Low Temperature Phase MnBi Research Articles

Cross-sectional surface analysis of magnetic domains, microstructures, and magnetic properties of the low-temperature phase MnBi prepared by low-temperature vacuum sintering

In this work, the low-temperature phase MnBi prepared by a low-temperature vacuum sintering process at 325 °C was studied. We found a significant increase in the energy product from 2.63 MGOe in the 12-h sintered sample to 3.64 MGOe in the 48-h sintered sample. This improvement is attributed to the solid-liquid diffusion process. Cross-sectional scanning electron microscopy (SEM) reveals that MnBi forms at the external surface of Mn particles and along interior surfaces, notably within cracks. Transmission electron microscopy further demonstrates that the Mn ratio increases and Bi decreases with distance from the crack. The selected area diffraction showed variations in the Mn ratio with distance from cracks and identified both Bi and MnBi phases in the MnBi layer. Magnetic force microscopy (MFM) analysis exhibited large phase shifts indicating repulsive or attractive forces in single ferromagnetic domains. This provides valuable insight into magnetic domains in the MnBi regions near Mn cracks. The MnBi formation model, developed for the vicinity of single cracks with uniform MnBi content, partly explains the magnetic interactions and phase shifts observed near these cracks. These findings provide significant insights into the MnBi microstructural and magnetic properties, potentially useful in tailoring and engineering magnetic structures.

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.

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.

Mechanochemical synthesis of MnBi/Fe3C@C exchange coupled hard magnetic nanocomposites

Despite the better suitability of rare earth permanent magnets for various applications, the scarcity of rare earths limits their application range. The, low temperature phase of MnBi is sought as a probable gap magnet without any rare earth elements. Further, the nanocomposites (soft/hard magnets) are known to provide better properties. In this study, Mn55Bi45/Fe3C@C nanocomposites with enhanced magnetic properties were prepared using cryogenic ball milling. The feasibility of utilizing Fe3C@C as a soft phase in hard-soft composites was explored. Fe3C@C was synthesized through the solvothermal technique, followed by calcination, resulting in a remarkable saturation magnetization (MS) of 150 emu/g. Different amounts of Fe3C@C (0, 5, 10, 15, and 20 wt.%) were added to Mn55Bi45 powder, and after cryo-milling, it was observed that the addition of 10 wt. % yielded the optimum results, with a 37% enhancement in MS and 58% increase in Mr values. The presence of exchange-coupled nanocomposites was confirmed through x-ray diffraction, transmission electron microscopy, and magnetic studies. This study not only paves the way for increasing the energy product of the MnBi system but also explores the potential of utilizing Fe3C@C as a soft phase—a relatively oxidation-resistant phase—in other hard magnetic systems.

Magnetocrystalline anisotropy of interstitially and substitutionally Sn-doped MnBi for high temperature permanent magnet applications

First-principles calculations were performed to calculate the electronic structures of low temperature phase (LTP) MnBi (Mn50Bi50) and substitutionally and interstitially Sn-doped MnBi [Mn50Bi25Sn25, (Mn0.5Bi0.5)66.7Sn33.3]. Brillouin function predicts the temperature dependence of saturation magnetization M(T). Sn substitution for Bi in MnBi (Mn50Bi25Sn25) changes the magnetocrystalline anisotropy constant (Ku) from −0.202 MJ/m3 (the in-plane magnetization) for LTP MnBi to 1.711 MJ/m3 (the out-of-plane magnetization). In comparison, the Ku remains negative but slightly decreases to −0.043 MJ/m3 when Sn is interstitially doped in MnBi [(Mn0.5Bi0.5)66.7Sn33.3]. The Curie temperature (TC) decreases from 716 K for LTP Mn50Bi50 to 445 K for Mn50Bi25Sn25 and 285 K for (Mn0.5Bi0.5)66.7Sn33.3. Mn50Bi25Sn25 has a lower magnetic moment of 5.034 μB/f.u. but a higher saturation magnetization of 64.2 emu/g than (Mn0.5Bi0.5)66.7Sn33.3 with a magnetic moment of 6.609 μB/f.u. and a saturation magnetization of 48.2 emu/g because the weight and volume of the substitutionally Sn-doped MnBi are smaller than the interstitially Sn-doped MnBi. The low Curie temperature and magnetization for Sn-doped MnBi are attributed to the high concentration of Sn. Thus, future study needs to focus on low Sn-concentrated MnBi.

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.

