MnBi Magnets Research Articles

Effect of high pressure torsion on magnetic properties and corrosion resistance of MnBi/NdFeB composite magnets

The MnBi/NdFeB composite magnets have been fabricated by high pressure torsion (HPT). The effects of NdFeB content, torsional pressure and torsional degree on the microstructures, magnetic properties, and corrosion resistance of the MnBi/NdFeB composite magnets are systematically investigated. The results show that the remanent magnetization (Mr) of MnBi/NdFeB composite magnets increases with the Nd2Fe14B content. A peak value of the maximum energy product ((BH)max) = 5.61 MGOe is obtained while the Nd2Fe14B content is 75 wt%. Addition of MnBi increases the coercivity temperature coefficient and the density of MnBi/NdFeB composite magnets. The MnBi/NdFeB composite magnets prepared by HPT show perpendicular anisotropy, which can be explained by the (0kl) texture in the perpendicular direction observed by TEM. With the increase of degree of HPT, the intrinsic coercivity (Hcj) difference between perpendicular direction and parallel direction is more obvious, the (0kl) texture is enhanced by the degree of HPT. The first-order reversal curve (FORC) demonstrates that MnBi/NdFeB composite magnets exhibits weaker exchange coupling compared with pure MnBi magnet. Besides, a remarkably improvement in electrochemical stability for the MnBi/NdFeB composite magnet is achieved with the MnBi added.

Fe-Ni based alloys as rare-earth free gap permanent magnets

L10-ordered FeNi phase has potential as a rare-earth free permanent magnet due to its large magnetization and high Curie temperature. However, low long-range crystal ordering and weak magnetocrystalline anisotropy (Ku) impede its practical use in a technologically relevant permanent magnet. Employing density functional and Monte Carlo simulations, we demonstrate the substantial improvements on structural and thermal stability, disorder-order phase transition, and intrinsic permanent magnetic properties of Fe−Ni structures. We propose that this is achievable through the complementary elemental substitutions by metal elements (M) and interstitial doping with 2p elements. Fe1-xMxNi with simple metal (M = Ga and Al) and metalloid (M = Ge and Si) elemental substitutions at x = 0.5 are broadly known as Heusler (L21) structures with no permanent magnet characteristics. On the contrary, we find that for x = 0−0.5, L10-type structure is energetically favored over the Heusler-type structures. The predicted structures exhibit Ku values up to approximately 2.1 MJ/m3, which is roughly three times that of FeNi phase (0.68 MJ/m3), where the absolute value of Ku depends on the degree of L10 crystal ordering. Interstitial doping with B elevates Ku further up to a maximum value of 3.9 MJ/m3 (at 0 K), leading to room-temperature intrinsic permanent magnetic properties, including maximum energy density product (BH)max, anisotropy field μ0Ha, and hardness parameter κ, comparable to the widely investigated MnBi and MnAl permanent magnets. These results may serve as a guideline in designing Fe−Ni based rare-earth free gap permanent magnetic materials that were previously overlooked.

Investigating the Effect of B, Sb, and Ge Elements on the Structural and Magnetic Properties of Rare Earth Elements -Free Permanent MnBi Magnets

The goal of this study was the investigation of the structural and magnetic characteristics of MnBi permanent magnets. In this regard, the mechanical milling and annealing processes were used for the synthesis of Mn45Bi50M5 (M= Sb, Ge, B) samples. Based on the results, the best annealing temperature and time for the formation of the highest value of MnBi ferromagnetic phase with optimum magnetic properties (Hc=1750 Oe and Ms= 12 emu/g) were recognized at 325 oC and 24 h, respectively. B, Ge, and Sb elements had a distractive effect on the stabilization of the MnBi ferromagnetic phase. Consequently, the samples containing B, Ge, and Sb showed lower magnetic characteristics in comparison with the Mn50Bi50 stoichiometric sample.

