Rare-earth Free Permanent Magnets Research Articles

Computational Exploration of Pd-Based Heusler alloys for permanent Magnets: Density Functional Theory and High-Throughput methods

In this work, we have explored the potential of a relatively new class of Pd-based Heusler alloys as permanent magnets. Our approach began with a high-throughput study using the Automatic Flow (AFLOW) electronic structure database to identify Pd₂YZ-type Heusler systems with tetragonal symmetry and high magnetization. For the alloys that passed our initial selection filters, we performed a detailed investigation of their magnetic properties using density functional theory (DFT) calculations. These calculations included assessments of magnetocrystalline anisotropy energy (EMCA) and exchange interactions. The selected candidates, which met all our criteria, were then subjected to Monte Carlo simulations to analyze the temperature dependence of their magnetization and determine their Curie temperatures (TC). Our results indicate that Pd₂FeNi and Pd₂FePt are promising candidates for permanent magnets. Pd₂FeNi exhibited an EMCA of 2.4 MJ/m3 and a TC of 504 K, while Pd₂FePt showed an EMCA of 4.11 MJ/m3 and a TC of 463 K. Both Curie temperatures are significantly above room temperature by more than 160 K, demonstrating the alloys’ potential for high-temperature applications and their overall suitability as efficient, rare earth-free permanent magnets.

Microstructure and coupling mechanisms in MnBi–FeSiB nanocomposites obtained by spark plasma sintering

Fabrication and extensive characterization of hard-soft nanocomposites composed of hard magnetic low-temperature phase LTP-MnBi and amorphous Fe70Si10B20 soft magnetic phase for bulk magnets are reported. Samples with compositions Mn55Bi45 + x⋅(Fe70Si10B20) (x = 0, 3, 5, 10, 20 wt.%) were prepared by spark plasma sintering of powder mixtures. Characterization has been performed by X-ray diffraction, scanning and transmission electron microscopy, magnetometry and 57Fe Mӧssbauer spectroscopy. It was shown that samples contain crystallized and nanometric LTP-MnBi phases with various elemental compositions depending on the degree of Bi clustering. Complex correlations between starting compositions, processes during fabrication, and functional magnetic characteristics were observed. Unexpected special situations of the relation between microstructure and magnetic coupling mechanisms are discovered. Exchange spring effects of different strengths occur, being very sensitive to morpho-structural and compositional features, which in turn are controlled by processing conditions. An in-depth analysis of related microscopic characteristics is provided. Results of this work suggest that fabrication by powder metallurgy routes, such as spark plasma sintering of hard and soft magnetic powder mixtures, of MnBi-based composites with exchange spring phenomena have a high potential in designing and optimization of suitable materials with tunable magnetic properties towards rare-earth–free permanent magnet applications.

Recent progress in research and development of rare-earth free iron nitride permanent magnet

The high cost of rare earth materials, unstable supply and high demand remain big problems to solve for permanent magnet industries. Thus α″-Fe<sub>16</sub>N<sub>2</sub> has been picked up as one of the most promising rare earth-free magnet candidates because of its use of environmental-friendly raw materials, confirmed giant saturation magnetic flux density (2.9 T), and reasonably high magnetic anisotropy constant (1.8 MJ/m<sup>3</sup>). Also, α”-Fe<sub>16</sub>N<sub>2</sub> metastable with film form exhibits high tetragonality (c/a=1.1) by interstitial introduction of N atoms, leading to a high magnetocrystalline anisotropy constant (K<sub>u</sub> = 1.0 MJ/m<sup>3</sup>). but synthesis of bulk form has lower properties than thin film (<230 nm) because of magnetic interaction, experimentally low anisotropy, and low density. In this paper, we review the synthetic methods of metastable α”-Fe<sub>16</sub>N<sub>2</sub> with powder and bulk form and discuss the approaches to enhance magnetocrystalline anisotropy of α”-Fe<sub>16</sub>N<sub>2</sub>.

