Low Temperature Phase MnBi Research Articles

Magnetic properties of low temperature phase MnBi of island structure

The magnetic and structural properties of island-structured LTP MnBi fabricated onto MgO single crystal substrates are discussed. The size and height of the “Volmer-Weber” type islands vary from place to place but are averagely a few microns and sub-microns, respectively. From the wetting angle (40 ∼ 60°) of those islands, the surface energy ΥMnBi of LTP MnBi is found to be 0.5∼0.8 J/m2. Those MnBi islands possesses the magnetic anisotropy constant Ku and saturation magnetization Ms close to those for bulk over a temperature range of 5 to 400 K. There seems to be a correlation between Ku and lattice constant c measured at 300K. The Ku is found to be inversely proportional to Ms5 over the temperature range from 5 K to 400 K, as compared to the Ms8 dependence for those fabricated onto fused silica glass substrates.

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.

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.

Investigation Into Magnetic Anisotropy of Low-Temperature Phase MnBi Thin Films

The temperature dependence of magnetic anisotropy in the low-temperature phase MnBi thin films is discussed in conjunctionwith saturation magnetization and lattice constants. It is experimentally found that $K_{u}$ is inversely proportional to the eighth power of the saturation magnetization $M_{s}$ . It is also found that the $K_{u}$ is proportional to the unit-cell volume. The calculation based on first principles of magnetic anisotropy energy as a function of lattice constants ( $a$ and $c$ ) and statistical simulations of its spin-temperature dependence is consistent with the experimental results. However, the spin-reorientation transition taking place at 100 K is not accounted for by the present calculation.

XRD, LPF and FTIR investigation of Mn-Bi alloy

High purity MnBi low temperature phase has been prepared and analyzed using X- ray diffraction, Lorentz-Polarization Factor and Fourier transforms infrared measurement. After synthesis of samples structural characterization has done on samples by X-ray diffraction, which shows that after making the bulk sample is in no single phase MnBi has been prepared by sintering Mn and Bi powder. 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 0.154nm (Cu K-alpha).and x-rays were detected using a fast counting detector based on Silicon strip technology (Bruker LynxEye detector). By Lorentz- Polarization Factor is affecting the relative intensity of diffraction lines on a powder form. The infrared absorption spectra of the alloys and intermetallic compound were measured at room temperature, in the wave number range 4000 to 400 cm−1 by a computerized spectrometer type Jasco FTIR-300 (JAPAN) using the KBr pellet technique. And by FTIR which shows absorption peaks of MnBi alloys.

Electronic Structure and Maximum Energy Product of MnBi

We have performed first-principles calculations to obtain magnetic moment, magnetocrystalline anisotropy energy (MAE), i.e., the magnetic crystalline anisotropy constant (K), and the Curie temperature (Tc) of low temperature phase (LTP) MnBi and also estimated the maximum energy product (BH)max at elevated temperatures. The full-potential linearized augmented plane wave (FPLAPW) method, based on density functional theory (DFT) within the local spin density approximation (LSDA), was used to calculate the electronic structure of LPM MnBi. The Tc was calculated by the mean field theory. The calculated magnetic moment, MAE, and Tc are 3.63 μB/f.u. (formula unit) (79 emu/g or 714 emu/cm3), −0.163 meV/u.c. (or K = −0.275 × 106 J/m3) and 711 K, respectively. The (BH)max at the elevated temperatures was estimated by combining experimental coercivity (Hci) and the temperature dependence of magnetization (Ms(T)). The (BH)max is 17.7 MGOe at 300 K, which is in good agreement with the experimental result for directionally-solidified LTP MnBi (17 MGOe). In addition, a study of electron density maps and the lattice constant c/a ratio dependence of the magnetic moment suggested that doping of a third element into interstitial sites of LTP MnBi can increase the Ms.