Engineering microstructure to improve coercivity of bulk MnBi magnet

MnBi is a candidate material for high-temperature magnets because of its increasing coercivity with increasing temperatures up to 255 °C. However, most efforts in fabricating bulk MnBi magnets have run into the problem of preserving the coercivity (Hcj) of its feedstock powders. About 70% of powder's Hcj would be lost during the densification process. Our micromagnetic modeling shows that the coercivity mechanism of the MnBi bulk magnet is controlled by nucleation of the reversal magnetization domains, and the large Hcj loss that occurred during the powder consolidation process can be attributed to the inter-grain magnetic coupling. To attain a high Hcj, the grains in the MnBi bulk magnet must be separated with a non-magnetic grain boundary phase (GBP). To validate this GBP hypothesis, we engineered MnBi bulk magnets with two different types of GBP. The first type of GBP was created in-situ by precipitating excessive Bi from the grains; the second type was created ex-situ by coating silicates on the feedstock powders before the consolidation. While both GBP work, the ex-situ approach resulted in a better Hcj due to a more uniform GBP distribution. The Hcj loss was reduced from 70% to 15%, and the (BH)max of a warm sintered bulk magnet reached 8.9 MGOe.

Engineering Microstructure to Improve Coercivity of Bulk MnBi Magnet

Engineering Microstructure to Improve Coercivity of Bulk MnBi Magnet

Synthesis and magnetic properties of Sb added MnBi magnets for applications at sub-zero temperatures

MnBi-LTP bulk magnets containing substitutional Sb element were fabricated using gas-atomizing process, surfactant-assisted wet magnetic compaction process, and thermal compaction process. Magnetic properties and microstructures were investigated according to the processing parameters. It was found that substitutional Sb element effectively refined the grain size of gas-atomized powders. The conventional ball milling process was able to pulverize these powders into fine particles with a single crystalline structure and minimize their phase decomposition, resulting in a (BH)max of 11.0 MGOe. By analyzing the effects of the compaction pressure on the density and magnetic alignment during the surfactant-assisted wet compaction process, Mn54Bi45.9Sb0.1 bulk magnet with Hc of 10.0 kOe at room temperature and temperature coefficient Hc of 0.68%/K was successfully fabricated. Furthermore, effects of Sb element on the temperature dependency of Hc in the bulk magnet were discussed.

On the Balance between Magnetization and Coercivity of MnBi Green Powders and Magnets Prepared Thereof

Currently, MnBi magnetic material is promising to be very potential for low-cost high-temperature permanent magnetic applications. The most attractive parameters are the high magnetocrystalline anisotropy energy and its positive thermal coefficient which allow a high coercivity of MnBi magnets working even at 150 – 200 oC. This advantage is paid by the moderate value of magnetization, so the high energy product of MnBi magnets depends on how one can keep a good balance between the remanent magnetization Mr and the coercivity bHc. The paper considers this problem and discusses how to regulate the processing route to keep this balance to enhance the performance of MnBi green powders and MnBi bulk magnets prepared thereof

Neutron study of magnetic correlations in rare-earth-free Mn-Bi magnets

We report the results of an unpolarized small-angle neutron scattering (SANS) study on Mn-Bi-based rare-earth-free permanent magnets. The magnetic SANS cross section is dominated by long-wavelength transversal magnetization fluctuations and has been analyzed in terms of the Guinier-Porod model and the distance distribution function. This provides the radius of gyration which, in the remanent state, ranges between about $220-240 \, \mathrm{nm}$ for the three different alloy compositions investigated. Moreover, computation of the distance distribution function in conjunction with results for the so-called $s$-parameter obtained from the Guinier-Porod model indicate that the magnetic scattering of a Mn$_{45}$Bi$_{55}$ sample has its origin in slightly shape-anisotropic structures.

Temperature-gradient-driven annealing process for formation of MnBi ferromagnetic phase