Deformation behavior and magnetic properties of equiatomic FeNi single crystals

An equiatomic Fe–Ni alloy in its disordered A1 structure is a soft magnetic alloy. On ordering to the L10 ordered phase, it shows significant magneto-crystalline anisotropy and a strong permanent magnet behavior and is of interest as a rare earth-free permanent magnet. However, synthesis of an L10 phase in a bulk form remains a challenge due to its low critical ordering temperature Tc and consequent extremely slow ordering kinetics. This phase is present in asteroids, and how it was formed remains unclear. The likely mechanism is enhanced diffusion kinetics due to extreme dislocation densities and vacancy concentrations produced by deformation during asteroid collisions and the presence of S. Prior to examining extreme deformation in an FeNi alloy comparable to that in asteroid collisions, low strain rate deformation behavior, magnetic properties, and the structure of undoped and S-doped [100]-oriented FeNi single crystals were carried out. Controlled deformation at a strain rate of 1 × 10−5/s showed the yield point to be 89 MPa, and the critical resolved shear stress was 25.7 MPa. The dislocation densities obtained were ∼1017/m2. The saturation magnetization value was ∼147–151 emu/g both before and after deformation, comparable to NdFeB magnets. Coercivity increased slightly from ∼0.04–0.4 to ∼5 Oe after deformation due to an increase in dislocation density. The x-ray diffraction scan of S-doped and deformed single crystals after annealing at 300 °C, just below Tc, showed no evidence of L10 order. These data serve as a baseline for extreme strain rate deformation where much higher dislocation densities and vacancy concentrations can be obtained to facilitate L10 order.

MagGen: A Graph-Aided Deep Generative Model for Inverse Design of Permanent Magnets.

A deep generative model based on a variational autoencoder (VAE), conditioned simultaneously by two target properties, is developed to inverse design stable magnetic materials. The structure of the physics-informed, property embedded latent space of the model is analyzed using graph theory. An impressive ∼96% of the generated materials are found to satisfy the target properties as per predictions from the target-learning branches. This is a huge improvement over approaches that do not condition the VAE latent space by target properties or that do not consider the connectivity of the parent materials from which the new materials are generated. This impressive feat is achieved by using a simple real-space-only representation that can be directly read from material cif files. Model predictions are finally validated by density functional theory calculations on a randomly chosen subset of materials. The performance of the present model is comparable or superior to that of models reported earlier. This model (MagGen) is applied to the problem of designing rare earth-free permanent magnets with promising results.

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

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

Leveraging available data for efficient exploration of materials space using Machine Learning: A case study for identifying rare earth-free permanent magnets

It is shown that even the relatively small amount of available materials data can be innovatively utilized to explore the materials space in order to identify materials with desired target properties. As an example of this, data from the novomag and Novamag databases are used to train random forest and neural network models which can predict thermodynamic stability, and magnetic properties of materials. Performance of these models are tested thoroughly and are found satisfactory. These models are subsequently used to interpolate within the above databases, and to extrapolate to parts of the materials composition and structure space not covered in these databases, to identify stable, magnetic materials that have large saturation magnetization and large easy-axis anisotropy. Screening 686 materials via the trained models, and subsequently performing first principles calculations, 21 new candidate materials for rare earth free permanent magnet are identified. Some of these materials have anisotropy constants as large as 5 and 6 MJ m−3, larger than that of the most widely used permanent magnet Nd2Fe14B. This simple approach can be used to screen materials with other functionalities in future.