Effect of composition and heat treatment on MnBi magnetic materials

The metallic compound MnBi is a promising rare-earth-free permanent magnet material, unique among all candidates for its high intrinsic coercivity (Hci) and its large positive temperature coefficient. The Hci of MnBi in thin-film or powder form can exceed 12 and 26kOe at 300 and 523K, respectively. Such a steep rise in Hci with increasing temperature is unique to MnBi. Consequently, MnBi is a highly sought-after hard phase for exchange coupling nanocomposite magnets. However, the reaction between Mn and Bi is peritectic, and hence Mn tends to precipitate out of the MnBi liquid during the solidification process. As result, when the alloy is prepared using conventional induction or arc-melting casting methods, additional Mn is required to compensate the precipitation of Mn. In addition to composition, post-casting annealing plays an important role in obtaining a high content of MnBi low-temperature phase (LTP) because the annealing encourages the Mn precipitates and the unreacted Bi to react, forming the desired LTP phase. Here we report a systematic study of the effect of composition and heat treatments on the phase content and magnetic properties of Mn–Bi alloys. In this study, 14 compositions were prepared using conventional metallurgical methods, and the compositions, crystal structures, phase content and magnetic properties of the resulting alloys were analyzed. The results show that the composition with 55at.% Mn exhibits both the highest LTP content (93wt.%) and magnetization (74emug−1 with 9T applied field at 300K).

Novel processing of high-performance MnBi magnets

Rare-earth-free MnBi magnets have attracted much attention recently due to their positive temperature coefficient of coercivity. In this work, the preparation, microstructure and magnetic properties of bulk MnBi magnets have been investigated. A low-temperature (−120 °C), low-energy ball-milling (LTLEBM) process has been adopted in the initial MnBi powder preparation that reduces the particle size to 1–5 μm from the 35–75 μm size of raw material powders of the MnBi low-temperature phase (LTP) (∼97 wt%) made by melting and annealing. The LTLEBM process has significantly suspended the decomposition of the LTP MnBi that occurs excessively during ordinary room-temperature ball milling. After the LTLEBM, the coercivity iHc of the MnBi powder was increased from 1 kOe to 12 kOe while the LTP content in the powder was retained as high as 95 wt%. The as-milled powders were then aligned in an 18 kOe field and warm-compacted into a dense bulk magnet at 300 °C for 10 min to reach a mass density of ∼8.4 g cm−3. The bulk magnets have a maximum energy product of 7.8 MGOe and coercivity of 6.5 kOe at room temperature. When the temperature is increased to 475 K, the coercivity is increased to 23 kOe.

Structure and magnetic properties of low-temperature phase Mn-Bi nanosheets with ultra-high coercivity and significant anisotropy

The microstructure, crystal structure, and magnetic properties of low-temperature phase (LTP) Mn-Bi nanosheets, prepared by surfactant assistant high-energy ball milling (SA-HEBM) with oleylamine and oleic acid as the surfactant, were examined with scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometer, respectively. Effect of ball-milling time on the coercivity of LTP Mn-Bi nanosheets was systematically investigated. Results show that the high energy ball milling time from tens of minutes to several hours results in the coercivity increase of Mn-Bi powders and peak values of 14.3 kOe around 10 h. LTP Mn-Bi nanosheets are characterized by an average thickness of tens of nanometers, an average diameter of ∼1.5 μm, and possess a relatively large aspect ratio, an ultra-high room temperature coercivity of 22.3 kOe, a significant geometrical and magnetic anisotropy, and a strong (00l) crystal texture. Magnetization and demagnetization behaviors reveal that wall pinning is the dominant coercivity mechanism in these LTP Mn-Bi nanosheets. The ultrafine grain refinement introduced by the SA-HEBM process contribute to the ultra-high coercivity of LTP Mn-Bi nanosheets and a large number of defects put a powerful pinning effect on the magnetic domain movement, simultaneously. Further magnetic measurement at 437 K shows that a high coercivity of 17.8 kOe and a strong positive temperature coefficient of coercivity existed in the bonded permanent magnet made by LTP Mn-Bi nanosheets.

