MnBi Alloys Research Articles

Transformation of in-plane to out-of-plane anisotropy in MnBi alloy for permanent magnet application: a First-principles study

The low-temperature phase (LTP) MnBi exhibits remarkable ferromagnetic properties at room temperature. However, below its Curie temperature (TC\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${T}_{C}$$\\end{document}), a phase transition occurs around 613 K due to diffusion of Mn into interstitial sites, raising concerns about its structural and magnetic properties. Furthermore, the presence of in-plane anisotropy in LTP-MnBi alloy at low temperatures raises concerns about its suitability for use in permanent magnet applications, even at higher temperature. Therefore, this study examines the structural and magnetic properties of pure LTP-MnBi and its successive Ni-doped and Fe-substituted alloys using first-principles study based on density functional theory (DFT). To prevent Mn diffusion into interstitial sites, Ni doping is employed. Additionally, the incorporation of Ni successfully addresses the in-plane anisotropy issue in LTP-MnBi, transforming it into out-of-plane anisotropy. Moreover, we explored the potential advantages of substituting Fe for one of Mn site. This substitution aims to improve the observed dynamical instability in Ni-doped alloy and to further enhanced the magnetocrystalline anisotropy energy (MAE) of the material, resulting in an MAE of 3.21 MJ/m3, along with a TC\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${T}_{C}$$\\end{document} of 523 K. Therefore, the coexistence of high MAE and moderate TC\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${T}_{C}$$\\end{document} in the Mn0.5Fe0.5Bi–Ni alloy presents viable option for its application in permanent magnet technology.

Influence of Sn addition on the phase formation, microstructure and magnetic properties of Mn-Bi alloy ribbons

In this work, the phase formation, microstructure and magnetic properties of Mn54Bi46−xSnx (x = 0, 1, 2, 3 and 4) alloy ribbons fabricated by melt-spinning method combined with annealing were investigated. The fraction of the low-temperature MnBi phase (LTP-MnBi) depends not only on the Sn concentration but also on the annealing regime. In the as-quenched ribbons, the fraction of the LTP-MnBi decreases monotonically from 38.1 to 21.1 wt% when the Sn concentration increases from 0 to 4 at%. Meanwhile, the fraction of this phase increases significantly from 69.0 to 83.2 wt% when the Sn concentration changes from 0 to 2 at% and tends to decrease upon further increasing the Sn concentration to 4 at% for the ribbons annealed at 250 °C for 16 h. Saturation magnetization, Ms, of the annealed samples changes consistently with the fraction of LPT-MnBi and reaches a maximum value of 70.4 emu/g in the sample with Sn concentration of 2 at%. The addition of Sn also aids in refining the grains, leading to an improvement in the hard magnetic properties of the material. The coercivity, Hc, of the alloy ribbons tends to increase as the Sn concentration increases and changes non-monotonically with annealing time. The maximum energy product, (BH)max, reached 5.5 MGOe in the sample with x = 2, demonstrating the promise of Mn-Bi based alloys in practical applications. The density functional theory calculations were also performed to clarify the effect of Sn addition. The calculated results confirmed the decrease in the lattice parameters with Sn substitution in the experimental data. The total magnetic moment per formula unit and the evaluated saturation magnetization are 3.62 µB and 78.28 emu/g, respectively, for Sn substitution of ∼3.1 at%. (Although the total magnetic moment decrease with doping, the saturation magnetization per unit mass could be improved.)

Magnetic property enhancement of rare-earth-free nanocrystalline LTP-MnBi melt-spun ribbons

Rare-earth-free low-temperature-phase MnBi alloys have attracted extensive attention, but achieving high-purity and high-performance remains a challenge. In this study, a low-temperature phase (LTP) MnBi alloy was prepared through melt spinning and vacuum annealing techniques. The crystalline structure, microstructure, and magnetic properties of MnBi alloys were systematically investigated by x-ray diffraction, scanning electron microscopy, and physical property measurement system. Results showed that roller speed effectively improved the purity and coercivity of MnBi alloys. The optimization of process parameters achieved a high saturation magnetization of 70.7 A m2/kg and a significant coercivity of 2.34 T (at 450 K). It revealed that microstructure and magnetic properties were closely associated with rolling speed processes. This work serves as an exemplar for the fabrication of high-performance LTP-MnBi alloys, exhibiting its potential for high-temperature applications.

