High-performance Permanent Magnets Research Articles

Nitrogenation of Nd(Fe,Mo)12 powders for sintered magnets

The intermetallic Rare Earth-Fe12 (REFe12) compounds are considered as the strongest candidates for alternative high-performance permanent magnet material, thanks to the combination of high magnetocrystalline anisotropy, high Curie temperature and moderate saturation magnetization. However, the NdFe12 based compounds suffer from a weak uniaxial magnetocristalline anisotropy at room temperature. The insertion of light elements, like nitrogen, induces a strong uniaxial anisotropy and an increase of the Curie temperature as well. Nevertheless, the nitrogenation of these compounds have proved to be tricky, as the reaction kinetics is very slow at moderate temperatures, whereas the decomposition of the 1–12 phase occurs at higher temperatures. In the present study the aim is to develop nitrogenated powders suitable for the manufacture of Nd(Fe,Mo)12N-based sintered magnets, starting from Nd(Fe,Mo)12 precursors prepared by strip-casting. The nitrogenation of Nd(Fe,Mo)12 powders was undertaken to analyze the mechanisms of reaction and to determine the experimental conditions allowing to avoid both the formation of α-Fe and the nitrogenation of the Nd-rich intergranular phase. Structural and magnetic investigations are performed on the parent alloys and the nitrogenated powders, allowing to identify the mechanism of the solid-gas reaction and to determine, for the first time to our knowledge, the nitrogen thermodynamic equilibrium pressure between the 1–12 phase and its nitride.

Accurate machine-learning predictions of coercivity in high-performance permanent magnets

Accurate machine-learning predictions of coercivity in high-performance permanent magnets

Development of a high-performance and cost-effective in-vacuum undulator.

In-vacuum undulators (IVUs), which have become an essential tool in synchrotron radiation facilities, have two technical challenges toward further advancement: one is a strong attractive force between top and bottom magnetic arrays, and the other is a stringent requirement on magnetic materials to avoid demagnetization. The former imposes a complicated design on mechanical and vacuum structures, while the latter limits the possibility of using high-performance permanent magnets. To solve these issues, a number of technical developments have been made, such as force cancellation and modularization of magnetic arrays, and enhancement of resistance against demagnetization by means of a special magnetic circuit. The performance of a new IVU built upon these technologies has revealed their effectiveness for constructing high-performance IVUs in a cost-effective manner.

Processability and Separability of Commercial Anti-Corrosion Coatings Produced by In Situ Hydrogen-Processing of Magnetic Scrap (HPMS) Recycling of NdFeB.

The recycling of NdFeB magnets is necessary to ensure a reliable and ethical supply of rare earth elements as critical raw materials. This has been recognized internationally, prompting the implementation of large-scale legislative measured aimed at its resolution; for example, an ambitious recycling quote has been established in the Critical Raw Materials Act Successful recycling in sufficient quantities is challenged by product designs that do not allow the extraction and recycling of these high-performance permanent magnets without excessive effort and cost. This is particularly true for smaller motors using NdFeB magnets. Therefore, methods of recycling such arrangements with little or no dismantling are being researched. They are tested for the hydrogen-processing of magnetic scrap (HPMS) method, a short-loop mechanical recycling process. As contamination of the recycled material with residues of anti-corrosion coatings, adhesives, etc., may lead to downcycling, the separability of such residues from bulk magnets and magnet powder is explored. It is found that the hydrogen permeability, expansion volume, and the chosen coating affect the viable preparation and separation methods as recyclability-relevant design features.

Preparation of High-performance Bulk Permanent Magnets by Doping Tb-Fe-B in Nd-Fe-B Magnets

Preparation of High-performance Bulk Permanent Magnets by Doping Tb-Fe-B in Nd-Fe-B Magnets

Dependence of Nucleation Field on the Size of Soft Phase in Magnetic Hard–Soft Exchange Coupling Nanocomposites

Hard–soft exchange coupling nanocomposites have critical applications in various important materials. The magnetic properties of nanocomposite permanent magnetic films improve with a higher nucleation field (Hns) of the soft magnetic phase. Hns is sensitive to the thickness (ds) of the soft magnetic layer. Understanding the dependence of Hns and irreversible field (Hirr) on ds, especially at the nanometric scale, is crucial for comprehending the magnetic mechanism and facilitating the design and preparation of high-performance nanocomposite permanent magnets. However, during the high-temperature deposition process, diffusion between hard and soft magnetic phases occurs, leading to the generation of other phases. This makes it challenging to accurately reflect the relationship between Hns and ds. To address this issue, we successfully fabricated high-quality SmCo5/Fe nanocomposite bilayer films with different soft magnetic thicknesses and high textures by controlling the preparation process. We conducted a quantitative analysis of the relationship between Hns and ds within the range of 2–40 nm. Based on the experimental results, we propose a new theoretical simulation formula that enhances the understanding of the characteristics at the interface between the soft magnetic and hard magnetic phases. The theoretical simulation results show that a thin softened hard layer of about 4–6 nm thickness exists at the interfacial region, which concurrently reverses with the soft magnetic phase during the demagnetization process. Our results offer the generality and critical basis for the further study of hard–soft nanocomposite magnetic materials.