Low-temperature phase MnBi compound: A potential candidate for rare-earth free permanent magnets

The low-temperature phase (LTP) MnBi is one of the few rare-earth free compounds that exhibit a large magnetocrystalline anisotropy energy in the order of 106J/m3. A large coercive field (μ0Hcj) above 1T can be obtained readily by reducing the crystallite size (D) through mechanical grinding (MG). The room-temperature Hcj values follow a phenomenological expression μ0Hcj=μ0Ha(δ/D)n where the anisotropy field (μ0Ha) is ∼4T, the Bloch wall width (δ) is 7nm and the exponent (n) is about 0.7 in our study. The grain refinement upon MG is accompanied by suppression of the spin reorientation transition temperature (TSR) from 110K to below 50K. The coercive field starts to exhibit positive temperature dependence approximately 50K above TSR and the room-temperature magnetic hardening induced by MG could partially be brought about by the lowered onset of this positive temperature dependence. The suppression of TSR by MG is likely to be induced by the surface anisotropy with which the 2nd order crystal field term is enhanced. One of the shortcomings of LTP-MnBi is its poor phase stability under the ambient atmosphere. The spontaneous magnetization decreases considerably after room-temperature aging for 1week. This is due to oxidation of Mn which leads to decomposition of the MnBi phase. Hence, the surface passivity needs to be established before this material is considered for a permanent magnet in practical uses. Another shortcoming is the limited spontaneous magnetization. The theoretical upper limit of the maximum energy product in LTP-MnBi remains only a quarter of that in Nd2Fe14B. Nevertheless, owing to the unique positive temperature dependence of the first-order anisotropy constant (K1), the hardness parameter (κ) of LTP-MnBi is enhanced above room temperature; κ reaches as large as 2.8 at 580K. This makes LTP-MnBi a possible candidate for the hard phase in rare-earth free nanocomposite magnets.

Appreciable Magnetic Moment and Energy Density in Single-Step Normal Route Synthesized MnBi

We study the structural and magnetic properties of the MnBi intermetallic compound. The LTP (Low Temperature Phase) MnBi compound is successfully synthesized in single step by vacuum encapsulation technique and rapid quenching from phase formation temperature. The phase purity and the magnetic moments of MnBi are highly dependent on heat treating schedule. The best phase purity and the magnetic moment are found for a heat-treated sample at 310 °C for 48 h. Rietveld fitted X-ray diffraction (XRD) patterns revealed that the studied MnBi compound is crystallized in hexagonal P63/mmc space group with minute presence of unreacted Bi and Mn phases. The scanning electron microscopy (SEM) study is carried out to visualize the grains morphology and phase identification. The bulk MnBi powder showed appreciable magnetic moment of ∼62 emu/g at 6 Tesla and maximum energy product BH max of 4.01 MGOe at 6 Tesla. The magnetic properties of synthesized MnBi show that it could be a potential candidate for rare earth free permanent magnets.

Magnetic properties of Mn–Bi melt-spun ribbons

Mn–Bi melt-spun ribbons with the low temperature phase (LTP) of MnBi were produced by melt-spinning and subsequent annealing. The as-rapidly quenched Mn–Bi melt-spun ribbons contained some LTP MnBi phase and exhibited a high coercivity exceeding 8kOe. Annealing of the melt-spun ribbons resulted in an increase in the amount of the LTP MnBi phase. A maximum remanence value of 42emu/g was achieved in Mn50Bi50 melt-spun ribbon annealed at 673K for 1h. High-temperature measurements revealed that the coercivity of the annealed Mn50Bi50 melt-spun ribbon increased with increasing ambient temperature. Although the Mn50Bi50 melt-spun ribbons showed a much smaller coercivity than Nd15Fe77B8 melt-spun ribbon at room temperature, it exhibited a higher coercivity at temperatures of 473K and higher. Therefore, the magnetic properties of Mn50Bi50 melt-spun ribbon are comparable to those of Nd–Fe–B melt-spun ribbon at an ambient temperature of 473K and become superior to those of Nd–Fe–B melt-spun ribbon at 573K.