Enhancing structural and magnetic properties of BaFe12O19 nanoparticles through co-substitution (Co-Mn): A promising approach for permanent magnet applications

In-depth study into the creation and improvement of rare-earth free permanent magnets, particularly ferrites, has been sparked by recent economic and environmental concerns. M-type barium hexaferrites (BaFe12O19, pure BHF) are a class of low-cost permanent magnets with an excellent curie temperature and good resistance to oxidation and corrosion that are significant from a technological perspective. In the current study, the experimental conditions are optimized to obtain barium hexaferrite (BaFe12O19) co-substituted with transition elements (Co and Mn) (BaFe12–2xCoxMnxO19) via the chemical co-precipitation method. In this framework, different substitution possibilities will be investigated. The structure, morphology, vibrational spectrum, and thermal stability of the generated nanoparticles were revealed using powder x-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). Rietveld refinement was used to estimate the average bond length and bond angle in order to determine the impact of superexchange interaction. Based on more precise structural characteristics, the atomic occupation and distribution of bonds are depicted from their charge density graph. For all the synthesized materials, the maximum energy product (BH)max and magnetic characteristics (VSM) were also investigated. The potential causes of this phenomenon were examined and discussed at length.

Comparative Study of Mechanical and Electrical Characteristics of High-Strength and Conventional Electrical Steel for EV Traction High-Speed Multilayer IPMSM Using Rare-Earth Free PM

In this article, the electrical and mechanical characteristics of multilayer interior permanent magnet synchronous motors (ML-IPMSMs) using ferrite permanent magnets (PMs) are compared when driven at high speeds with respect to the type of electrical steel. In high-speed operation, the mechanical and electrical characteristics of ML-IPMSMs vary depending on the number of PM layers, PM shape, number of bridges, and bridge thickness. Additionally, ferrite PMs have a lower coercive force than neodymium (Nd) PMs; therefore, irreversible demagnetization (ID) must be considered, and it is affected by the number of PM layers and PM shape of ML-IPMSMs. Thus, when designing a multilayer IPMSM using ferrite PMs for electric vehicle (EV) traction, all characteristics should be considered. Compared to conventional electrical steel (CES), high-strength electrical steel (HSES) offers advantages in mechanical properties, allowing for greater flexibility in ML shape. Consequently, it is possible to design a shape with improved electrical performance when compared to CES by adjusting the ML shape. In this article, all characteristics of each ML-IPMSM using HSES and CES are reviewed via finite element analysis (FEA), and the benefits of HSES applied motors for high-speed driving motors are presented. To verify the validity of the review content in this article, experiments are conducted on the manufactured motor, and FEA results are compared with the test results.

Effects of quenching on the microstructure and magnetic properties of MnBi nanopowders synthesized by low-oxygen induction thermal plasma

Fine and magnetically hard MnBi powders with limited surface oxidation were unprecedentedly synthesized with a high-throughput low-oxygen induction thermal plasma (LO-ITP) system for rare-earth-free permanent magnets (REF-PMs) application. Furthermore, by inducing quenching during the LO-ITP process, one can effectively reduce the as-processed Mn-Bi particle size from 63.9 to 49.8 nm and promote the formation of magnetic hard low-temperature phase of MnBi (LTP-MnBi, 72 to 84 wt%) with enhanced surface diffusion during post-annealing. The dominant LTP-MnB in the quenched and post-annealed MnBi powders was confirmed with detailed microstructure analysis. With higher weight fraction of LTP-MnBi, the quenched and post-annealed MnBi powders showed improved magnetic properties with a higher saturation magnetization (56.7 vs. 60.1 Am2/kg) and stronger coercivity (188.4 vs. 371.0 kA/m) than the powders without quenching. This work paved a new path for the mass production of high-purity and fine LTP-MnBi powders for synthesizing REF-PMs and/or related sustainable energy and automotive applications.

The Influence of Preparation Parameters on the Morphology and Magnetic Properties of Fe-N Powders Obtained by the Gas Atomization Method

α″-Fe16N2 materials are of increasing interest for their applications in products such as rare earth-free permanent magnets. The lack of a method of mass production for powders as raw materials delays the preparation of such magnets. Through employing the gas atomization method, we managed to prepare α″-Fe16N2 powders whose morphology and magnetic properties were tailored by the preparation parameters. As a result of optimizing the preparation parameters (ejection temperature and pressure, ejection nozzle diameter, and atomization pressure), we managed to prepare powders with a size of about 30 μm and a content of 31% α″-Fe16N2 phase. The value of the saturation magnetization (234.8 emu/g), the reasonable coercivity value (970 Oe) presented by the prepared powders, and the opportunity of scaling up approaches based on the preparation of powders via gas atomization support the feasibility of preparing α″-Fe16N2 powders at an industrial level.

