Ferromagnetic Τ-phase Research Articles

Structural and magnetic properties of rapidly solidified and Bi-bonded hot compacted Mn[formula omitted]Al45C1.7Ti[formula omitted] magnets

This study examines the impact of Ti doping on the phase formation and magnetic properties of Mn-Al-C alloys. Using melt spinning, we synthesized Mn53.3−xAl45C1.7Tix alloys (x values ranging from 0 to 1.5) producing high purity ϵ-phase ribbons. The optimal annealing treatment for the formation of ferromagnetic τ-phase is 20 min at 550 °C for all compositions. However, Ti-doped samples showed the formation of β-phase due to partial phase decomposition, and TiC precipitates were present at high Ti concentrations. Upon doping, the Curie temperature was increased from 557K (Ti free sample) to 600K. To improve the coercivity, a combination of ball milling and hot compaction, introducing Bi as a metallic binder, have resulted in increased coercivity from 0.18 T to 0.33 T. Bi addition lead to formation of a grain boundary phase aiding densification. We demonstrate how modifying composition and processing can improve the magnetic properties of Mn-Al based magnets.

Crack reduction in laser powder bed fusion of MnAl(C) using graphene oxide coated powders

MnAl(C) is a promising candidate as a rare earth free magnet. When processing MnAl(C) in laser powder bed fusion (L-PBF) the high cooling rates can retain the high temperature ε-phase which can then be annealed at low temperatures to yield the ferromagnetic τ-phase. However, MnAl(C) has been shown to be difficult to print using L-PBF and the material is prone to severe cracking. In this study, we have investigated how the addition of a graphene oxide (GO) coating on the powders can affect the processability of MnAl(C) and properties of the printed parts. MnAl(C) powders were coated with 0.2 wt.% GO using a wet-chemical process. The addition of GO reduced crack formation in the printed parts, and also influenced the degree of ⟨001⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle {001} \\rangle$$\\end{document} texture along the build direction. After printing, densities of 93% and 87% could be achieved for the reference and 0.2 wt.% GO, respectively. Furthermore, a 35% reduction of cracking was calculated from image analysis, comparing printed samples produced from coated and non-coated powders. Both powders formed mostly the ε-phase but some two-phase regions with a mix of γ- and ε-phase could be observed in the as-printed parts, but seemed to be more prominent in the uncoated reference samples and could also be linked to cracks. The τ-phase together with smaller amounts of secondary phases was obtained after heat treatment at 560 °C for 5 min for both samples. Vibrating sample magnetometry was used to measure the magnetic properties, the reference had a remanence of 33 Am2/kg and a coercivity of 139 kA/m, and the 0.2 wt.% GO sample showed a similar remanence of 30 Am2/kg and coercivity of 130 kA/m. These results show that GO coating is a viable method to reduce detrimental cracking in L-PBF MnAl without reducing the magnetic performance of the material.

Targeted hydrolysis and decrepitation of Mn3AlC precipitates: a route to a novel precursor of rare earth free MnAl–C permanent magnets

In this work, Mn–Al–C alloys consisting of ϵ-MnAl and carbides of the types Mn3AlC and Al4C3 were produced and exposed to water at 80 °C for 35 d. This led to the hydrolysis and decrepitation of the carbides and ultimately, of the material. The hydrolysis behaviour was studied by combined mass spectrometry, thermogravimetry and differential scanning calorimetry. Microstructural studies with scanning electron microscopy and x-ray diffraction were carried out. X-ray computed tomography was used to study the evolution of cracks on a macroscopic scale. As the Mn3AlC precipitates were present as parallel platelets inside the ϵ-grains, the hydrolysis resulted in parallel voids separating regions of ϵ with the same orientation. This ϵ-skeleton could be easily milled into ϵ-flakes, which could be mechanically aligned so that their <001> directions were parallel. The ϵ-powder was transformed into the ferromagnetic τ-phase via heat treatment and is therefore suitable as a novel precursor material for the production of anisotropic τ-MnAl–C-based permanent magnets. Employing such a textured, ϵ-phase, precursor powder in an extrusion process could result in increased texture quality of the τ-phase with correspondingly enhanced magnetic properties.