Research progress of magnetic anisotropy enhancement mechanism of high-performance La-Co co-substituted M-type permanent magnet ferrites

<sec>La-Co co-substituted M-type ferrite, which was first reported at the end of the 20th century, as the cornerstone of high-performance permanent magnet ferrites, has received increasing attention from researchers around the world. The unquenched orbital moments of Co<sup>2+</sup> play a pivotal role in enhancing the uniaxial anisotropy of M-type ferrites. However, a comprehensive understanding of its microscopic mechanism remains elusive. In order to meet the increasing performance requirements of ferrite materials, it is imperative to clarify the mechanism behind the enhancement of magnetic anisotropy, and at the same time seek the guiding principles that are helpful to develop high-performance product quickly and economically. But its mechanism at a microscopic level has not been explained. This review comprehensively analyzes various studies aiming at pinpointing the crystal sites of Co substitution within the lattice. These investigations including neutron diffraction, nuclear magnetic resonance, and Mössbauer spectroscopy can reveal the fundamental origins behind the enhancement of magnetic anisotropy, thereby providing valuable insights for material design strategies aiming at further enhancing the magnetic properties of permanent magnet ferrites.</sec><sec>The exploration of co-substitution sites has yielded noteworthy findings. Through careful examination and analysis, researchers have discovered the complex interplay between Co ions and the lattice structure, revealing the mechanisms of enhanced magnetic anisotropy. The current mainstream view is that Co ions tend to occupy more than one site, namely the 4<i>f</i><sub>1</sub>, 12<i>k</i>, and 2<i>a</i> sites, all of which are located within the spinel lattice. However, there have also been differing viewpoints, implying that further exploration is needed to uncover the primary controlling factors influencing Co occupancy. It is worth noting that the identification of specific Co substitution sites, especially the spin-down tetrahedron 4<i>f</i><sub>1</sub>, has achieved targeted modifications, ultimately fine-tuning the magnetic properties with remarkable precision.</sec><sec>Furthermore, the reviewed research emphasizes the pivotal role of crystallographic engineering in tailoring the magnetic characteristics of ferrite materials. By strategically manipulating Co substitution, researchers have utilized the intrinsic properties of the lattice to amplify magnetic anisotropy, thereby unlocking new avenues for the advancement of permanent magnet ferrites.</sec><sec>In conclusion, the collective findings outlined in this review herald a promising trajectory for the field of permanent magnet ferrites. With a detailed understanding of Co-substitution mechanisms, researchers are preparing to open up new avenues for developing next-generation ferrite materials with enhanced magnetic properties.</sec>

Facile Synthesis of Nd2Fe14B Hard Magnetic Particles with Microwave-Assisted Hydrothermal Method

The synthesis of Nd-Fe-B magnetic powders via chemical techniques presents significant promise, but poses challenges due to their inherent chemical instability. In this investigation, Nd-Fe-B hard magnetic particles were synthesized utilizing an eco-friendly and simple microwave-assisted hydrothermal synthetic method. The technique involves the synthesis of the Nd-Fe-B oxide precursor using the microwave-assisted hydrothermal method, followed by reduction–diffusion using CaH2. The microwave-assisted hydrothermal technique presents a viable approach for preparing Nd-Fe-B precursor particles, offering advantages such as time and energy efficiency and environmental sustainability. The synthesized Nd-Fe-B particles demonstrated a coercivity of up to 2.3 kOe. These magnetic particles hold significant potential for use in high-performance permanent magnets, and can effectively contribute to developing high-energy density exchange-coupled nanocomposite magnets. This study also offers valuable insights into the design and synthesis of additional magnetic materials based on rare earth elements.