Enhancing the performance of rare-earth free permanent magnets: A computational study of nitrogen and oxygen defects in CoPt, CoFePt2, and CoNiPt2 alloys

This work presents a computational investigation utilizing density functional theory and Monte Carlo simulations to study the influence of nitrogen and oxygen defects on the structural and magnetic properties of CoPt, CoFePt2, and CoNiPt2 alloys. The goal of this study was to explore their potential applications as alternatives to rare-earth permanent magnets. Our findings have revealed that the incorporation of nitrogen and oxygen defects significantly influence the magnetic properties of these alloys. The incorporation of defects yields negative formation energies, thereby confirming the structural stability of the defective structures. In addition, phonon dispersion calculations further confirmed the dynamical stability of the defective structures with lower formation energies. Notably, the Curie temperature (Tc) of CoFePt2 increased upon the addition of nitrogen, while the magnetocrystalline anisotropy energy (EMCA) of CoNiPt2 was enhanced by the presence of oxygen. In contrast, no improvement in the saturation magnetization was observed. These results provide valuable insights toward the development of rare-earth-free permanent magnets with tailored magnetic properties.

Temperature dependence of semi-hard iron silicide rare-earth free magnet

Searching for rare-earth free permanent magnet attracts extensive research interests due to diverse technological applications and other subtle issues. Here, the temperature dependent magnetic properties of Fe5SiC structure are explored. The Fe5SiC has a critical temperature of 710 K with perpendicular magnetic anisotropy. The magnetic anisotropy constant and coercive field are monotonically decreased with increasing temperature. For instance, the magnetic anisotropy constant is 0.42 MJ m−3 at zero Kelvin and reduced to 0.24 MJ m−3 and 0.06 MJ m−3 at 300 K and 600 K. The coercive field becomes 0.7 T at 0 K. With increasing the temperatures, it is suppressed to 0.42 T and 0.20 T at 300 K and 600 K, respectively. Overall, the Fe5SiC system has a (BH)max of 417 kJ m−3 at zero Kelvin. The (BH)max is decreased at high temperature. Nonetheless, we obtain the (BH)max of 234 kJ m−3 at 300 K. Since the Fe5SiC shows better permanent magnetic (PM) property than the conventional ferrites and also CeCo5. This finding may indicate that the Fe5SiC can be a potential candidate as a Fe-based gap PM between ferrite and Nd–Fe–B (or Sm–Co) at room temperature.

Tripling in coercivity (Hc) for the thermal and structural highly stabilized MnAlBi-C rare earth free permanent magnet via Bi doping

The limited coercivity (Hc) and metastability of the ferromagnetic τ phase MnAl-C magnet have constrained its magnetic performance and application potential. Bi was doped into the Mn54Al44-xBixC2 alloy system in this study. The doped Bi could effectively stabilize the metastable τ phase and inhibit the non-magnetic Al-rich phase formation. The ferromagnetic τ phase ratio increased dramatically as the doped Bi increase, with a peak Ms of 81 emu/g was achieved for the post-annealed Mn54Al43.6Bi0.4C2 alloy at 19 kOe. High Hc ferromagnetic LTP MnBi phase forms within the Mn54Al44-xBixC2 (x = 0.2, 0.4, 0.6) alloys when it been annealed at 300℃ for 24 h, the samples tripled or even quadrupled in Hc compared with the initial synthesized alloy. The doped Bi dissolved within lattice caused the τ phase thermally stability, structural stability improves substantially; 0.4 Bi doped Mn54Al43.6Bi0.4C2 alloy exhibited a combination of high Ms and Hc even after been ball milled for a long period of 15 and 20 h. The (Ms, Hc) achieved a value of (59.6 emu/g, 3526 Oe) for the 15 h milled sample and (51.5 emu/g, 4546 Oe) for the 20 h milled sample at 19 kOe, respectively; which is desirable for its application as a high-performance rare earth free permanent magnet.