The high content of ferromagnetic phase content δ is crucial in establishing high performance of MnBi magnets, which is the potential alternative for NdFeB magnetic material in high-temperature applications. Because of the peritectic solidification, the ferromagnetic phase of as-cast MnBi alloys is low in content and always assisted with Mn and Bi phases, and therefore, δ enhancement must be done using the annealing process. Since the ferromagnetic phase of MnBi is formed only at the temperature lower than 340 °C, the isothermal low-temperature anneal can enhance δ only moderately. In this paper, the temperature-gradient-driven annealing process has been applied to accelerate the formation of the MnBi ferromagnetic phase. This annealing process is based on the advection-assisted diffusion process which allows to accelerate the formation of ferromagnetic phase and to enhance its content δ and thus the spontaneous magnetization Ms of MnBi alloys. By this annealing process at 290 °C superposed by the temperature gradient gradT = 1–3 °C/cm for 20 h, in comparison with the isothermal annealed sample, the content δ is enhanced from the value of 60.6 to 89.9 wt% resulting the Ms enhancement from 43.0 to 64.1 emu/g. The mentioned annealing process was described by using the time-dependent one-dimensional diffusion–advection equation. The obtained analytical solution allows controlling the parameters of the process. So, the temperature-gradient-driven annealing process is promising for the large-scale production of high-LTP content MnBi alloys.

Magnetic and microstructural properties of anisotropic MnBi magnets compacted by spark plasma sintering

In this study, we present that anisotropic MnBi bulk magnets can be obtained using spark-plasma-sintering technique (SPS). Grain misalignment was suppressed by either external field or addition of appropriate polymer binder. Various microstructures were effectively achieved during SPS processing, and lead to distinct magnetic performance. Microstructure analysis reveals that the coercive field of the SPS-compacted magnets declines strongly with increasing grain size. Bi-rich residue clearly plays a role in magnetic decoupling to enhance the coercive field, whereas the exchange coupling mediated by excess Mn reduces it dramatically. In addition, the amount and distribution of Mn and Bi residue has a profound influence on the thermal magnetic stability of sintered MnBi magnets. We suggest that fast consolidation methods like SPS technique enable to tailor the microstructure of bulk magnetic products to meet specific application requirements.

Phase Transformations in Mn–Al and Mn–Bi Magnets by Repeated Heat Treatment

A stepped heating cycle in a tube furnace at 1000 °C for 1 h followed by holding at 400 °C for 1 h directly increases the amount of ferromagnetic τ-phase of manganese–aluminum (Mn–Al). Repeating this heating regime for 2 more times, further increase in the amount and uniform distribution of the τ-phase is obtained at the expense of the e-phase. The magnetization and coercivity are increased after each heating cycle, and the effect is more pronounced when 2% carbon (C) is added to Mn–Al. When the similar heat treatment cycle is applied to manganese–bismuth (Mn–Bi), the ferromagnetic α-phase is detected with larger magnetizations, but substantial Mn and Bi are segregated from the alloy. This solid-state diffusion in tube furnace offers a facile route to improve rare-earth-free magnets.

Microstructure, improved magnetic properties, and recoil loops characteristics for MnBi alloys

Abstract MnBi is a potential candidate for permanent magnets that can be used in high-temperature applications due to the large magnetocrystalline anisotropy and unique positive temperature coefficient of coercivity. In this work, MnBi magnets with a high content of low temperature phase (LTP) were prepared by arc melting followed by heat treatment. The high coercivity of 7.11 kOe at 300 K and 21.97 kOe at 500 K can be obtained for MnBi ingots, respectively. Two-step thermal annealing can promote the transformation from the Mn and Bi phases to form LTP MnBi, allowing a maximum remanence of 38.3 emu/g to be acquired. Open recoil loops for MnBi ingots have never been reported and are worthy of investigation. It was found that the non-uniform distribution of the Mn phase, Bi phase, LTP MnBi, and even the amorphous phase, as well as their large variation in size, could produce a non-negligible local demagnetization field with non-uniform distribution and resulted in open recoil loops. Nearly closed recoil loops can be achieved by improving the non-uniform distribution of the microstructure. The dependence of the magnetic properties and the recoil loops on the microstructure was investigated in detail.