Cu-doping induced tuning of magnetic properties and phase transformation in MnBi alloys

This study investigates the effect of Cu doping on the magnetic properties of MnBi alloy at sub-ambient temperatures. The Curie temperatures (Tc) were observed at 175.2 and 161.6 K for Mn52Bi45Cu3 and Mn51Bi45Cu4 alloys, respectively. The Mn55Bi45 sample demonstrates a notable saturation magnetization (Ms) of 72 emu/g, reaching 90% of the theoretical value, indicating the successful synthesis of high-purity ferromagnetic low-temperature phase MnBi. Compared to Mn55Bi45, all Cu-doped samples exhibit higher coercivity (Hc) within the temperature range of 10–200 K. In particular, the sample with 4 at. % Cu exhibits an Hc of 7.4 kOe at 10 K, which is 37 times higher than that of the MnBi sample (0.2 kOe), and it remains significantly higher than that of the 0–3 at. % Cu-doped samples, even at 200 K. Further details of this investigation are presented in this paper.

Microstructure and magnetic properties of MnBi powders prepared by various ball milling processes

The MnBi intermetallic compound with high purity of low-temperature phase MnBi (LTP-MnBi) and high maximum energy product (BH)max were successfully prepared using various ball milling processes. The effects of ball milling speed and duration on the microstructures, phase compositions, and magnetic properties of MnBi alloys were systematically investigated. The particle size of MnBi and the purity of LTP-MnBi gradually decreased with increasing ball milling speed. An optimized ball milling speed of 175 rpm resulted in MnBi powder with the (BH)max of 9.22 MGOe and a squareness (M r/M s) of 93.7%. Furthermore, the ball milling duration was optimized to observe its impact on the MnBi alloy. As the duration increased, the average particle size of MnBi decreased from 27 μm at 0 h to 1.84 μm at 10 h. Simultaneously, the coercivity (H c) increased from 0.7 T at 0 h to 1.7 T at 10 h. Ultimately, the powder ball milled at 175 rpm for 5 h exhibited the highest (BH)max of 11.3 MGOe.

The effect of Ge substitution on phase transformation and magnetic properties of MnBi alloys

The Mn55Bi45-xGex intermetallic compound with x ≤ 4 was prepared and compared by annealed ribbons and powder metallurgy. Substituting Ge in the MnBi alloy significantly alters the phase transformation temperatures, enhances the coercivity (Hc), and improves the squareness. As the Ge concentration increased from 0 at.% to 4 at.%, both the crystalline size of low-temperature phase MnBi in annealed ribbons and ball-milled powders demonstrated a gradual decrease, while lattice strain exhibited an upward trend. The Hc of Mn55Bi41Ge4 powder reaches 14.1 kOe, which is 33 % higher than that of pure MnBi powder, making MnBi-Ge an ideal candidate magnet for the next generation of rare-earth-free magnets.

Effects of Mo doping on the phase transformation behaviors and the magnetic properties of rare-earth-free MnBi alloy

The high-performance rare-earth-free low-temperature phase Mn–Bi-based permanent magnet alloys (LTP-MnBi) have gained widespread attention, but it still presents a challenge. Here, the LTP-MnBi alloy with Mo element doping was prepared by using arc-melting and subsequent high-vacuum annealing. The crystalline structure, phase transformation behavior, and magnetic properties were systematically investigated. The result showed that Mo element doping significantly improved the stability of the LTP in MnBi alloy. The investigation revealed that the disparity in atomic radius between Mo and Mn, Bi elements played a pivotal role in the formation and stability of the LTP-MnBi. With the increase of Mo content in the LTP-MnBi alloy, the coercivity value remarkably increased from 8.1 kOe to 11.7 kOe with a decrease of saturation magnetization from 71.6 emu/g to 56.5 emu/g, and the maximum energy product gradually reduced from 10.6 MGOe to 7.9 MGOe. The underlying mechanism revealed that magnetic properties were closely related to the fraction and crystal structure of LTP-MnBi. Therefore, magnetic properties of Mo-doped MnBi alloy make them an attractive candidate among the rare-earth-free permanent magnets.