Influence of glycine addition on formation and magnetic properties of MnBi prepared by vacuum sintering technique

An alternative way to increase a low-temperature phase MnBi (LTP-MnBi) content synthesized by low-temperature vacuum sintering is by using Mn powder with small particle sizes. To reduce the particle size by ball milling, glycine was added to prevent particle agglomeration and possible oxide formation. The ball-milled Mn and Bi powders were fixed at an atomic ratio of 1:1. The ball-milling time in glycine was between 1 and 3 hrs. The average particle size reduces from 400 µm (original) to 35-40 µm and 15-20 µm after grinding for 1 and 3 hr, respectively. Sintering of the mixtures was carried out at 275 °C for 12 hours at a vacuum pressure of less than 5×10−8 mbar. The LTP-MnBi samples sintered from 1 hr (MnG1Bi) and 3 hr (MnG3Bi) glycine-added Mn grinding times were investigated by XRD, SEM, XPS, and VSM. The coercivity values (H c) are 3.95 ± 0.04 and 2.35 ± 0.03 kOe for MnG1Bi and MnG3Bi, respectively. The saturation magnetization (M s) of MnG1Bi and MnG3Bi is relatively low (~0.18 emu/g) compared to the samples prepared by conventional ball-milling, i.e., without adding glycine in grinding. The XRD results show a few percentages of MnBi content in all samples, suggesting that glycine addition could prevent MnBi formation. The coverage of hydrocarbon (i.e., NH2-CH2-COOH) groups on Mn particles could prevent the diffusion mechanism during liquid-phase sintering. Moreover, it was found that the oxygen and carbon content in the MnGxBi were much higher than the conventional ball-milled MnBi.

Effects of compression on magnetic properties of MnBi prepared by liquid-phase sintering

In this work, the Mn and Bi mixture prepared with and without compression were sintered in a vacuum at 325°C and cooled down naturally to obtain low-temperature phase MnBi. For non-compressed samples, the MnBi content increases with sintering time which could be described by the diffusion process. The average maximum energy product ((BH)max) values were 3.67, 3.05, and 4.77 MGOe for MnBi sintered for 24, 48, and 96 hr, respectively, consistent with their phase identifications. For the compressed samples, the MnBi phase is significantly low in the 24-hr sintered samples compared to the samples sintered for 48 and 96 hr. This resulted in an average (BH)max of 0.43, 3.90, and 3.23 MGOe for the compressed samples sintered for 24, 48 and 96 hr, respectively. The results suggest that compression of the Mn and Bi mixture affects the flow of liquid Bi during sintering and, thus, hinders the formation of MnBi. The effects of the compression were reduced for the prolonged sintered samples, suggesting changes in the internal structure as the sintering proceeded.

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.

Effects of the C interstitial doping on the magnetic properties of LTP MnBi

Experimental and theoretical investigations on the structural and magnetic properties of the MnBi and MnBiC hard magnetic phases are presented. X-ray diffraction patterns showed that the highest concentration of MnBi low-temperature phase (LTP) was obtained for annealing at 400 °C for 12 h. X-ray photoemission measurements (XPS) of the core levels (Mn 2p and 3d; Bi 4d and 5p, respectively) show chemical shifts by C addition, providing evidence that the C atoms enter the MnBi structure in the vicinity of both type of metallic atoms. C atoms were assumed to occupy the 2d interstitial crystal sites of the hexagonal NiAs structure type, as suggested in earlier studies. The theoretical calculations show that C addition enhances slightly the magnetic moment of the samples compared to pristine LTP MnBi, in agreement with our magnetic measurements. Also, following both theoretical and experimental investigations, we obtained increased magnetocrystalline anisotropy energy (MAE) by adding C as interstitial dopant.

Suppressing antiferromagnetic coupling in rare-earth free ferromagnetic MnBi-Cu permanent magnet

Rare-earth free, ferromagnetic MnBi shows a positive temperature coefficient of coercivity from room temperature to 400 K and energy product (BH)max of 17.7 MGOe at 300 K. However, MnBi undergoes a first-order structural phase transformation from a ferromagnetic low-temperature phase (LTP) to a paramagnetic high-temperature phase at 613 K below the Curie temperature (Tc) of 716 K. The transformation is attributed to Mn diffusion into the interstitial site of LTP MnBi unit cell. Interstitial Mn antiferromagnetically couples with the Mn at lattice 2a site, lowering the magnetization. Cu-occupied bipyramidal sites are investigated as a possible means to suppress Mn diffusion into the bipyramidal sites using first-principles calculations based on the density functional theory. Saturation magnetization, magnetocrystalline anisotropy constant (K), and Tc of (Mn0.5Bi0.5)100−xCux (x = 0–33) are reported. The magnetocrystalline anisotropy changes to the out-of-plane direction (x = 13) from the in-plane direction (x = 0.0). Tc decreases gradually to 578 K at x = 33 from 716 K at x = 0.0. The calculations show a slightly lower (BH)max of 15.6 MGOe while it is expected that Cu-occupied interstitial sites will significantly suppress Mn diffusion and raise the temperature of the phase transformation.

