3 h. However, Ms decreases from 57.2 emu/g for ingot to 35.6 emu/g for 5 h milled sample, the 15.4% reduction of the quality fraction occupied by the magnetic MnBi phase is consistent with the decrease in magnetization by about 37.6% in powder specimen. The ratio Mr/Ms value and Ms decrease linearly after milling time extends to 2 h, and the plummeting of Ms and Mr/Ms value is mainly attributed to the decomposition of LTP MnBi. The magnetic properties for specimens after different time of ball milling are shown in Table 2. Furthermore, increase of defects on the particle's surface also plays significant role, which act as nucleation centers for reversed magnetization, further studies will be needed to identify the effect of surface defects on magnetization and coercivity [21,37].
Acknowledgment This work was supported by the Graduate Science and Technology Innovation Project of Chongqing University of Science & Technology (YKJCX1620202, YKJCX1620205 and YKJCX1620209), China, the National Natural Science Foundation of China (51201191), Chongqing Research Program of Basic Research and Frontier Technology (cstc2016jcyjA0232, cstc2017jcyjAX0378, cstc2014jcyjA50010 and cstc2015jcyjA50004), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1601321 and KJ1709202), the Program for Innovation Teams in University of Chongqing, China (Grant No. CXTDX201601032) and Research Foundation of Chongqing University of Science & Technology (CK2015Z12). References [1] O. Gutfleisch, M.A. Willard, E. Brück, et al., Magnetic materials and devices for the 21st century: stronger, lighter, and more energy efficient, Adv. Mater. (Deerfield Beach, Fla.) 23 (7) (2011) 821–842. [2] J.M.D. Coey, Hard magnetic materials: a perspective, IEEE Trans. Magn. 47 (12) (2011) 4671–4681. [3] T. Dutta, K.H. Kim, M. Uchimiya, et al., Global demand for rare earth resources and strategies for green mining, Environ. Res. 150 (2016) 182–190. [4] K.Y. Zhu, S.Y. Zhao, S.L. Yang, et al., Where is the way for rare earth industry of China: an analysis via ANP-SWOT approach, Resour. Policy 49 (2016) 349–357. [5] L. Wang, B.Q. Zhong, T. Liang, et al., Atmospheric thorium pollution and inhalation exposure in the largest rare earth mining and smelting area in China, Sci. Total Environ. 572 (2016) 1–8. [6] S. Cao, M. Yue, Y.X. Yang, et al., Magnetic properties and thermal stability of MnBi/ NdFeB hybrid bonded magnets, J. Appl. Phys. 109 (7) (2011), 07A740-07A740-3. [7] M. Sagawa, in: Proceedings of the 21st Workshop on REPM and Their Applications, Slovenia, 2010, p. 183. [8] M.J. Kramer, R.W. Mccallum, I.A. Anderson, et al., Prospects for non-rare earth permanent magnets for traction motors and generators, JOM 64 (7) (2012) 752–763. [9] V.V. Nguyen, N. Poudyal, X.B. Liu, et al., High-performance MnBi alloy prepared using profiled heat treatment, IEEE Trans. Magn. 50 (12) (2014) 1–6. [10] N.V. Rama Rao, A.M. Gabay, W.F. Li, et al., Nanostructured bulk MnBi magnets fabricated by hot compaction of cryomilled powders, J. Phys. D. Appl. Phys. 46 (26) (2013) 1–5. [11] T. Chen, W.E. Stutius, The phase transformation and physical properties of the MnBi and Mn1.08Bi compounds, IEEE Trans. Magn. 10 (3) (1974) 581–586. [12] D.T. Zhang, S. Cao, M. Yue, et al., Structural and magnetic properties of bulk MnBi permanent magnets, J. Appl. Phys. 109 (7) (2011), 07A722-07A722-3. [13] P. Kharel, V.R. Shah, R. Skomski, et al., Magnetism of MnBi-based nanomaterials, IEEE Trans. Magn. 49 (7) (2013) 3318–3321. [14] Y.B. Yang, X.G. Chen, S. Guo, et al., Temperature dependences of structure and coercivity for melt-spun MnBi compound, J. Magn. Magn. Mater. 330 (3) (2013) 106–110.
4. Conclusion High purity MnBi ingot has LTP phase up to 89.1 wt. % was obtained through arc-melting and 25 h of annealing, then anisotropic particles with an average size of less than 1 μm can be effectively obtained through ball milling and magnetic field alignment. The process is compatible with current industrial production and easy to implement. Magnetically aligned MnBi powders display strong c-axis orientation, enhanced coercivity and ratio Mr/Ms value. With milling time extending, the coercivity 325
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