Hardening of Cobalt Ferrite Nanoparticles by Local Crystal Strain Release: Implications for Rare Earth Free Magnets.

In this work, we demonstrate that the reduction of the local internal stress by a low-temperature solvent-mediated thermal treatment is an effective post-treatment tool for magnetic hardening of chemically synthesized nanoparticles. As a case study, we used nonstoichiometric cobalt ferrite particles of an average size of 32(8) nm synthesized by thermal decomposition, which were further subjected to solvent-mediated annealing at variable temperatures between 150 and 320 °C in an inert atmosphere. The postsynthesis treatment produces a 50% increase of the coercive field, without affecting neither the remanence ratio nor the spontaneous magnetization. As a consequence, the energy product and the magnetic energy storage capability, key features for applications as permanent magnets and magnetic hyperthermia, can be increased by ca. 70%. A deep structural, morphological, chemical, and magnetic characterization reveals that the mechanism governing the coercive field improvement is the reduction of the concomitant internal stresses induced by the low-temperature annealing postsynthesis treatment. Furthermore, we show that the medium where the mild annealing process occurs is essential to control the final properties of the nanoparticles because the classical annealing procedure (T > 350 °C) performed on a dried powder does not allow the release of the lattice stress, leading to the reduction of the initial coercive field. The strategy here proposed, therefore, constitutes a method to improve the magnetic properties of nanoparticles, which can be particularly appealing for those materials, as is the case of cobalt ferrite, currently investigated as building blocks for the development of rare-earth free permanent magnets.

Large magnetic anisotropy in Co–Fe–Ni–N ordered structures: a first-principles study

Material design of promising rare-earth free permanent magnet requires tailoring and controlling the intrinsic magnetic properties namely large saturation magnetization μ 0 M s, giant uniaxial magnetic anisotropy K u, and high Curie temperature T C. Based on first-principles electronic structure calculations, we present a detailed analysis for the intrinsic magnetic properties of Co x Fe1−x Ni and Co x Fe1−x NiN0.25 ordered structures. We predict an enhanced structural stability with improved K u ranging from 1.53–2.29 MJ m−3 for Co x Fe1−x NiN0.25 ordered structures, with the exception of CoNiN0.25 having planar anisotropy. Detailed analysis of the predicted large K u, based on perturbation theory and electronic structure calculations, is attributed to the cumulative effect of contribution from the increased tetragonal distortion and induced orbital distortion from the simultaneous Co substitution and interstitial N-doping. By tailoring the K u, we may create efficient and affordable PMs, bridging the gap between commonly used ferrite and high-performance Nd–Fe–B magnets.

Combined experimental and DFT studies of Co82Zr12V6-xBx melt-spun ribbons to investigate structure and magnetic properties

For the development of rare-earth free permanent magnets with better performance to cost ratio, V and B dopings were employed to enhance the hard magnetic properties of Zr-Co-V-B alloys. The effect of V and B microalloying additions on the magnetic properties, phase stability and microstructure of the metastable Co5Zr phase have been investigated with experimental measurements together with first principles based on the DFT (Density Functional Theory) calculations. This study also investigates the effects of annealing at different temperatures on the intrinsic and extrinsic magnetic properties of Co82Zr12V6-xBx melt-spun ribbons. Rapidly solidified Co82Zr12V6-xBx (x = 1, 2, 3) alloy ribbons were produced by melt-spinning. For as-spun Co82Zr12V6-xBx (x = 1, 2, 3) ribbons, the coercivityHcdecreases from 2.34 kOe to 0.008 kOe with an increasing × value from 1 to 3.The amorphous alloy ribbons were annealed in a vacuum furnace at a series of temperatures of 600 °C, 650 °C, 700 °C and 750 °C each for 30 min. The XRD analysis showed the presence of two soft magnetic phases (fcc-Co and Co23Zr6), accompanied with a hard phase Co5Zr in ribbons thus leading to the desired hard/soft structure and the amount of both (hard and soft) phases increased with the annealing temperature. Coercivity was found to increase upon annealing treatment. The phase stabilities of the Co82Zr12V6-xBx ribbons were calculated from the total energy by the DFT calculations in this work. The calculated magnetic anisotropy energies have been compared to that of experimental coercivity values. Annealed sample Co82Zr12V5B1 showed the maximum coercivity value of 3.58 kOe due to the formation of a high volume fraction of hard magnetic phase (Co5Zr) and evenly distributed finer grains throughout the matrix. However, annealing under the same conditions resulted in lower coercivity of about 0.088 kOe and 1.61 kOe for Co82Zr12V3B3 and Co82Zr12V4B2 samples, respectively. Although sample Co82Zr12V4B2 exhibited higher remanence (42.80 emu/g) and maximum magnetization value with the applied field of 17 kOe (i.e. M17 = 77.70 emu/g) compared to Co82Zr12V3B3 sample.