Structural and magnetic properties of Sb substituted LTP Mn-Bi annealed ribbons

We optimized the composition of Mn-Bi alloy, examined the effects of third element substitution of Sn, Cu, Al, and Sb on the magnetic properties, and investigated the microstructures and magnetic properties of Sb substituted Mn-Bi annealed ribbons. The optimized composition was Mn54Bi46, and it was found that the Sb substitution had the most positive effects on the magnetic properties among the substitution elements. Thus, we accomplished a detailed investigation on Mn54Bi46-xSbx (0.0 ≤ x ≤ 5.0) annealed ribbons. The fracture planes of the annealed ribbons revealed that the grain size drastically decreased from a few tens of microns to submicrons close to the single domain size, which substantially enhanced the coercivity (Hc), remanent magnetization (Mr), and maximum energy product ((BH)max). As a result, as the Sb content changed from 0 to 1.5, the Mr, Hc, and (BH)max of randomly oriented samples increased significantly from 14.7 emu/g, 0.50 kOe, and 0.15 MGOe to 37.2 emu/g, 8.15 kOe, and 3.27 MGOe, respectively. Furthermore, the temperature dependence of Hc on the Sb substitution showed significant enhancement over a wide temperature range from 223 to 378 K, suggesting that Mn-Bi alloy can be used in electrical vehicles.

Effect of magnetic field annealing on magnetic properties and corrosion resistance of MnBi alloy

Herein, MnBi alloys are prepared using melt spinning and magnetic-field annealing techniques, and the effects of magnetic-field annealing on the microstructure, magnetic properties, magnetisation behaviour, and corrosion resistance of MnBi are systematically investigated. The results show that the saturation magnetisation (Ms) of the Mn55Bi45 ribbons increases with increasing magnetic field annealing temperature and time. Annealing above Curie temperature (Tc) enhances the intrinsic coercivity (Hcj) of the MnBi alloy. Moreover, Mn55Bi45 ribbons with a (BH)max of 2.15 MGOe at room temperature are successfully fabricated after annealing at 673 K for 20 min. The first-order reversal curve (FORC) demonstrate that the Mn55Bi45 ribbons after magnetic-field annealing exhibits weaker exchange coupling compared with the Mn55Bi45 ribbons after nonmagnetic-field annealing. Additionally, the corrosion rate of the Mn55Bi45 ribbons after magnetic-field annealing is lower than that of the Mn55Bi45 ribbons after nonmagnetic-field annealing. Finally, the changes in the grain size and content of the MnBi phase are investigated.

A core–shell magnet Mn70Bi30 grown at seeds in magnetic fields and its impacts on its spin-dynamics, Curie point and other tailored properties

The MnBi alloys is a model series of rare-Earth free magnets for surge of technologies of small parts of automobiles, power generators, medical tools, memory systems, and many others. The magnetics stem primarily at unpaired Mn-3d5 spins (a 4.23 μ B moment) align parallel via an orbital moment 0.27 μ B of Bi-5d106s2p3 in a crystal lattice. Thus, using a surplus Mn (over Bi) in a Mn70Bi30 type alloy designs a spin-rich system of duly tailored properties useful for magnetics and other devices. In this view, we report here a strategy of a refined alloy powder Mn70Bi30 can grow into small crystals of hexagonal (h) plates at seeds as annealed in magnetic fields (in H2 gas). So, small h-plates (30 to 50 nm widths) are grown up at (002) facets, wherein the edges are turned down in a spiral (≤2.1 nm thicknesses) in a core–shell structure. The results are described with x-ray diffraction, lattice images and magnetic properties of a powder Mn70Bi30 (milled in glycine) is annealed at 573 K for different time periods, so to the Mn/Bi order at the permeable facets (seeds). Duly annealed samples exhibit an enhanced magnetization, M s → 70.8 emu g−1, with duly promoted coercivity H c → 10.810 kOe (15.910 kOe at 350 K), energy–product 14.8 MGOe, and the crystal-field-anisotropy, K 1 → 7.6 × 107 erg cm−3, reported at room temperature. Otherwise, M s should decline at any surplus 3d5-Mn spins order antiparallel at the antisites. Enhanced Curie point 658.1 K (628 K at Mn50Bi50 alloy) anticipates that a surplus Mn does favor the Mn–Bi exchange interactions. Proposed spin models well describe the spin-dynamics and lattice relaxations (on anneals) over the lattice volume (with twins) and spin clusters.