IMPROVEMENT OF MAGNETIC PROPERTIES OF MnBi POWDERS PREPARED BY LOW-ENERGY BALL MILLING

Recently MnBi magnetic material attracts a large attention due to its potential for high-temperature permanent magnetic applications. Although its sponatenous magnetization is moderate, Ms ~ 8.2 kG74 emu/g but its large coercivity, iHc > 10 kOe, which results in the theoretical value of energy product (BH)max ~16.8 MGOe. The MnBi single phase is difficult to be prepared by using conventional techniques, such as the arc-melting, melt-spinning and sintering because of the big difference between the melting temperatures of Bi (544 K) and Mn (1519 K). Furthermore, the magnetic properties of magnets are strongly dependent on processing. The heat treatment of arc-melted alloys, the ball milling of annealed alloys, and the bulk magnets fabrication were found to have large effects on (BH)max of MnBi magnets. In this work, we report the effects of decomposition of MnBi LTPlow temperature phase (LTP) into Bi and Mn during low-energy ball milling (LEBM) carried out in xylene and silicon oil protection solvent environments and its influence on the magnetic properties of green MnBi as-milled powders. In both solvents, by LEBM for 120 - 150 min, MnBi arc-melted and annealed alloys were ground into fine particles of 0.5 – 5 µm to increase iHc up to 5 kOe. By LEBM for 120 min, the viscous silicon oil constrained the decomposition of MnBi (LTP) keeping Ms around 56 emu/g instead of 42 emu/g of in-xylene LEBM powders.

Effects of intergranular phase on the coercivity for MnBi magnets prepared by spark plasma sintering

MnBi magnets with a high content of low temperature phase (LTP) and excellent magnetic properties were prepared by spark plasma sintering (SPS) using ball milling powders as precursors without magnetic purification. A complicated intergranular phase, which contains Mn phase, Bi phase, MnO phase, and even amorphous phase in MnBi magnets, was characterized and reported systematically. It was found that the formation of intergranular phase which was contributed by ball milling precursors and sintering mechanism, jointly, had important influence on the magnetic properties. The appropriate content of intergranular phase was beneficial in improving the coercivity due to the strong magnetic isolation effects. The optimum magnetic properties with Mr=26.0 emu/g, Hci= 7.11 kOe and (BH)max=1.53 MGOe at room temperature, and a maximum value Hci= 25.37 kOe at 550 K can be obtained. Strongly favorable magnetic properties make SPSed MnBi magnets an attractive candidate material for small permanent magnets used in high-temperature applications.

HIGHLY ANISOTROPIC MnBi MAGNETS

The rare-earth-free MnBi magnetic material is promising for high-temperature (150 200 oC) application of permanent magnets because of its large magnetocrystalline energy and especially the positive thermal coefficient of coercivity (dHc/dT > 0). Because of the moderate value of the spontaneous magnetization Ms 74 emu/g, the anisotropy of MnBi bulk magnets should be investigated to enhance the remanence Mr. With large ratio Mr/Ms and appropriate microstructure, the squareness of MnBi magnets should have high value leading the remanent coercivity bHc close to the intrinsic coercivity iHc, thus enhancing the energy product (BH)max. The paper presents an approach to loading and compacting of MnBi powders in the 18 kOe magnetic field oriented perpendicular to the pressing direction where MnBi grains can be freely rotated and oriented parallel to the field direction. Based on the energy minimization of the assembly of magnetized grains, the compacting pressure was chosen to optimize two parameters, the mass density and the coercivity iHc of magnets. The prepared MnBi bulk magnet had 8.4 g/cm3, Mr/Ms 0.92, 0.89 and (BH)max reached 8.4 MGOe.

Effects of microstructures on the performance of rare-earth-free MnBi magnetic materials and magnets

Since the solidification of MnBi alloys is peritectic, their microstructures always consist of the starting phases of Mn and Bi and the productive phase MnBi. The high performance of MnBi bulk magnets requires appropriate routes of preparing MnBi powders of high spontaneous magnetization Ms and large coercivity iHc as well a route of producing bulk magnets thereof. In these routes, the microstructures of arc-melted alloys, annealed alloys and magnets strongly related to the quality of powders and the performance of magnets. The paper proves that: i) The microstructure of fine Mn-inclusions embedded in the matrix of Bi is preferred for arc-melted alloys to realize the rapid evolution of the ferromagnetic phase inside them during their sequent annealing process; ii) The time-controlled annealing process plays a key role in controlling the microstructure with the main ferromagnetic phase matrix, in which the rest of Mn and the Bi accumulations are embedded; iii) The cold (in-liquid-nitrogen) ball milling annealed alloys is required for preparing a high quality powders with the preferred sub-micrometer microstructure without a Bi-decomposition; iv) The short-time warm compaction is crucial to fabricate dense, highly textured bulk magnets with the micrometer microstructure. The realization and control of these preferred microstructures figured in these routes enhance the chance of preparing MnBi bulk magnets with the energy product (BH)max larger than 8 MGOe.