Unexpected Magnetic Domain Behavior in LTP-MnBi

Low-temperature-phase MnBi (LTP-MnBi) has attracted much interest as a potential rare-earth-free permanent magnet material because of its high uniaxial magnetocrystalline anisotropy at room temperature, K ≈ 107, ergs/cc, and the unusual increase of anisotropy with increasing temperature, with an accompanying increasing coercive force (HC) with temperature. However, due to the complex Mn-Bi phase diagram, bulk samples of LTP-MnBi with the optimum saturation moment, ~75-76 emu/g have been achieved only with zone-refined single crystals. We have prepared polycrystalline samples of LTP-MnBi by induction melting and annealing at 300°C. The moment in 70 kOe is 73.5 emu/g, but HC is only 50 Oe. This is quite surprising-the high saturation moment indicates the dominating presence of LTP-MnBi. Therefore, an HC of some significant fraction of 2K/Ms ≈ 30 kOe would seem reasonable in this polycrystalline sample. By examining “Bitter” patterns, we show that the sample is composed ~ 50-100 μm of crystallites. The randomly oriented crystallites exhibit the variety of magnetic domain structures and orientations expected from the hexagonal-structured MnBi with its strong uniaxial anisotropy. Clearly, the reversal of magnetization in the sample proceeds by the low-field nucleation of reversed magnetization in each crystallite, rather than by a wall-pinning mechanism. When the annealed sample was milled into fine particles, increased by several orders of magnitude, as expected.

Temperature dependences of structure and coercivity for melt-spun MnBi compound

Low temperature phase MnBi compound was prepared by annealing the melt-spun amorphous MnBi ribbons. Temperature dependences of structure, magnetization and coercivity were investigated using neutron/X-ray powder diffraction and magnetic measurements. It is found that the lattice parameter c/a ratio of the MnBi increases with the increasing temperature and finally reaches a maximum of 1.433 around 600K. No structural phase transition is observed at 633K. Especially, the lattice constants of MnBi at 700K are a=b=4.30919Å, and c=6.17521Å, which are not in accordance with the previous data for bulk MnBi. Magnetic moment of Mn atom tends to lie in the ab-plane at 10K, and turns to align along c-axis around 90K. The growth of LTP MnBi grains is observed above 500K, which increases the content of MnBi. The coercivity of MnBi shows a positive temperature coefficient, reaches a maximum of 2.5T at 540K, and decreases to 1.8T at 610K. The temperature dependence of the coercivity is related to the change of magnetocrystalline anisotropy, and shows strong dependence on the sizes of MnBi particles.

Mn-Bi Magnetic Powders With High Coercivity and Magnetization at Room Temperature

Mn55Bi45+x at.% Mn ( x=0, 7 and 11) magnetic powders were prepared by low energy ball milling technique. The x-ray diffraction pattern showed that the samples mainly contain over 90% of the low temperature Mn-Bi phase for x = 0 and 7 with a very small amount of Bi phase. Microstructural analysis showed that the amount of sub-micron particles increases with milling time. The milling time dependent magnetic properties were studied for all the compositions. An intrinsic coercivity (Hci) of 12.2 kOe, remanent magnetization (4 πMr) of 7.1 kG, and energy product (BH)max of 11 MGOe have been obtained for the 8 hours milled Mn55Bi45+7 at.% Mn sample. These Mn-Bi powders have potential use as precursor for rare earth free permanent magnets.

Preparation and magnetic properties of MnBi

MnBi with low temperature phase was fabricated by melt-spinning and subsequently annealing. The influence of quenching speeds, compositions and annealing conditions on the formation of low temperature phase MnBi was systematically investigated. It was found the amorphous MnBi ribbons could transform into low temperature phase by heat treatment in a temperature range of 533–593 K. The coercivity of MnBi was greatly improved by porphyrization, and exhibited a positive temperature coefficient. The maximum energy product BHmax of the anisotropic bonded magnet is 7.1 MGOe (56 kJ/m3) and 4.0 MGOe (32 kJ/m3) at room temperature and 400 K.

