Rare-earth Free Permanent Magnets Research Articles

Production of net-shape Mn-Al permanent magnets by electron beam melting

The main goal of this work is the adoption of additive manufacturing for the production of inexpensive rare-earth free MnAl-based permanent magnets. The use of more advanced binder-free additive manufacturing technique such as Electron Beam Melting (EBM) allows obtaining fully-dense magnetic materials with advanced topology and complex shapes. We focus on the feasibility of controlling the phase formation in additively manufactured Mn-Al alloys by employing post-manufacturing heat treatment. The as-manufactured EBM samples contain 8% of the desired ferromagnetic τ-MnAl phase. After the optimized annealing treatment, the content of the τ-phase was increased to 90%. This sample has a coercivity value of 0.15 T, which is also the maximum achieved in conventionally produced binary MnAl magnets. Moreover, the EBM samples are fully dense and have the same density as the samples produced by conventional melting density.

Interdiffusion Induced High Coercivity of Rare-Earth Free Alnico Thin Films

Rare-earth free Alnico permanent magnet is composed of spinodally decomposed ferromagnetic L10 Fe-Co rich (α1) phase embedded in paramagnetic or weak ferromagnetic L12 Al-Ni rich (α2) matrix. We deposited α1 phase alloy (Fe52Co35.8Al5.6Ni5.3Ti0.9Cu0.4) on (111) oriented Pt(100 nm)/TiO2/SiO2/Si substrate to investigate coercivity enhancement induced by the α1 and substrate interdiffusion through annealing. It is demonstrated that as-deposited α1 film is magnetically soft with coercivity ~0.05 kOe, while its coercivity increased to 5.4 kOe after annealing at 800 oC. This is attributed to interdiffusion between α1 and Pt. Therefore, the occurrence of large magneto-crystalline anisotropy tetragonal L10 (Co, Fe)Pt.

Enhancement of maximum energy product in exchange-coupled BaFe12O19/Fe3O4 core-shell-like nanocomposites

Recent economic and environmental concerns have prompted intensive research on the development and optimisation of rare-earth free permanent magnets, in particular of ferrites. M-type barium hexaferrites (BaFe12O19, BaM) are a type of technologically important, low-cost permanent magnet, with high Tc and high resistance to oxidation and corrosion. Their magnetic performance can be improved upon by exploring exchange-coupling mechanisms, to increase their competitiveness with existing rare-earth magnets. The present investigation explores core-shell-like BaM/Fe3O4 nanocomposites, where BaM flake-like particles where prepared via the sol-gel auto-combustion method, and then coated by magnetite spinel nanoparticles via a hydrothermal method, requiring no post-heat treatment. We show how optimised hard to soft magnetic phase ratio and preparation conditions lead to a significant enhancement to the saturation magnetization and remanence, and consequently to an increase of over 75% in the maximum energy product, compared to the parent BaM hexagonal ferrite compound.

Tunable magnetic anisotropy in nanostructured permanent magnet: A micromagnetic study

Abstract The optimization of the magnetic energy of nanowires assembly is a delicate balance between the magnetic properties of each nanowire, their organization and their density inside the bulk assembly. As a matter of fact, from the simulation point of view, a dense packed structure of Co nanowires with high magnetocrystalline anisotropy and perfect alignment is the best solution to obtain a high magnetic energy and thus a powerful rare earth free permanent magnet. However, experiments have been shown that this is not a trivial task since the misalignment of nanowires inexorably occurs and likely reduces (more than expected) the magnetic properties of the final nanostructured assembly. Experiments also suggest that when the hardness and/or relative density (%) of the ensemble increases, the magnetic properties decreases casting the seeds of doubt on the realistic possibility of using these sintered materials in a new rare earth technology. Inside this frame, we report here the effect of a realistic misalignment reproducing the one observed experimentally inside the nanowires assembly. We show how, dipolar interactions and macroscopic shape contribute in different ways to destroy the magnetic energy of the bulk nanostructured material. However, we report the critical percentage of misalignment and disorder still affordable to keep the magnetic properties of the bulk material. We also demonstrate that these nanostructured materials can be interesting for their high macroscopic magnetic anisotropy. Last but not least, we show that the macroscopic shape of the nanowires assemblies presents very different critical sensibility to the misalignment/disorder of the arrangement. In particular, we report how a bi-dimensional arrangement of nanowires can be more magnetically stable compared to the tri-dimensional one. All these results give insights on the future strategies to optimize the magnetic properties of cobalt-based nanostructured material.