Optimization of Ca–Al–Mn–Si Substitution Level for Enhanced Magnetic Properties of M-Type Sr-Hexaferrites for Permanent Magnet Application

Enhanced hard magnetic properties were achieved in M-type hexaferrite by optimizing the substitution levels of Mn, Al, and Si for Fe, and Ca for Sr within SrFe12O19. The addition of Al–Si–Mn effectively controlled crystallite growth, resulting in an increased coercivity (HC), while causing a decrease in the remanent magnetization (4πMr). A higher Ca content exhibited a trend of increasing the sintered density but decreasing the 4πMr and HC. The optimized composition, considering both the 4πMr and HC, was determined to be Sr0.8Ca0.2Fe10.2Mn0.1Al0.2Si0.1O19−d, with a sintered density of 4.84 g/cm3, 4πMr = 2.22 kG, and HC = 5.10 kOe. This result demonstrates the achievement of isotropic magnets with controlled crystal growth and densification without additional sintering additives. This development is promising, as this enhancement could be achieved without the use of cobalt, an expensive but essential ingredient in high-performance permanent magnets.

Elastic constants of equiatomic Fe–Ni Invar alloy single crystal

In a disordered fcc structure, the equiatomic Fe–Ni alloy is of great interest as a soft magnetic Invar alloy with a near zero or low thermal expansion coefficient. In the L10 ordered phase, it is of interest as a potential high-performance permanent magnet alloy. In this work, the first detailed measurements of the elastic constants in a [001]-oriented Fe-50 at. % Ni alloy single crystal under a no magnetic field condition were made using the resonant ultrasound spectroscopy technique. The elastic constants measured allow one to understand the nature of atomic bonding in a material and the mechanical response to applied stress. The single crystal sample having a rectangular parallelepiped shape was prepared from a single crystal grown using the vertical Bridgman technique. The elastic constants C11, C12, and C44 were obtained from the resonant frequencies observed over a frequency range of 100–900 kHz. Using these C11, C12, and C44 values, Young’s modulus, shear modulus, bulk modulus, and Poisson’s ratio were calculated to be 99.1 GPa, 73.4 GPa, 176.7 GPa, and 0.20, respectively. The large anisotropy factor of 3.29 highlights the strong directional dependence of the material’s mechanical behavior.

High-performance anisotropic SmCo-based magnet with novel microstructure

A novel microstructure of magnetically anisotropic SmCo-based magnet with high-performance is reported. The magnet consists of SmCo7-H (TbCu7-hexagonal type), Sm2Co17–H (Th2Ni17-hexagonal type) and SmCo3-R (SmCo3-rhombohedral type) phases. The maximum magnetic energy product of the magnet is 231.69 kJ/m3, and the intrinsic coercivity is 1005.47 kA/m. An outstanding intrinsic coercivity temperature coefficient (β) of −0.125%/K between 298 and 773 K is obtained, which is very close to the β of commercial high-temperature Sm2Co17-based sintered magnets. The initial magnetization curve indicates that the coercivity mechanism is controlled by a domain wall pinning mechanism. The SmCo3-R lamellar phase may be a potential pinning center or self-pinning center. The microstructure of the magnet is different from that of any previous SmCo-based magnets. These findings provide a new idea for preparing high-performance SmCo-based permanent magnets.

Phase equilibria in iron-rich Sm–Fe–Ti and Sm–(Fe,Co)–Ti alloys at 1100–1200 °C

Iron-rich corners of ternary Sm–Fe–Ti phase diagram at 1100 °C and quasi-ternary Sm–Fe0.8Co0.2–Ti phase diagrams at 1100 and 1200 °C are constructed based on experimental investigation of equilibrated alloys with electron probe microanalysis, X-ray diffraction and thermomagnetic analysis. In addition, the upper boundaries of the temperature ranges of Sm(Fe,[Co,]Ti)12 and Sm3(Fe,[Co,]Ti)29 phases are determined with differential thermal analysis to update earlier rough estimates. The existence of a high-temperature phase of the Th2Ni17 type, originally reported by Ivanova et al. [J. Alloys Compd. 224 (1995) 29], is confirmed. In the Sm–Fe0.8Co0.2–Ti system, the composition and equilibria of this hexagonal phase are established for 1200 °C; it is Sm-depleted (≈9.8 at% Sm) compared to the 2:17 stoichiometry and it coexists with the rhombohedral 2:17 phase. The magnetic anisotropy of the cobalt-substituted Th2Ni17-type phase is planar, with the easy magnetization direction parallel to [100]. Equilibrium between a Sm-rich liquid phase and the 1:12 phase, which is important for the development of new high-performance permanent magnets, is absent up to 1000 °C, but does exist at 1100 °C (for the 1:12 phase with at least 8.7–8.9 at% Ti) and at 1200 °C (for the 1:12 phase with as little as 7.4 at% Ti). The development of magnets may be complicated, however, by an observed tendency of the high-temperature liquid to solidify into ferromagnetic phases including the Th2Ni17-type phase. The Curie temperatures of the α-(Fe,Ti), Sm(Fe,Ti)12, Sm3(Fe,Ti)29 and rhombohedral Sm2(Fe,Ti)17 phases are not only increased by the partial Co substitution for Fe, but their dependence on the Ti concentration is changed by this Co substitution from positive (or, for the 1:12 phase, zero) to negative values.

