MnBi Alloys Research Articles

Fabrication of MnBi alloys with high ferromagnetic phase content: effects of heat treatment regimes and dopants

The low-cost and rare-earth-free MnBi alloys were focused for investigation during last 5 years as a promising alternative for high-cost rare-earth-containing Nd–Fe–B in high-temperature permanent magnet applications. Because of the peritectic nature of solidification, MnBi alloys containing high ferromagnetic phase content could not be easily prepared for massive scale. In this paper, the roles of heat treatment regimes including the zone-travelling under temperature gradient, multi-time annealing (MTA) and the doping soft ferromagnetic Fe65Co35 phase in preparing MnBi alloys with high ferromagnetic phase content were investigated. The results showed that, for the case of applying the temperature gradient assisted zone-travelling technique, the ferromagnetic phase content in prepared alloys reached 85 wt%. By using the MTA technique, the ferromagnetic phase content is raised up 96 wt% when MnBi alloys were annealed three times at 300 °C for 20 h. The doping of Fe65Co35 as a soft ferromagnetic phase stabilized the ferromagnetic phase in the obtained MnBi1−x(Fe65Co35)x alloys (x = 0.1, 0.2, 0.3) with the magnetization measured at 13.5 kOe of 63.1, 70.8 and 80.5 emu/g, respectively.

Studies on preparation and properties of low temperature phase of MnBi prepared by electrodeposition

MnBi alloy with low-temperature phase (LTP) is a promising candidate as rare-earth-free permanent magnet for its high intrinsic coercivity and the unique positive temperature coefficient. Many works have investigated on the preparation of the LTP-MnBi using physical methods, while there are few reports about the successful fabrication using electrodeposition method although this low cost process has been used for many material preparations so far. The main difficulty is the large negative reduction potential of manganese ions and on the basis the big difference between the reduction potentials of manganese ions and bismuth ions. In this paper, we reduced the reduction potentials of manganese ions and bismuth ions firstly and on this basis, the MnBi films with different ratios of manganese and bismuth were prepared using different deposited potentials. After annealed treatment of 380 °C, there appears the low temperature phase in XRD pattern for the sample in which the ratio of manganese and bismuth is 54:46. This is the first report in electrodeposited method so far and the corresponding coercivity reaches 1450 Oe measured by the physical property measurement system (PPMS).

Microstructure and magnetic properties of MnBi alloys with high coercivity and significant anisotropy prepared by surfactant assisted ball milling

Abstract MnBi alloys with high content of low temperature phase (LTP) MnBi and excellent magnetic properties were prepared by surfactant assistant ball milling (SABM), using annealed ribbons as precursors. Effects of the SABM processes on the phase constituent, microstructure, and magnetic properties were investigated in detail. It is found that the reduction of saturation magnetization is mainly attribute to the decomposition of LTP MnBi into Mn phase, Bi phase and MnO phase, with increasing the ball milling time. Meanwhile, due to the fine grain size and increased magnetic isolation effects between LTP MnBi grains, the coercivity increases monotonously with prolonging the ball milling time to 10 h for Mn55Bi45 powders. Through compression molding under the magnetic field, based on the SABM powders, the optimum anisotropic bonded magnet displays the maximum energy product (BH)max of 9.06 MGOe at room temperature, and a value of 7.05 MGOe at 380 K can be still achieved. In addition, the magnetic hardening mechanism of bonded magnets can be well explained by the strong pinning model. Strongly favorable magnetic properties make bonded MnBi magnets an attractive candidate material for small permanent magnets used for temperature applications up to 380 K.