Additive Manufacturing of MnAl(C)-Magnets

Permanent magnets are becoming more and more relevant for modern society. As the most widely used permanent magnets contain rare-earth elements, the increased dependence on these strategic elements is worrisome, and the pursuit for rare-earth free alternatives has become a strategic goal in many countries. The metastable and ferromagnetic τ-phase that forms in the MnAl(C) system is one of the most promising alternatives, and since its discovery, major efforts have been made to improve its performance and realize its full potential. One major factor that has prevented a widespread commercialization of MnAl(C) permanent magnets is their relatively low coercivity. Here, we demonstrate that additive manufacturing, using laser powder bed fusion, can be used to produce MnAl in its high-temperature polymorph (ε, hcp), which can be subsequently transformed, through post-heat treatments to the ferromagnetic τ-phase. Although we successfully obtained a preferential orientation of the ε-phase with &lt;001&gt; parallel to the build direction, this did not translate into a strong preferential orientation in the τ-phase, thus indicating that the phase transformation occurs by the migration of incoherent interfaces. The MnAl(C) samples are characterized by a density of ≈4.4 g/cm3, a saturation magnetization of 39.3 Am2/kg, a coercivity of 168 kA/m, and a remanence of 17.5 Am2/kg.

Composition and property optimization of rare-earth-free Mn-Al-C magnet by phase stability and magnetic behavior analysis

Mn-Al-C system offers a possibility of rare-earth-free permanent magnets with the reduced temperature-dependent deterioration of magnetic properties. The Mn-Al-C alloy composition and magnetic properties have been optimized via calculations of electronic structures and phase stability of L 1 0 -ordered ferromagnetic τ-phase (Mn 0 . 5 Al 0 . 5 ) 100−x C x . The WIEN2k program package, Vienna Ab-initio simulation package (VASP), and Alloy Theoretic Automated Toolkit (ATAT) were used to calculate and identify the optimal carbon content for the most stable τ-phase of the L 1 0 -structured Mn-Al-C. We used the Brillouin function and Callen-Callen semiempirical relation to obtain the saturation magnetization and magnetocrystalline anisotropy constant at elevated temperatures. It was found that a carbon content of 2.33 at% gives the most stable τ-phase L 1 0 (Mn 0 . 5 Al 0 . 5 ) 100−x C x that has the lowest formation energy and highest saturation magnetization among the studied carbon contents (x = 0–3.03 at %). The magnetocrystalline anisotropy constant at 0 K increases with increasing the carbon content. Therefore, the carbon-doped Mn-Al becomes magnetically harder than the pure Mn 50 Al 50 . However, the anisotropy constant decreases at 300 K as the carbon content increases. The Curie temperature decreases to 590 K at x = 2.33 from 685 K at x = 0.0. The estimated saturation magnetization was approximately 130 emu/g at 300 K, leading to 18 MGOe under B s = B r and H ci > B r /2. Therefo r e, it is highly probable that Mn-Al-C potentially fills the gap between 10 and 30 MGOe magnets. The results in this study quantify and explain the reason for widely studying the approximately 2 at% carbon-doped Mn-Al systems. • A rare-earth-free, ferromagnetic L10 τ-phase (Mn0.5Al0.5)100-xCx was designed by first-principle calculations. • The calculated carbon content of 2.33 at% gives the lowest formation energy. • Theoretically optimized carbon content was in good agreement with experimental results. • The results explain the reason for widely studying the approximately 2 at% carbon-doped Mn-Al systems. • Rare-earth-free Mn-Al-C potentially fill the gap between 10 and 30 MGOe magnets.

Perpendicularly magnetized ferromagnetism in Mn/Al bilayer thin films on Si substrates induced by temperature dependent ion beam mixing