Structural and magnetic properties of nanocrystalline Zr-Co-Nb-Si-B rare earth free permanent magnets fabricated by spark plasma sintering

Abstract Rare earth free nanocrystalline Zr16Co78-xNbxSi3B3 (x = 0–3) bulk magnets were fabricated from melt spun ribbons with high density by employing spark plasma sintering (SPS) technique. The effect of SPS temperature on structural and magnetic properties was investigated. The hard magnetic Zr2Co11 phase which was present in the as-spun ribbons retained after SPS, indicating no phase decomposition. The magnetic properties were found to depend on consolidation temperature and Nb concentration. Optimum magnetic properties of coercivity of 3.3 kOe with saturation magnetization of 52 emu/g were obtained for x = 1 magnet consolidated at 600 °C. SPS magnets display a moderate magnetic texture and the coercivity mechanism is governed by combination of both domain wall pinning and nucleation of reverse domains.

Designing Rare-Earth Free Permanent Magnets in Heusler Alloys Via Interstitial Doping

Based on high-throughput density functional theory calculations, we investigated the effects of light interstitial H, B, C, and N atoms on the magnetic properties of cubic Heusler alloys, with the aim to design new rare earth free permanent magnets. It is observed that the interstitial atoms induce significant tetragonal distortions, leading to 32 candidates with large (> 0.4 MJ/m3) uniaxial magneto-crystalline anisotropy energies (MAEs) and 10 cases with large in-plane MAEs. Detailed analysis following the the perturbation theory and chemical bonding reveals the strong MAE originates from the local crystalline distortions and thus the changes of the chemical bonding around the interstitials. This provides a valuable way to tailor the MAEs to obtain competitive permanent magnets, filling the gap between high performance Sm-Co/Nd-Fe-B and widely used ferrite/AlNiCo materials.

Designing rare earth free permanent magnets: insights from small Co clusters.

With the ultimate goal of rational design of permanent magnets without rare earth elements, we study the magnetic properties of Co4 clusters doped with six different group 14 and group 15 elements, using first principles electronic structure methods, based on density functional theory. For the first time, low energy isomers and magnetic moments of pure Co4 and Co6 clusters, and the doped clusters are identified through a global search employing an evolutionary algorithm. The magnetic anisotropy energy (MAE) of these clusters is then calculated. No correlation is found between the type of dopant atom and MAE. Through further analyses we establish that MAE is largely determined by the sum of absolute values of the spin moments on the individual atoms, and the HOMO-LUMO gap of the system. The importance of these in designing new permanent magnetic materials is discussed.

Measured and calculated properties of B-doped τ-phase MnAl – A rare earth free permanent magnet

Abstract The metastable tetragonal τ-phase MnAl has been doped interstitially with B through a drop synthesis method creating the (Mn0.55Al0.45)B0.02 compound. The as-casted samples were annealed, quenched and thereafter ball-milled and relaxed in order to decrease grain size and reduce the number of crystallographic defects. The Curie temperature of the quenched sample was estimated to 655 K. The magnetization, coercivity and anisotropy were analyzed with respect to flash-milling time, relaxation time and temperature. The results show that (Mn0.55Al0.45)B0.02 could be directly obtained from drop synthesis. The highest measured saturation magnetization of 393 kA/m (measured at ±1440 kA/m) was achieved with a relaxation process after 1.5 h milling, giving a theoretical maximum energy product of 48 kJ/m3. The highest value of the coercivity was 355 kA/m achieved by flash-milling for 10 h. However, the high coercivity was achieved at an expense of low saturation magnetization.