Morphology of Bi<sub>2</sub>O<sub>3</sub> Nanowires and Nanoflowers in the Synthesis of MnBi Alloys

High temperature phase (HTP) MnBi alloys were formed using the arc-melting method. The drastic difference in the melting points of Mn and Bi resulted in non-homogeneity. The MnBi, Mn, Bi and O were detected by energy dispersive spectrometry (EDS). The field emission scanning electron microscope (FESEM) revealed the morphology of each phase. The rod-like and flower-like nanostructures were consistent with Bi2O3 as indicated by EDS and X-ray diffactometry. The HTP MnBi was transformed to the low temperature phase (LTP) following the annealing process. The remaining Bi and Mn are susceptible to oxidation leading to the subsequent formation of Bi2O3 as well as MnO. Whereas LTP MnBi alloys are useful for their hard magnetic properties, Bi2O3 nanowire is receiving attention for potential applications in optoelectronic devices.

Effects of Ga-doping on the microstructure and magnetic properties of MnBi alloys

The low temperature phase Mn55Bi45-xGax (x = 0, 1, 3, 5, and 10) alloys were prepared by induction melting process with subsequent low temperature annealing. The effects of Ga-doping on the crystal structure and magnetic properties of the alloys were systematically studied. The room temperature coercivities of Mn55Bi45-xGax after ball milling increased from 1.43 T for x = 0 to 1.66 T for x = 5, while the saturation magnetization decreased from 60.7 Am2/kg (x = 0) to 45.1 Am2/kg (x = 5). The maximum energy product (BH)max of Mn55Bi44Ga powders reached 7.87 MGOe. The Curie temperature of the Mn55Bi45-xGax alloys increased from 633 K to 658 K with increasing Ga concentration in the range of 0 ≤ x ≤ 5.

Coercivity mechanism of rare-earth free MnBi hard magnetic alloys

In this work, we present and discuss results concerning the hard magnetic behavior of rare earth-free MnBi alloys obtained by suction casting technique. The physics of coercivity for these type of alloys is based on the nucleation process of reverse domains, which in turn is determined by the alloy microstructure features such as phase distribution, morphology, grain size and in particular, defects, which are characteristic ofreal materials. The microstructure of the as-cast alloy presented here comprises the formation of the Low Temperature Intermetallic Phase (LTIP)-MnBi, interspersed within Bi- and Mn-rich areas. A considerable intrinsic coercivity field of 238 kA/m together with a saturation magnetization of 0.04 T were observed. The nucleation controlled mechanism of this alloy was described in terms of the Kronm¨uller equation, which incorporates the detrimental effect of microstructure defects through fitting parameters associated to reduced intrinsic magnetic properties at grain size boundaries, interfaces and local demagnetizing fields. A notorious switching of coercivity mechanism associated with domain wall pinning was found to be produced upon annealing of the alloy at 583 K for 24 hrs, yielding a drastic reduction of coercivity (down to 16 kA/m). The key microstructural feature determining the switching of coercivity mechanism is the formation/suppression of Bi-rich areas, which promotes the nucleation and growth of LTIP.

Investigation of structural and magnetic properties of Al and Cu doped MnBi alloy

Alloys of Mn-Bi, MnBi-Al and MnBi-Cu were prepared by arc-melting followed by homogenization treatment. All the alloys exhibited hexagonal NiAs crystal structure. Weight fraction of the Low Temperature Phase (LTP) MnBi phase in the Mn-Bi alloy was found to be 88% with a magnetization of 51 emu/gm at 20 kOe, whereas Al and Cu doping resulted in the decrease of the weight fraction of the LTP MnBi phase as well as saturation magnetization. The coercivity is found to increase with the increase in the doping concentration. Upon ball milling the alloys, phase decomposition and a decrease in the magnetization were observed in undoped Mn-Bi alloy. On the other hand, there was no significant phase decomposition and change in the magnetization in doped MnBi alloys due to ball milling. A change in the magneto structural phase transition temperature of MnBi was observed with doping, which is attributed to the lattice modifications occurring in the MnBi system. Doped and undoped Mn-Bi alloys shows increase in coercivity with temperature. Details of investigation are presented in this paper.