Effect of Mg content on the microstructure and magnetic properties of rare-earth-free MnBi alloys

The low-temperature phase MnBi (LTP-MnBi) has become one of the most promising rare-earth-free permanent magnetic alloys owing to its unique positive temperature coefficient of coercivity. However, so far, the maximum energy product of MnBi alloys is still lower than its theoretical value. In this study, Mn55Bi45-xMgx (x = 0, 1, 2, 3, and 4) alloys were prepared by induction melting followed by an annealing process. The effects of Mg content on the microstructure and magnetic properties of MnBi alloys were investigated. The results showed that the partial substitution of Mg for Bi inhibited the formation of LTP-MnBi. With the increase of Mg content, the coercivity increases from 8.1 kOe for x = 0 to 10.0 kOe for x = 4, but the saturation magnetization decreased from 71.7 emu/g for x = 0 to 61.3 emu/g for x = 4. Finally, the calculated maximum energy product of 11.1 MGOe was obtained in the annealed Mn55Bi43Mg2 powder.

Magnetic Properties of the Mn55Bi45/Nd2Fe14B Hybrid Magnetic Alloys

The (Mn55Bi45)100−x/(Nd2Fe14B)x hybrid magnetic alloys were prepared by the ball milling of the combined annealed Mn55Bi45 powders and Nd2Fe14B powders. The magnetic properties at room temperature and elevated temperature were investigated. It was found that the saturation magnetization and the coercivity at room temperature increased significantly with the increasing Nd2Fe14B content. The enhanced energy product of 10.8 MGOe and 11.5 MGOe were obtained in (Mn55Bi45)40/(Nd2Fe14B)60 and (Mn55Bi45)20/(Nd2Fe14B)80. At elevated temperatures (350 K), the coercivities of 16.6 kOe and 16.1 kOe were obtained with Nd2Fe14B content of 20 wt.% and 40 wt.%, which were higher than those at room temperature. The temperature coefficients of coercivity of (Mn55Bi45)80/(Nd2Fe14B)20 and (Mn55Bi45)60/(Nd2Fe14B)40 were calculated to be positive, owing to the coercivity temperature characteristics of MnBi alloy. Finally, the energy products remained 10.5 MGOe and 10.1 MGOe in (Mn55Bi45)40/(Nd2Fe14B)60 and (Mn55Bi45)20/(Nd2Fe14B)80 at 350 K, which exhibited potential for high temperature applications.

Facile method of raising the LTP content in Mn-Bi alloys by using sequential separation techniques for Bi and Mn

Mn-Bi alloys, especially the Low Temperature Phase belonging to the P63/mmc crystallographic space group are an attractive alternative for replacing the rare-earth permanent magnets especially at elevated temperatures. The encountered issues during the fabrication are driven by phase segregation effects and inhomogeneities that are difficult to control or eliminate, thus leading to low experimental reproducibility and uncorrelated results for structural and magnetic properties. To overcome these issues, we propose an innovative method to increase the content of LTP by integrating an intermediary separation process during the manufacturing of Mn-Bi melt-spun ribbons. The method consists of two steps: one for Bi excess removal and the second for Mn excess removal. Bi excess removal is achieved by using a system of quartz tubes arrays though gravitational separation. This method is suitable for obtaining Mn-Bi ribbons with high content LTP, as the removal of Mn excess from the alloy occurs during the ribbons formation through the melt spinning technique. As the method employs the removal of the excess unalloyed metals, the global weight yield is in range of 53–64 wt%, but it can be increased by recovering the metallic elements and reprocess them. After a proper thermal treatment of the as quenched Mn-Bi melt-spun ribbons, over 99.0 wt% is represented by the LTP crystallographic phase characterized by saturation magnetization (MS) values of 70.6 emu/g.

Spontaneous exchange bias in high energy ball milled MnBi alloys

Low temperature phase (LTP) MnBi has received considerable interest as a prospective “Gap Magnet” between the ferrites and rare-earth magnets. The ball-milled Mn55+xBi45−x (x = 0, 1, 3, 5, 8) powders exhibited a positive spontaneous exchange bias (SEB) at room temperature, which is yet to be reported for this system. The vacuum arc melted alloys were homogenized and then subjected to high-energy ball milling. The pinning of ferromagnetic MnBi spins by localized antiferromagnetic clusters (due to Mn interstitials, anti-site disorder, and compositional fluctuations) is proposed to be the cause for SEB phenomenon. A noticeable coercivity of around 7.5 kOe at ambient temperature was achieved for x = 8 sample after 8 h of milling. The maximum exchange bias field (HEB) of around 1.2 kOe at room temperature has been observed for x = 2 after 4 h and x = 8 after 8 h of milling. Apart from hard magnets, this phenomenon opens up the possibility of its application in magnetoresistive random access memory (MRRAM) devices and spintronics.