Magnetic and Structural Properties of L10 Mn–Ga–Al Epitaxially Grown Thin Films of Island Structure

Magnetic and Structural Properties of L1₀ Mn–Ga–Al Epitaxially Grown Thin Films of Island Structure

Evidence of exchange coupling in τ-MnAlC/FeCo system

Abstract The τ -MnAlC/FeCo system has potential to be applied as rare earth-free permanent magnet, due to exchange coupled between hard and soft magnetic phases as alternative for the development of high energy product permanent magnets. In this work the hard-magnetic phase Mn53.3Al45C1.7 ( μ 0Hc = 0.37 T of coercive force, Mr = 23.52 Am2/kg of remanent magnetization, Ms = 52.94 Am2/kg of saturation magnetization and (BH)max = 3.89 kJ/m3 of maximum energy product) was mixed with the soft magnetic phase Fe65Co35 ( μ 0Hc = 0.007 T and Ms = 221.33 Am2/kg) using a solid-state procedure. The magnetic materials were sintered using different ratios of hard and soft magnetic phases: 95:5, 90:10, 85:15 and 80:20; and different sintering temperatures (300, 400, 500 and 600 °C) in order to improve the magnetic properties. The magnetic exchange coupling between MnAlC and FeCo was proved by the hysteresis loop and its corresponding Thamm-Hesse analysis. The best result was obtained for MnAlC/FeCo (95/5 wt%) sintered at 500 °C during 30 min, for which the magnetic properties were μ 0Hc = 0.277 T, Ms = 76.43 Am2/kg and (BH)max = 5.57 kJ/m3.

Microstructure and coercivity in alnico 9

Magnetic property enhancement of alnico, a rare-earth free permanent magnet, is highly dependent on both the initial microstructure and the evolution of the spinodal decomposition (SD) inside each grain during the heat treatment process. The size, shape and distribution of the magnetic FeCo-rich (α1) phase embedded in continuous non-magnetic AlNi-rich (α2) phase as well as a minor Cu-enriched phase residing in between are shown to be crucial in controlling coercivity. Phase and magnetic domain morphology in a commercial alnico 9 alloy was studied using a combination of structural characterization techniques, including scanning electron microscopy, electron backscatter diffraction, aberration-corrected scanning transmission electron microscopy and Lorentz microscopy. Our results showed that casting created structural nonuniformity and defects, such as porosity, TiS2 precipitates and grain misorientation, are heterogeneously distributed, with the center section having the best crystallographic orientation and minimal defects. The optimal spinodal is a “mosaic structure”, composed of rod-shape α1 phase with {1 1 0} or {1 0 0} planar faceting and diameter of ∼30–45 nm. There is also a Cu-enriched phase residing at the corners of two 〈1 1 0〉 facets of the α1 phase. It was observed that grain boundary phase reverse magnetization direction at lower external magnetic field compared to the SD region inside the grain. Improving grain orientation uniformity, reducing detrimental grain boundary phase volume fraction, and the branching of the α1 rods, as well as their diameter, are promising routes to improve energy product of alnico.

Development of permanent magnet MnAlC/polymer composites and flexible filament for bonding and 3D-printing technologies

Searching for high-performance permanent magnets components with no limitation in shape and dimensions is highly desired to overcome the present design and manufacturing restrictions, which affect the efficiency of the final devices in energy, automotive and aerospace sectors. Advanced 3D-printing of composite materials and related technologies is an incipient route to achieve functional structures avoiding the limitations of traditional manufacturing. Gas-atomized MnAlC particles combined with polymer have been used in this work for fabricating scalable rare earth-free permanent magnet composites and extruded flexible filaments with continuous length exceeding 10 m. Solution casting has been used to synthesize homogeneous composites with tuned particles content, made of a polyethylene (PE) matrix embedding quasi-spherical particles of the ferromagnetic τ-MnAlC phase. A maximum filling factor of 86.5 and 72.3% has been obtained for the composite and the filament after extrusion, respectively. The magnetic measurements reveal no deterioration of the properties of the MnAlC particles after the composite synthesis and filament extrusion. The produced MnAlC/PE materials will serve as precursors for an efficient and scalable design and fabrication of end-products by different processing techniques (polymerized cold-compacted magnets and 3D-printing, respectively) in view of technological applications (from micro electromechanical systems to energy and transport applications).