Coercivity enhancement and magnetization process in Mn55Bi45 alloys with refined particle size

The MnBi permanent magnetic alloys with high coercivity was prepared by a low-energy ball milling process. The coercivity of the MnBi alloys can be controlled by tuning the particle size and the size distribution of the MnBi alloys powders through the milling process. The size-dependent coercivity of the particles was expressed by a function of Hc = 106.17−0.56·log(D), where the particle size (D) was supposed to be lager than the single-domain size. With an optimized treatment, a room-temperature coercivity of ∼1.9 T was achieved in the MnBi powders. The magnetization curves and micromagnetic analyses indicated that the magnetization process of the MnBi alloys was governed by the coherent magnetization rotation. The hardening mechanism was corresponding to the nucleation process in the anisotropic MnBi powders with refined particle size.

Microstructural evolution and phase transformation kinetics of MnBi alloys

The low temperature phase (LTP) compound of MnBi is a promising, rare-earth metal free, and permanent magnetic material. However, it is difficult to obtain pure LTP MnBi, which results from a peritectic reaction between Mn and Bi atoms. In this study, the microstructural evolution and phase transformation kinetics of MnxBi100-x (x = 50, 55, 60) alloys were systematically investigated. Choosing an appropriate Mn content (Mn55Bi45) was found to promote the formation of the LTP MnBi. The results of the phase transformation kinetics indicated that the nucleation, growth and soft impingement processes are involved in the phase transformation process. In addition, in the initial stage of the LTP phase transformation, it was shown that the diffusional transformation was governed by two- and three-dimensional nucleation and growth, as well as a single mechanism of diffusion-controlled growth, which describes the entire transformation. At the final stage of the phase transformation, diffusion controlled growth, with soft impingement effects, dominates the process. The phase transformation kinetics analysis was confirmed with the X-ray diffraction and scanning electron microscopy results. This work implies that the local activation energy, Ec(α), and local Avrami exponent, n(α), can be applied to characterize the phase transformation behaviors of the LTP MnBi alloy.

Effect of heat treatment and ball milling on MnBi magnetic materials

MnBi alloy was prepared using arc melting, and was then heated at various temperatures and times. The alloy was ball milled for various lengths of time, following a heat treatment at 573 K for 20 h. The effects of the heat treatment and the ball milling on the magnetic performances of the material were investigated by analyzing the phases, the particle sizes, and the grain sizes. Results showed that the mass percentage of the LTP MnBi phase increased as the heat treatment time increased. The mass percentage initially increased and then decreased as the heat treatment temperature increased. The saturation magnetization increased quickly as the mass percentage of the LTP MnBi increased following the heat treatment. The value rose as high as 71.39 emu g−1 at 573 K for 30 h. The magnetization decreased, due to the decomposition of MnBi phases after ball milling. The coercivity increased simultaneously, due to the grain refinement, the presence of stresses, defects, and an amorphous phase. This value was improved from 0.09 to 14.65 KOe after ball milling for 24 h.

Preparation of highly pure α-MnBi phase via melt-spinning

High concentration of the magnetically hard α phase in the Mn-Bi alloys is important for the development of these alloys as rare-earth-free permanent magnets. Among several explored manufacturing methods, melt-spinning followed by annealing is known to be suitable of producing the most pure α structure. In this work, a series of melt-spun Mn100-xBix alloys was prepared with x = 43 – 51 at a wheel speed of 67 m/s by ejecting the alloys through orifices 0.17 mm and 0.27 mm in diameter. The smaller orifice diameter favored formation of an amorphous phase in the as-spun alloys as well as a higher saturation magnetization Ms in the alloys subsequently annealed at 300 °C. Although the most pure, 98 vol.%, α phase was obtained for the off-stoichiometric Mn55Bi45 composition, the Ms of this material was lowered, possibly because the excess Mn atoms induced antiferromagnetic coupling in the α phase. The highest Ms of 78 emu/g was obtained for a composition closer to the Mn50Bi50 stoichiometry, despite the slightly lower purity of α phase. Evolution of the room-temperature coercivity with the formation of the α phase in the melt-spun alloys was studied for the Mn55Bi45 ribbons compacted at 275 °C; the coercivity values of around 1 kOe obtained through this simple procedure are not sufficient for permanent magnet applications.