Synthesis and Characterization of Mnbi Magnetic Alloy via Two Different Processing Routes

Manganese bismuth alloy has gained importance due to its rare earth free elements, positive temperature coefficient and unique magnetic properties. Low temperature phase (LTP) MnBi was successfully prepared by arc melting with subsequent heat treatments and melt spinning technique followed by heat treatment for different durations. LTP MnBi formation was confirmed using XRD analysis and microstructural characterization of the samples was done using field emission scanning electron microscope. MnBi with greater LTP amount was formed by melt spinning route when compared with its counter arc melted one. Magnetic energy density of LTP MnBi formed by melt spinning technique with different heat treatment time was studied.

Preparation of low-temperature phase MnBi by sintering in vacuum

A simple vacuum sintering system was set up to prepare low-temperature phase manganese-bismuth compound (LTP-MnBi). A mixture of Mn and Bi powders with 1:1 atomic ratio was sintered at 275 °C for 3, 6, 9 and 12 hours. The morphology of the sintered materials was investigated by SEM. The sintered product was further identified by XRD and energy dispersive spectroscopy and found to be LTP-MnBi, Mn and Bi. Sintering in vacuum could prevent the formation of manganese oxides. The magnetic properties of the sintered materials were characterized by using a vibrating sample magnetometer. The coercivity and the saturated magnetization were found to be 2.5 kOe and 42.4 emu/g, respectively. The maximum energy product of this magnetic materials was about 1.7 MGOe.

Effect of phase purity on enhancing the magnetic properties of Mn-Bi alloy

High purity MnBi magnetic low temperature phase (LTP) were prepared via induction melting, annealing and low energy ball milling. The effects of synthetic parameters and chemical compositions on phase purity and magnetic properties were systematically investigated. A homogenization heat treatment process was employed to increase the fraction of LTP MnBi, which showed notably high saturation magnetization (Ms) of up to 74.8 Am2/kg with the purity of 97 wt% LTP MnBi. The purity of MnBi is strongly related to the amount of Mn and residual Bi in the annealed ingot. By controlling the residue, the highest phase fraction was obtained with the atomic ratio of Mn56Bi44. The maximum energy product (BH)max was recorded 11.7 MGOe under an applied magnetic field of 1.5 T at room temperature, making MnBi a promising rare-earth-free permanent magnet.

Phase transition, structural and magnetic properties studied in Y doped low temperature phase MnBi

Phase transition, structural and magnetic properties studied in Y doped low temperature phase MnBi

RETRACTED ARTICLE: Study on the structure and magnetic properties of Gd-doped LTPMnBi

Series samples MnBi1-xGdx (x = 0, 0.04, 0.08, 0.12, and 0.16) were prepared and studied the structure and magnetic properties. The Rietveld refinement results showed that the low-temperature phase MnBi content in these samples is above 78% and the doped atoms occupied the position of Bi in the MnBi crystal structure. The Debye temperature is founding to decrease with increasing doping. The Curie temperature, paramagnetic to ferromagnetic transition temperature, thermal hysteresis, saturation magnetization, and coercive force of the samples change significantly with increasing doping content. Within a temperature range of 300–500 K, a positive coercivity temperature coefficient β existed. This work helps MnBi-Gd as a candidate for high-temperature magnetic material applications and contributes to the development of tailoring magnetic properties.

Coercivity mechanism and FORC analysis of MnBi-based permanent alloy

The Mn55Bi45−xZrx (x = 0, 1, 3 and 5) permanent alloys are prepared by melt-spinning process with subsequently annealing. The microstructure evolution of Mn-Bi-Zr alloys was systematically studied by XRD, DSC and TEM, their coercivity mechanism was discussed by Kronmüller equation and magnetic behaviors were analyzed by FORC. The XRD results showed the proportion of the low temperature phase (LTP) MnBi in the as quenched MnBi ribbons increased with the addition of Zr, and improved by annealed treatment. According to the Kronmüller equation, the coercivity mechanism for the Zr-free and Zr-doped MnBi alloys is explained by the nucleation of reverse domains. The FORCs diagram showed that more than one major peak appeared for x = 0 sample, demonstrating the dispersive distribution of the interaction, resulting from isolated LTP MnBi grains by the Mn matrix from TEM HAADF image, and the dispersive distribution of the interaction led to collapsed hysteresis loop. Comparing to the Zr-free sample, one stronger peak is located at the lower Hc in the x = 1 sample, indicating a concentrated distribution of the interaction, which resulted in a single-phase magnetized loop and the lower coercivity.