Solution processed MnBi-FeCo magnetic nanocomposites

Advanced permanent magnets based on rare-earth-free MnBi intermetallic alloys are considered energy-critical materials due to their applications in high temperature power electronics and green energy-related generators and motors, owing to their positive temperature coefficient of magnetic anisotropy. However, a direct method to achieve high saturation magnetization, without the significant loss of coercivity, is critical for attaining high performance MnBi magnets. Here, we demonstrate the synthesis and processing of magnetic nanocomposites, consisting of metal-redox MnBi nanoparticles and electro-spun FeCo nanowires. The composition ratio, processing dependent magnetism, and increased coercivity with increasing temperature, were studied in MnBi-FeCo nanocomposites. The magnetic performance of nanocomposites was dictated by interfacial coupling between magnetically hard MnBi and semi-hard FeCo nanowires, as well as the composition ratio and processing conditions. Solution processed MnBi-FeCo nanocomposites allow the potential for the development of high temperature and high performance rare-earth-free permanent magnets.

Processing of MnBi bulk magnets with enhanced energy product

We report magnetic properties and microstructure of high energy-product MnBi bulk magnets fabricated by low-temperature ball-milling and warm compaction technique. A maximum energy product (BH)max of 8.4 MGOe and a coercivity of 6.2 kOe were obtained in the bulk MnBi magnet at room temperature. Magnetic characterization at elevated temperatures showed an increase in coercivity to 16.2 kOe while (BH)max value decreased to 6.8 MGOe at 400 K. Microstructure characterization revealed that the bulk magnets consist of oriented uniform nanoscale grains with average size about 50 nm.

Mechanical and electrical properties of low temperature phase MnBi

Low temperature phase (LTP) manganese bismuth (MnBi) is a promising rare-earth-free permanent magnet material due to its high intrinsic coercivity and large positive temperature coefficient. While scientists are making progress on fabricating bulk MnBi magnets, engineers have begun considering MnBi magnets for motor applications. Physical properties other than magnetic ones could significantly affect motor design. Here, we report results of our investigation on the mechanical and electrical properties of bulk LTP MnBi and their temperature dependence. A MnBi ingot was prepared using an arc melting technique and subsequently underwent grinding, sieving, heat treatment, and cryomilling. The resultant powders with a particle size of ∼5 μm were magnetically aligned, cold pressed, and sintered at a predefined temperature. Micro-hardness testing was performed on a part of original ingot and we found that the hardness of MnBi was 109 ± 15 HV. The sintered magnets were subjected to compressive testing at different temperatures and it was observed that a sintered MnBi magnet fractured when the compressive stress exceeded 193 MPa at room temperature. Impedance spectra were obtained using electrochemical impedance spectroscopy at various temperatures and we found that the electrical resistance of MnBi at room temperature was about 6.85 μΩ m.

Investigation of magnetic properties of MnBi/Co and MnBi/Fe65Co35 nanocomposite permanent magnets by micro-magnetic simulation

Demagnetization curves of anisotropic nanocomposite MnBi/Co and MnBi/Fe65Co35 permanent magnets were investigated by micro-magnetic finite element method. Effects of volume ratio, deviation degree of orientation and intrinsic magnetic properties of the soft magnetic phase on the magnetic properties of the magnets were investigated. From the viewpoint of practical applications, to meet the requirement of hardness parameters, ĸ=K/(μ0MS2)1/2>1, the calculation maximum (BH)max of MnBi/Co and MnBi/Fe65Co35 magnets are about 199kJ/m3 (V(Co)=22vol%) and 196kJ/m3 (V(FeCo)=14vol%), respectively, indicating their good potential in application. Compared with single phase MnBi magnet, the (BH)max of nanocomposite MnBi/Co and MnBi/Fe65Co35 magnets increases by 66% and 63%, respectively. The remanence and coercivity of MnBi/Co nanocomposite magnets reduce as appearing a deviation degree of orientation, result of greatly decrease of the magnetic energy product.