Switching of Coercivity Process in MnBi Alloys

MnBi-based alloys represent an interesting choice for developing rare earth-free permanent magnets due to the high magnetocrystalline anisotropy of their characteristic low-temperature intermetallic phase (LTIP) with hexagonal structure. In this work, we discuss the switching of coercivity mechanism in MnBi alloys by modulation of their phase distribution and microstructure. As-cast MnBi alloys obtained by suction-casting technique exhibited LTIP interspersed within Bi- and Mn-rich areas. A noticeable coercivity field of 282 kA/m was observed. The coercivity mechanism for this alloy was explained in terms of the nucleation of reverse domains after saturation, by means of the Kronmuller equation, which incorporates the detrimental effect of microstructure defects through fitting parameters associated to reduced intrinsic magnetic properties at grain size boundaries, interfaces, and local demagnetizing fields. Subsequent annealing at 583 K for 24 h produced a marked reduction of coercivity (down to 16 kA/m), reflecting a switching of coercivity process from nucleation to pinning of domain walls. The key microstructural feature determining this variation is the formation/suppression of Bi-rich areas, which promotes the nucleation and growth of the initial MnBi intermetallic phase.

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.

An Approach for Preparing High-Performance MnBi Alloys and Magnets

MnBi alloys were arc-melted and solidified by different cooling rates, which resulted in microstructures with different sizes of Mn grains embedded in the matrix consisting of Bi and MnBi phases. By annealing at an appropriate temperature and duration, the Mn grains become MnBi grains caused by their combination with the surrounding Bi. This process enhances the MnBi content leading to the large spontaneous magnetization (M s) of alloyed samples. The coercivity (i H c) of alloys is determined mostly by the MnBi grain distribution formed during the annealing process. Consequently, an alloy of large M s and i H c can be produced by controlling the required cooling rate as it figures␣in the arc-melting process and the appropriate annealing. The high-performance MnBi alloy was cold ball-milled into fine powders that were used for preparing MnBi bulk magnets by aligning in an 18 kOe-field followed by warm-compaction at 280°C and 12 MPa for 10 min. The magnet is highly anisotropic with an energy product of 7 MGOe.

Preparation and Magnetic Properties of MnBi/Co Nanocomposite Magnets

The method of synthesis and the magnetic properties of MnBi/Co nanocomposite magnets prepared with a combination of the magnetically hard MnBi alloy and semi-hard Co nanowires (CoNWs) have been investigated. The MnBi alloys were produced by arc-melting and temperature-gradient-driven annealing techniques. The CoNWs with high spontaneous magnetization M s (125 emu/g) and large aspect ratio α (5 ÷ 10) were synthesized by the solvothermal method. The nanocomposite MnBi/Co powder mixtures were cold ball-milled, aligned in an 18-kOe-field and warm-compacted into bulk magnets at 300°C under a uniaxial pressure of 2000 psi for 10 min. The magnetization and coercivity of the nanocomposite magnets were improved due to the intrinsic high magnetization and shape anisotropy of the CoNWs. The energy product, (BH)max, of the MnBi/Co nanocomposite magnets with 15 wt.% CoNWs reached its highest value of 4.8 MGOe. The simulation of magnetic properties of MnBi/Co magnets is also discussed in detail.

CaO-matrix processing of MnBi alloys for permanent magnets

The possibility to suppress agglomeration of MnBi alloy particles during milling and their unwanted sintering during subsequent annealing was explored by embedding the particles in CaO through co-milling. A 15 h annealing of the micron-sized MnBi particles embedded in the CaO matrix at 300 °C is not accompanied by sintering or growth of the particles while it significantly increases their coercivity – presumably by healing the milling-induced crystal defects. After separation from the CaO matrix, the annealed MnBi powder combines a calculated energy product of 10 MGOe with a room-temperature coercivity of 14.4 kOe. At the same time, the partial loss and degradation of the MnBi low-temperature phase during warm compaction of the powders makes the effect of the CaO-matrix annealing less pronounced in the case of fully dense magnets; the residue from the solvents employed for the removal of the CaO might have contributed to the decline of the properties. Still, a relatively high room-temperature coercivity of 8.5 kOe was obtained for the fuslly-dense MnBi magnet exhibiting an energy product of 5.3 MGOe.