The perpendicularly magnetized τ-phase was achieved in Mn/Al bilayer thin films by ion beam mixing using 400 keV Xe ions. The effect of annealing and irradiation on the structural, microstructural, and magnetic properties of Mn/Al bilayer thin films was investigated in the present study. Mn/Al bilayer thin films (thickness ∼97 nm) were deposited on silicon substrates by the evaporation technique. These as-deposited films were irradiated to the fluences of 5 × 1016 and 1 × 1017 ions cm−2 at three substrate temperatures (Room temperature ∼300 K, 450 K, and 623 K). As-deposited films were also annealed at 450 K and 623 K for 5 h. The ferromagnetic τ-phase was enhanced in annealed and irradiated thin films confirmed from synchrotron x-ray diffraction patterns, MFM images, and magnetic hysteresis curves. The average grain size estimated from the AFM study was found in the range of 78–192 nm. The domain patterns obtained in MFM images confirmed the ferromagnetic nature of the annealed and irradiated films. The hysteresis curves measured at 5 K show the maximum coercivity (7111 Oe) for the sample annealed at 450 K and the highest value of saturation magnetization (60 emu cc−1) for the film irradiated to the fluence of 1 × 1017 ions cm−2 at the substrate temperature of 450 K. Perpendicularly magnetized τ-phase was achieved in the films irradiated to the fluence of 1 × 1017 ions cm−2 at different temperatures. The formation of the ferromagnetic phase by ion beam mixing can be attributed to the combined effect of ballistic mixing and thermal spike mechanisms. This study suggests that the ferromagnetic properties of MnAl thin films can be modified by selecting the proper annealing and irradiation conditions to make them useful for applications in spintronics and magnetic memory devices.

Dual Acceleration of ε-τ Transformation in Mn–Al Induced by Zn-Addition and In-Magnetic-Field Annealing

Combined accelerating effects of ε–τ phase transformation by in-magnetic-field annealing and Zn-addition were investigated for Mn53Zn2Al45 in magnetic fields up to 15 T. The in-field annealing for Zn-added sample accelerated the ε–τ transformation 16 times faster than Mn55Al45 at annealing temperature of 623 K. The reduction of magnetization after finishing ε–τ transformation was also more rapid than that of Mn55Al45. However, magnetic-field-induced acceleration of ε–τ phase transformation and suppression of precipitation of β-phase were observed for Zn-added alloy Mn53Zn2Al45 annealed at 573 K. The obtained results suggested that the β-phase stabilized against both τ- and ε-phases by Zn-addition. Therefore, although the ε–τ transformation was efficiently accelerated by in-field annealing of Mn53Zn2Al45, more precisely control of annealing time is required for obtaining high fraction of ferromagnetic τ-phase.

Recent developments of manganese-aluminium as rare-earth-free magnets

This article reviews findings and progresses in the past decade on manganese-aluminium (MnAl) based magnets as the interest has been revived to fulfill their potential as commercial magnets. The challenges in developments of these rare-earth-free magnets are to acquire a high remanence and coercivity from the ferromagnetic τ-phase in MnAl alloys. To this end, the phase transformation to this τ-MnAl with L10 body centered tetragonal structure has been promoted by a variety of methods and a few percents of carbon (C) is often added to prevent the phase decomposition. Magnetization and coercivity are not only influenced by the phase composition but also the microstructure. The fabrication processes and factors affecting the phase and microstructure are therefore covered. Finally, the productions of bulk MnAl magnets are addressed.

Fabrication of bulk τ MnAl–C magnets by hot-pressing from ε-phase gas-atomized and milled powder

Isotropic nanocrystalline MnAlC particles with quasi-spherical shape were synthesized by gas-atomization being afterwards milled for a short time of 60 s. Gas-atomized and milled particles were compacted by the hot-pressing method to obtain bulk MnAlC magnets. An adequate combination of the parameters during hot-pressing made it possible to fabricate bulk MnAlC magnets with a simultaneous transformation of ε-phase into the ferromagnetic τ-phase. The study has allowed for correlating the morphology, microstructure and magnetic properties of the bulk magnets obtained from the different powder samples. Hot-pressing experiments performed on milled ε-phase based powders resulted in an increased coercivity as a consequence of a reduced crystallite size (about 25 nm), induced strain (about 0.40%) and the formation of both β-Mn phase and Mn3AlC. This single-step approach has been compared with a two-step process comprising annealing of the gas-atomized ε-phase to develop the τ-phase as a first step followed by compaction via hot-pressing in a second step. Comparable magnetic properties obtained for the bulk magnets resulting from this two-steps route validates the single-step approach followed in this study. This work shows that gas-atomization, flash-milling and hot-pressing of ε-phase MnAlC powder is an efficient path towards the fabrication of bulk τ-MnAlC magnets.

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.