Site dependent enhancement of magnetic anisotropy in 4d and 5d impurity doped α″-Fe16N2: A first principles study

Using the first principles method, we studied the electronic structure and magnetocrystalline anisotropy of site dependent 4d, 5d element doping in α″-Fe16N2. We found that different Fe sites contributed differently for magnetocrystalline anisotropy. For instance, on d-site doping, we obtained perpendicular magnetocrystalline anisotropy while the h-site doping resulted in an in-plane magnetocrystalline anisotropy in all the studied systems. The impurity doping induces local lattice distortions near the impurity site. However, the volume of the cell and total magnetic moment of the doped systems were not much affected. This local lattice distortion together with the spin-orbit coupling effect associated with heavy 4d, 5d element mainly contributed to enhancing the magnetocrystalline anisotropy. The enhancement of the magnetocrystalline anisotropy results in almost 15 to 67% enhancement of the coercivity for 4d element doping while we found 80 to 137% enhancement for 5d element doping. The maximum energy products were also enhanced compared to the pure α″-Fe16N2. The maximum enhancement was observed in Rh and Pt doped systems where the energy products were 119–120 MGOe. These results may suggest that substitutional doped α″-Fe16N2 system can be used as potential rare earth free permanent magnet.

Synthesis of hard magnetic Mn3Ga micro-islands by e-beam evaporation

The permanent magnet industry heavily depends on Nd-Fe-B and Sm-Co alloys because of their high-energy product and high room temperature coercivity. Main ingredient for having such superior magnetic properties compared to other known ferromagnetic materials is rare earth elements (Nd, Sm, Dy…). However recent worldwide reserve and export limitation problem of rare earths, shifted researchers’ focus to rare earth free permanent magnets. Among many alternatives (FePt, Zr2Co11, FeNi …), Mn-based alloys are the most suitable due to abundance of the forming elements and trivial formation of the necessary hard phases. In this study, Mn3Ga micro islands have been prepared. Mn3Ga owes its hard magnetic properties to tetragonal D022 phase with magnetic anisotropy energy of 2 MJ/m3. Thin films and islands of Cr/MnGa/Cr layers have been deposited on Si/SiO2 wafers using combination of e-beam and thermal evaporation techniques. Cr has been used as buffer and cover layer to protect the sample from the substrate and prevent oxidation during annealing. Annealing under Ar/H2 forming gas has been performed at 350oC for 10 min. Nano thick islands of 25, 50 and 100 μm lateral size have been produced by photolithography technique. Room temperature coercivity of 7.5 kOe has been achieved on 100 μm micro islands of Mn3Ga. Produced micro islands could be a rare earth free alternative for magnetic memory and MEMS applications.

In-situ magnetic force microscopy analysis of magnetization and demagnetization behavior in Al3+ substituted Sr-hexaferrite

The sintering temperature of an Al3+ substituted Sr-hexaferrite composite was systematically varied from 1180 °C to 1280 °C resulting in different microstructures. The grain size was found to range from a few hundred nanometers to several hundred micrometers depending on Al content and sintering temperature. Adding an Al substituted powder to a commercial powder increased the coercivity from 360 mT to 470 mT, at the same time, decreasing remanence from 350 mT to 305 mT. Magnetization and demagnetization processes from the thermally demagnetized state (TDS) and DC-demagnetized state (DCD) have been investigated systematically by in-situ magnetic force microscopy (MFM) under magnetic field. From the surface domain contrast a polarization was derived which quantitatively matches the global i.e. bulk polarization obtained by superconducting quantum interface device (SQUID) magnetometry. The shape of the initial polarization curve and the polarization from the DCD state were correlated with the in-situ MFM data revealing a distinctly different magnetization behavior depending on grain size. The presented results enable a better understanding of local nucleation mechanisms, global influences of pinning centers and further opportunities to improve rare earth (RE) free permanent magnets based on ferrites.