Structural and magnetic properties of bulk MnBi permanent magnets

Structural and magnetic properties of bulk nanostructural Mn100−xBix (x = 40, 45, 52) permanent magnets prepared by spark plasma sintering technique were studied. The effect of the Mn/Bi ratio on the MnBi low temperature phase (LTP) formation and its magnetic properties were investigated. An increase of the bismuth amount in the magnets leads to better formation of LTP, resulting in the improvement of both magnetization (Ms) and remanence (Mr), but decreasing the coercivity (Hc) of the magnets. At room temperature, Ms increases from 27.87 emu/g for Mn60Bi40 to 45.31 emu/g for Mn48Bi52, whereas Hc decreases from 12 to 7.9 kOe. The microstructure of Mn48Bi52 magnet is composed of fine and uniform grains with an average size of 140 nm as shown in the TEM image. The Mn48Bi52 magnet shows a high Hc of 19 kOe at 423 K, indicating a strong positive temperature coefficient of coercivity for the MnBi magnet.

MnBi nanostructures: Size dependence of magnetostructural transition and matrix templating

Rapidly solidified MnxBi(1−x) (x=0.05 Mn5, x=0.10 Mn10) alloys form as NiAs-type low-temperature phase MnBi nanoparticles in a Bi matrix. Microstructural differences underlie diverse magnetic behavior: Mn5 has well-separated nanorods along the basal plane axial directions of Bi, while Mn10 consists of MnBi nanorods and submicron-sized MnBi grains. The Curie transition of Mn10 is TC=630K with a second-order thermodynamic character, while Mn5 nanorods exhibit a reduced and hysteretic TC=520K. Magnetic field annealing fosters (00n) alignment of the MnBi phase. Mn10 also shows alignment of the Bi matrix by a proposed templating mechanism.

Effect of magnetic field on the TC and magnetic properties for the aligned MnBi compound

In the compound MnBi, a first-order transition from the paramagnetic to the ferromagnetic state can be triggered by an applied magnetic field and the Curie temperature increases nearly linearly with an increase in magnetic field by ∼2 K/T. Under a field of 10 T, T C increases by 20 and 22 K during heating and cooling, respectively. Under certain conditions a reversible magnetic field or temperature induced transition between the paramagnetic and ferromagnetic states can occur. A magnetic and crystallographic H– T phase diagram for MnBi is given. Magnetic properties of MnBi compound aligned in a Bi matrix have been investigated. In the low temperature phase MnBi, a spin-reorientation takes place during which the magnetic moments rotate from being parallel to the c-axis towards the basal plane at ∼90 K. A measuring Dc magnetic field applied parallel to the c-axis of MnBi suppresses partly the spin-reorientation transition. Interestingly, the fabricated magnetic field increases the temperature of spin-reorientation transition T s and the change in magnetization for MnBi. For the sample solidified under 0.5 T, the change in magnetization is ∼70% and T s is ∼91 K.

Alignment and analyses of MnBi∕Bi nanostructures

A Mn5Bi95 alloy was rapidly solidified into a mixture of nanocrystalline Bi and metastable Bi(Mn). Heating the ribbons to temperature T=525K in a dc magnetic field causes formation and c-axis alignment of low-temperature phase (LTP) MnBi nanorods along the applied field direction. Nanorod alignment increases with increased magnetic field, with a calculated alignment half-angle of 47° for a sample heated to 520K at 50kOe. In situ magnetization changes suggest that nanorod alignment is achieved by rotation of MnBi particles. Particle alignment enables the measurement of the MnBi nanorod spin reorientation temperature of 100K, the same as its bulk counterpart.