Investigation of phase selection hierarchy in Mn–Al alloys

Primarily attributed to the formation of the ferromagnetic τ-phase, near equiatomic composition of Mn–Al have recently received much attention in permanent magnet industry. Several mechanisms have been proposed in literature for the τ-phase formation but controversy still arises regarding the dominating mode. In the current work, MnAl-based alloys having different compositions in a range between Mn50.5Al49.5 and Mn57Al43 have been studied by means of in-situ high energy X-ray diffraction, differential scanning calorimetry (DSC) and magnetic measurements. Synchrotron and DSC experiments showed the dependence of the τ-MnAl on the intermediate disordered ε′-phase. Alloys having 53.4 at% or less Mn (S1, S2) followed a transformation route of ε+ε'→τ→β+γ2 upon annealing. Alloys having more than 53.4 at% of Mn had only ε-phase. High energy X-ray diffraction patterns showed that ε-phase directly transformed into stable phases in the absence of ε′-phase. It is observed that ε′ not only promoted the ferromagnetic τ-phase but also stabilized it by delaying the nucleation of stable phases.

Irradiation induced enhancement of ferromagnetic τ-phase in MnAl alloy thin films on Si substrate

Ferromagnetic L10 MnAl phase was achieved in as-grown MnAl alloy thin films by using 250 keV H+ ion irradiation without conventional post annealing. The magnetic properties and crystallinity of MnAl thin film got modified with increase in irradiation fluence. The magnetic coercivity was found to be maximum (2500 Oe) at the fluence of 5 × 1014 ions cm−2. The hysteresis curve for the sample irradiated by H+ ions with fluence of 5 × 1016 ions cm−2 exhibited highest saturation magnetization (130 emu/cc) among all irradiated films. The increase in irradiation fluence led to larger grain growth as confirmed from AFM results. The magnetic domain pattern observed by MFM images confirmed the presence of ferromagnetic τ-phase in irradiated samples. The present study establishes that the H+ ion-irradiation can be an effective technique to tailor the structural as well as magnetic properties of MnAl thin films.

Magnetic properties of Mn54Al46C2.44/Sm2Fe17N3 and Mn54Al46C2.44/Fe65Co35 composites

Ferromagnetic τ-phase Mn54Al46C2.44 particles were synthesized, and its composites with commercial Sm2Fe17N3 and synthesized Fe65Co35 powders were fabricated. Smaller grain size than the single domain size of the Mn54Al46C2.44 without obvious grain boundaries and secondary phases is the origin for the low intrinsic coercivity. It was confirmed that the magnetic properties of the Mn54Al46C2.44 can be enhanced by magnetic exchange coupling with the hard magnetic Sm2Fe17N3 and soft magnetic Fe65Co35. The high degrees of the exchange coupling were verified by calculating first derivative curves. Thermo-magnetic stabilities of the composites from 100 to 400 K were measured and compared. It was demonstrated that the Mn54Al46C2.44 based composites containing Sm2Fe17N3 and Fe65Co35 could be promising candidates for future permanent magnetic materials with the proper control of purity, magnetic properties, etc.

Manufacturing of intermetallic Mn-46%Al by laser powder bed fusion

Laser powder bed fusion (LPBF) provides an excellent opportunity to use custom powders for complex objects without extensive machining. This opportunity is attractive for brittle and hard intermetallics, but is challenging due to cracking, anisotropy, and the formation of non-equilibrium phases. The present investigation is focused on a development of the process parameters for pre-alloyed Mn-46 at.%Al gas atomized intermetallic powder, which is a promising magnetic material. A hierarchical approach involving optimization of the process parameters for a single track, a single layer, and then a 3D specimen was applied. The manufacturing of single tracks was performed at scanning speeds of 0.06-3.4 m/s and laser powers of 50-350 W. Test parameters guaranteeing stable single track with constant width and height, and sufficient remelting depth were selected for further manufacturing. Surface morphology, chemical composition, crack density and distribution, and the microstructures in the final materials were investigated. It was shown that the consists mostly of the ε-phase with some amounts of equilibrium γ2 and β phases and the ferromagnetic τ-phase. The presence of the ε-phase shows a potential to use heat treatment to form τ-phase magnetic phase in AM Mn-46 at.%Al. Future investigations will clarify the applicability of LPBF to manufacture Mn-46%Al for magnetic applications.