Consolidation of cobalt nanorods: A new route for rare-earth free nanostructured permanent magnets

Rare-earth free permanent magnets were produced by consolidation of cobalt nanorods synthesized by the polyol process exhibiting a mean diameter in the range 10–30 nm. Compactions of magnetically prealigned rod assemblies at various pressures and temperatures were carried out to make dense materials. Bulk magnets exhibiting a very good mechanical strength and an energy product as high as 65 kJ m−3 were obtained. The best results were obtained when the compaction conditions were soft enough to preserve the morphology and alignment of the rods in the final material, as revealed by X-ray diffraction and neutron scattering. For the first time the bottom-up approach is convincingly reported to produce bulk magnets without the addition of any matrix, the obtained nanostructured materials exhibit coercivity much higher than the AlNiCo magnets and can fill the performance “gap” between hexaferrites and rare-earth based magnets.

Micromagnetic Simulations for Coercivity Improvement Through Nano-Structuring of Rare-Earth-Free L10-FeNi Magnets

In this work we investigate the potential of tetragonal L1$_0$ ordered FeNi as candidate phase for rare earth free permanent magnets taking into account anisotropy values from recently synthesized, partially ordered FeNi thin films. In particular, we estimate the maximum energy product ($BH$)$_\mathrm{max}$ of L1$_0$-FeNi nanostructures using micromagnetic simulations. The maximum energy product is limited due to the small coercive field of partially ordered L1$_0$-FeNi. Nano-structured magnets consisting of 128 equi-axed, platelet-like and columnar-shaped grains show a theoretical maximum energy product of 228 kJ/m$^3$, 208 kJ/m$^3$, 252 kJ/m$^3$, respectively.

A new structure of the environment-friendly material fe16n2

In this paper, a new compound c-Fe16N2 is obtained using a new crystal structure prediction software CALYPSO (Crystal Structure Analysis by Particle Swarm Optimization). The compound has a structure of C2/m (the twelfth space group), and a magnetic moment of 17 µB per unit cell. The enthalpy of c-Fe16N2 is - 8.620 eV/atom, which is little to that of α''-Fe16N2 (-8.287 eV/atom), so the former is more stable from the view of energy. The average magnetic moment of c-Fe16N2 is 2.13 µB per iron atom which is small to that of α''- Fe16N2 (2.8 µB). The c-Fe16N2 is a new rare earth-free permanent magnet. No rare earth means no environmental pollution in the the rare earth mining. The α''-Fe16N2 has a highest saturation magnetic flux density, but it is difficult to mass production, because there are other several N-Fe iron compounds (such as Fe3N, Fe4N) in the obtained resultants. The newly found compound c-Fe16N2 may replace α''-Fe16N2 because the former can be easily obtained compared to the latter, and there maybe a new way to mass production of α''-Fe16N2 from c-Fe16N2.

Bulk combinatorial analysis for searching new rare-earth free permanent magnets: Reactive crucible melting applied to the Fe-Sn binary system

The reactive crucible melting method is known to be an efficient and low-cost bulk combinatorial synthesis technique to search for new phases. In this work, we tested this technique by investigating the Fe-Sn binary system. For high-throughput characterization, this synthesis technique was combined with energy dispersive x-ray spectroscopy as well as magneto-optical Kerr microscopy. The latter was used for identification of desired phases with uniaxial magnetic anisotropy. Reliability of the reactive crucible method was evaluated by comparison of the phase composition forming in the reactive crucible with phases appearing in conventionally melted samples and with the phases represented in the reported Fe-Sn phase diagram. It was found that the Fe5Sn3 phase, existing in the equilibrium phase diagram at 800 °C and forming in conventionally melted alloys, does not exist in the diffusion zone of the reactive crucible. The problem of ‘missing phases’ is discussed. In addition, we have shown that the anisotropy energy obtained by quantitative analysis of the uniaxial domain structure of Fe3Sn2 phase, gives the value of 1 MJ/m3, whereas K1 evaluated by conventional magnetometry is ∼ 0.07 MJ/m3 only, by this demonstrating how erroneous the assessment of K1 from domain structure can be. Finally, a new unit cell is proposed for Fe5Sn3 phase, which is commensurately modulated by the orthorhombic unit cell with lattice parameters of a = 4.221 Å, b = 7.322 Å, c = 5.252 Å and the space group Pbcm(α00)0s0 with a modulation vector of q = (½,0,0).

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.