(Fe,Co)2(P,Si) rare-earth free permanent magnets: From macroscopic single crystals to submicron-sized particles

While rare-earth magnets exhibit unchallenged hard-magnetic properties, looking for alternatives based on inexpensive elements of non-critical supply remains of utmost interest. Here, we demonstrate that (Fe,Co)2(P,Si) single crystals combine a large magnetocrystalline anisotropy (K1 ≈ 0.9 MJ m−3 at 300 K), high Curie temperatures (TC up to 560 K) and an appreciable saturation specific magnetization (101 A m2 kg−1) leading to a theoretical |BH|max ≈ 165 kJ m-3, making them promising candidate materials as rare-earth-free permanent magnets. Our comparison between (Fe,Co)2P and (Fe,Co)2(P,Si) single crystals highlights that Si substitution reduces the low-temperature magnetocrystalline anisotropy, but strongly enhances TC, making the latter quaternary alloys most favorable for room temperature applications. Submicron-sized particles of Fe1.75Co0.20P0.75Si0.25 were prepared by a top-down ball-milling approach. While the energy products of bonded particles are to this point modest, they demonstrate that permanent magnetic properties can be achieved in (Fe,Co)2(P,Si) quaternary alloys. This work correlates the development of permanent magnetic properties to a control of the microstructure. It paves the way toward the realization of permanent magnetic properties in (Fe,Co)2(P,Si) alloys made of economically competitive Fe, P and Si elements, making these materials desirable for applications.

Hexaferrite-based permanent magnets with upper magnetic properties by cold sintering process via a non-aqueous solvent

The incessant technological pursuit towards a more sustainable and green future depends strongly on permanent magnets. At present, their use is widespread, making it imperative to develop new processing methods that generate highly competitive magnetic properties reducing the fabrication temperatures and costs. Herein, a novel strategy for developing dense sintered magnets based on Sr-hexaferrites with upper functional characteristics is presented. An innovative cold sintering approach using glacial acetic acid as novelty, followed by a post-annealing at 1100 °C, achieves a densification of the ceramic magnets of 92% with respect to the theoretical density and allows controlling the particle growth. After the cold sintering process, a fraction of amorphous SrO is identified, in addition to a partial transformation to α-Fe2O3 as secondary crystalline phase. 46 wt% of SrFe12O19 remains, which is mostly recuperated after the post-thermal treatment. These findings do not significantly modify the final structure of ferrite magnets, neither at short- nor long-range order. The innovative process has a positive impact on the magnetic properties, yielding competitive ferrite magnets at lower sintering temperatures with an energy efficiency of at least 25%, which opens up a new horizon in the field of rare-earth free permanent magnets and new possibilities in other applications.

Special Issue: Advances in Computational Electromagnetics

Recent advances in computational electromagnetics (CEMs) have made the full characterization of complex magnetic materials possible, such as superconducting materials, composite or nanomaterials, rare-earth free permanent magnets, etc [...]