Anisotropic single-variant of (Mn54Al46)97C3

The Mn-Al permanent magnet is considered as a candidate to fill the gap between the best ferrite and Nd–Fe–B magnets. Anisotropy is hard to be obtained due to the formation of randomly oriented multi-variants of the ferromagnetic τ-phase. In this work, an anisotropic single-variant is achieved through constrained martensitic transformation of the (Mn54Al46)97C3 single crystal, which is based on the principle of magnetoelastic energy. This work guides a new approach for developing anisotropic Mn-Al permanent magnets.

Effect of the Crystallization Conditions of Alloy Mn54Al43C3 on the Magnetic Properties of Mechanically Milled Powder

Results of formation of a ferromagnetic τ-phase and an antiferromagnetic e-phase in alloys of type Mn54Al43C3 obtained by induction and arc melting and by rapid hardening of the melt are presented. The magnetic hysteresis properties of powders produced by high-power mechanical milling of the synthesized alloys are studied. The optimum temperature conditions for transformation of the e-phase into a ferromagnetic τ-phase are determined for the alloys and for their ultrafine powders. The resulting powders of Mn54Al43C3 have an ultimate specific magnetization σ m = 81 (G · cm3)/g and a coercive force H c = 3 kOe in a magnetic field of 12 kOe.

Effects of Annealing Temperature and Magnetic Field on the $\varepsilon\hbox{--}\tau$ Phase Transformation in Mn-Al Alloys

Magnetic field effects on the transformation from the hexagonal close-packed e-phase to the ferromagnetic L1 0 τ-phase were evaluated for Mn-Al alloys annealed at 573, 623 and 673 K in magnetic fields up to 15 T. The phase transformation from the e-phase to the τ-phase was enhanced by the magnetic field for the 623 K annealing temperature. These in-field annealing effects are explained by the Zeeman energy of the ferromagnetic τ-phase.

Magnetization and Intrinsic Coercivity for τ-phase Mn54Al46/α-phase Fe65Co35Composite

We have synthesized ferromagnetic τ-phase Mn 54 Al 46 /α-phase Fe 65 Co 35 composite by annealing a mixture of paramagnetic e-phase Mn 54 Al 46 and ferromagnetic α-phase Fe 65 Co 35 particles at 650℃. The volume fraction ( f h ) of hard τ-phase Mn 54 Al 46 of the composite was varied from 0 to 1. During the annealing, magnetic phase transformation occurred from paramagnetic e-phase to ferromagnetic τ-phase Mn 54 Al 46 . The magnetization and coercivity of the composite monotonically decreased and increased, respectively, as the f h increased. These results are in good agreement with our proposed composition dependent coercivity and modified magnetization equations.

A comparison of τ-MnAl particulates produced via different routes

MnAl alloys are very promising rare-earth-free permanent magnets. Nanocrystalline microstructures can have beneficial effects on the properties of magnetic MnAl alloys. In the present work we examined multiple routes to process MnAl alloys and studied the effects of milling on Mn-46 at.% Al powders. Mn54Al46 was produced via gas atomization, melt spinning, and rapid solidification rate processing. It was then mechanically milled using a water-cooled Union Process attritor for times up to 20 h. X-ray diffraction patterns showed the presence of mostly the high-temperature ε-phase with significant amounts of the equilibrium γ2 and β phases in both the cast and milled particulates. The powders were annealed for various temperatures and times in order to obtain the ferromagnetic τ-phase. Magnetic measurements of the optimally annealed powders showed a coercivity of 3.62 kOe and saturation magnetization of 59.8 emu g−1 for mechanically milled gas-atomized powder annealed at 500 °C for 30 min.

Spark plasma sintering of Mn–Al–C hard magnets

Structural and magnetic characterization of isotropic Mn–Al–C bulk samples obtained by spark plasma sintering (SPS) is reported. This technique, to the best of our knowledge, has not been used for preparation of Mn–Al-based permanent magnets previously. Transformation from the parent ϵ-phase to the ferromagnetic τ-phase occurred on heating in the process of sintering. The phase constitution of the melt-spun precursors and consolidated samples was determined by x-ray diffraction. Magnetic hysteresis loops were recorded using a vibrating sample magnetometer. The compositional dependence of the coercivity, magnetization and density of the sintered materials is analysed. To combine good magnetic properties with proper densification, further optimization of the production parameters is necessary.