XRD and XANES study of some Cu-doped MnBi materials

High purity MnBi low temperature phase has been prepared and analyzed using X- ray diffraction (XRD) and X-ray absorption near edge structure (XANES) measurements. The X-ray diffraction measurements were carried out using Bruker D8 Advance X-ray diffractometer. The X-rays were produced using a sealed tube and the wavelength of X-ray was 154 nm (Cu K-alpha). and X-rays were detected using a fast counting detector based on Silicon strip technology (Bruker LynxEye detector)[1]. and the X-ray absorption spectra has emerged as a powerful technique for local structure determination, which can be applied to any type of material. The X-ray absorption measurements of two Cu-doped MnBi alloys have been performed at the recently developed BL-8 Dispersive EXAFS beam line at 2.5 GeV Indus-2 synchrotron at RRCAT, Indore, India[2]. The X-ray absorption near edge structure (XANES) data obtained has been processed using data analysis program Athena. The energies of the K absorption edge, chemical shifts, edge-widths, shifts of the principal absorption maximum in the alloys have been determined.

Preparation and Magnetic Properties of MnBi Alloy and its Hybridization with NdFeB

MnBi alloys were fabricated by arc melting and annealing at 573 K. The heat treatment enhanced the content of the low-temperature phase (LTP) of MnBi up to 83 wt%. The Bi-excess assisted LTP MnBi alloys were used in the hybridization with the Nd-Fe-B commercial Magnequench ribbons to form the hybrid magnets (100-x)NdFeB/xMnBi, x = 20, 30, 40, 50, and 80 wt%. The as-milled powder mixtures of Nd-Fe-B and MnBi were aligned in a magnetic field of 18 kOe and warm-compacted to anisotropic and dense bulk magnets at 573 K by 2,000 psi for 10 min. The magnetic ordering of two hard phase components strengthened by the exchange coupling enhanced the Curie temperature (T c ) of the magnet in comparison to that of the powder mixture sample. The prepared hybrid magnets were highly anisotropic with the ratio M r /M s > 0.8. The exchange coupling was high, and the coercivity iHc of the magnets was ~11-13 kOe. The maximum value of the energy product (BH) max was 8.4 MGOe for the magnet with x = 30%. The preparation of MnBi alloys and hybrid magnets are discussed in details.

Metal-Redox Synthesis of MnBi Hard Magnetic Nanoparticles

High coercivity MnBi alloy is a promising candidate as earth abundant permanent magnet for energy-critical technologies. We report here a new metal-redox method to synthesize colloidal MnBi nanoparticles, exhibiting a saturation magnetization of 49 emu/g and coercivity of 15 kOe. It is shown that the magnetic properties of the MnBi nanoalloys can be readily modified by precursor stoichiometry, temperature ramp rate, and reaction temperature, making it a versatile scalable strategy for generation of MnBi.

Influence of jet milling process parameters on particle size, phase formation and magnetic properties of MnBi alloy

The effect of jet milling conditions on phase decomposition, particle size and magnetic properties of MnBi alloy were studied. It is observed that jet milling pressure plays a critical role in controlling the decomposition of MnBi phase, particle size and associated magnetic properties. As the jet pressure increases, more amount of MnBi phase decomposes into Bi and Mn phases. The weight fraction of MnBi phase decreases from 81%, for the starting powder, to 43% by increasing the milling pressure to a high value of 80psi. Particle size decreases rapidly to about 1μm on jet milling and even though the average particle size does not change noticeably with further increase of milling pressure, the particle size distribution appears to be different. Moderate milling pressure has resulted in powders with narrow particle size distribution, low decomposition of MnBi phase and smooth demagnetization curves. All the jet milled powders displayed high coercivity (Hc) values of about 18kOe with saturation magnetization in the range of 41–51emu/g.