Insight on the correlation between bimodal-sized grain structure and tensile properties of extruded low-alloyed Mg–Sn–Bi–Mn alloy

In this study, a novel low-alloyed Mg–2Sn–2Bi-0.2Mn (wt.%) alloy is designed and extruded successfully at 523 K. Microstructural analysis reveals that a bimodal-sized grain structure is obtained due to the abnormal dynamic recrystallized (DRXed) grain growth. The results indicate that the studied alloy exhibits a good balance between strength and ductility with a tensile yield strength (TYS) of 299.7 MPa, an ultimate tensile strength (UTS) of 382.6 MPa, and an elongation (EL) of 21.2%. Besides, the underlying strengthening mechanism and ductility enhancement are analyzed and discussed in terms of bimodal-sized grain structure.

Modifying magnetic properties of MnBi with carbon: an experimental and theoretical study

MnBi and MnBi-based materials have been investigated as prospective rare-earth-free permanent magnets with moderate energy product. One of the main issues with MnBi synthesis is the presence of residual Bi in the sample which reduces the net magnetization. We have found that MnBi synthesized in the presence of carbon substantially reduces the amount of residual Bi, improving its saturation magnetization. We have synthesized Mn55Bi45 and Mn55Bi45C x samples using arc melting and high-vacuum annealing. The room temperature x-ray diffraction patterns indicate that both Mn55Bi45 and Mn55Bi45C x crystallize in the hexagonal NiAs-type structure. The Rietveld analysis of the x-ray patterns shows that the amount of residual Bi reduces from 16 wt.% for Mn55Bi45 to 5 wt.% for Mn55Bi45Cx. The high-field (3 T) magnetizations measured at room temperature are 61 emu g−1 and 66 emu g−1 for Mn55Bi45 and Mn55Bi45C x , respectively. To understand the role of C in enhancing the magnetization of MnBi, we carried out the first-principles calculations of both stoichiometric and nonstoichiometric MnBi alloys, which suggests that the increase of magnetization in Mn55Bi45C x may be due to the coating of MnBi grains with C.

Microstructure and magnetic properties of anisotropic Mn-Bi powder prepared by low energy ball milling assisted with polyvinylpyrrolidone

The excellent anisotropic Mn55Bi45 powder was prepared by arc melting and surfactant-assisted low-energy ball milling (LEBM). The microstructure and magnetic properties of the alloy were investigated. The ionic polyvinylpyrrolidone (PVP) was selected as a surfactant. The results show that the addition of PVP can improve the low-temperature phase (LTP) content and magnetic properties of the Mn55Bi45 alloy. The coercivity (Hcj) of the anisotropic Mn55Bi45 powder is 14.7 kOe, the maximum energy product ((BH)max) is 7.1 MGOe and the saturation magnetization (Ms) is 57.5 emu/g when it is 10 wt% PVP and after 3 h ball milling. The angle dependence of coercivity shows that mechanism of anisotropic Mn-Bi alloy follows to the Stoner-Wohlfarth model.

Spontaneous Exchange Bias in High Energy Ball Milled Mnbi Alloys

Spontaneous Exchange Bias in High Energy Ball Milled Mnbi Alloys

Microstructure and Magnetic Properties of Fe-doped MnBi Permanent Magnet Alloys

The melt-spinning method combined with annealing technology was used to prepare Mn-based permanent alloys in the paper. The influences of Fe-doping on the microstructure and magnetic performances of the MnBi alloys were studied. According to the results of XRD pattern, the fraction of the LTP-MnBi in the as spun MnBi ribbons increases with the doping of Fe element, a significant increase of the LTP-MnBi is due to annealing treatment. The magnetization at the 20 kOe field of MnSUB55/SUBBiSUB40/SUBFeSUB5/SUB melt-spun ribbons increases to 45 emu/g after being annealing. With the addition of Fe, the coercivity HSUBcj/SUB of the annealed ribbons increases first and then decreases. The results of δM plots show that the reduction of HSUBcj/SUB for the MnSUB55/SUBBiSUB40/SUBFeSUB5/SUB melt-spun ribbons can be attributed to enhanced exchange coupling interaction between MnBi LTP particles. The grain refinement contributes directly to an enhanced exchange coupling for the addition of 5 at% Fe from TEM images. The dependence of coercivity on temperature for the annealed MnBi alloy melt-spun ribbons is discussed.

Preparation and magnetic properties of MnBi alloy and MnBi /Fe hybrid magnets

Preparation and magnetic properties of MnBi alloy and MnBi /Fe hybrid magnets