Modified Nonlinear Active Disturbance Rejection Control for PMSM Speed Regulation With Frequency Domain Analysis

Strong robustness and smooth speed are two crucial goals of the high-performance permanent magnet synchronous motor (PMSM) drive systems. Conventional active disturbance rejection control (ADRC) strategy generally increases the bandwidth of the extended state observer (ESO) to improve the unsatisfactory disturbance rejection ability, which will inevitably amplify the measurement noise. To address this tough issue, a nonlinear extended state observer (NESO) using finite-time technique is constructed in this paper to enhance the anti-disturbance property. On this basis, the NESO with small bandwidth is sufficient to guarantee strong robustness without sacrificing noise suppression performance. Utilizing the frequency sweep method, the advantages of the proposed NESO based modified ADRC (MADRC) system can be analyzed in terms of frequency domain characteristics, which is more practical than traditional theoretical analysis from a mathematical point of view. Meanwhile, a quasi-resonant (QR) controller is combined with MADRC system to attenuate the main harmonic component of torque ripple for smooth speed. Experimental results are carried out to verify the effectiveness of the proposed control scheme and frequency sweep analysis method.

Origin and inhibition on quasi-periodic coarse grain regions in hot-deformed Nd-Fe-B magnets

Hot-deformed Nd-Fe-B magnets with nanocrystalline grains are promising contenders for high-performance permanent magnets. However, the quasi-periodic coarse grain regions at ribbon interfaces counteract the positive effects on magnetic properties to some extent. Up to now, the ambiguous formation mechanism of coarse grain regions impedes the development of advanced process to obtain uniform microstructure and excellent properties in hot-deformed Nd-Fe-B magnets. To address this issue, systematical studies have been carried out on the grain growth behaviors from melt-spun ribbons to bulk hot-deformed magnets. In this work, we have confirmed that abnormal grain growth occurred in the wheel-side of melt-spun ribbons under high temperature of hot deformation process, due to the ultra-fine grain size with high activation and lack of isolation by Nd-rich phase. Correspondingly, low-temperature heat treatment was applied on melt-spun ribbons to optimize the distribution of grain size, resulting in the significant inhibition on coarse grain regions in hot-deformed magnets.

Microwave-assisted chemical synthesis of SmCo5 magnetic particles with high coercivity

The hard-magnetic SmCo5-based materials with high coercivity have an enormous range of high-value-added technological applications. We reported the novel synthesis of SmCo5 particles with desired particle size by a microwave-assisted combustion process. The process consists of a combination of Sm-Co oxide precursor preparation by microwave-assisted combustion, and the following calcium hydride reduction-diffusion. The microwave-assisted combustion offers a time and energy-saving, and environmentally-friendly methodology to form the homogenous Sm-Co oxide precursor nanoparticles. This high-quality Sm-Co oxide precursor paves the way for achieving high coercivity in SmCo5 magnetic materials. A high coercivity up to 39.2 kOe was achieved in the aligned SmCo5 particles. These SmCo5 magnetic particles show promising prospects for high-performance permanent magnet applications and can be utilized for the development of exchange-coupled nanocomposite magnets with high energy density. This work also provides instructive information for designing and synthesizing rare-earth-based magnetic materials.

A combined-pole permanent magnet synchronous motor incorporating nanocomposite magnets

The development of rare earth permanent magnet materials has made outstanding contributions to the advancement of high performance permanent magnet synchronous motors (PMSM). However, the non-renewable and strategic protection of rare earth resources leads to price fluctuations and supply risk of rare earth permanent magnets, limiting electrical vehicle development and popularity. In this paper, we optimized the design of the nanocomposite magnet microstructure and proposed a combined-pole interior nanocomposite permanent magnet synchronous motor (CP-INPMSM), combining nanocomposite magnet and PMSM. The simulation results show that the no-load back-electromotive force and torque of the motor are improved. This design can reduce the usage of rare-earth permanent magnet materials and improve the electromagnetic performance of the motor.

Ce-substituted (Sm,Ce)2Fe17N3 compounds: A promising class of high-performance permanent magnets

In this work, the effects of partial substitution of Ce for Sm in (Sm1−xCex)2Fe17N3 (x = 0.2–0.8) compounds on their intrinsic and extrinsic magnetic properties have been estimated. With the × progressively increases from 0.2 to 0.8, the μ0Ms of (Sm1−xCex)2Fe17N3 only decreases from 1.52 T to 1.43 T and the anisotropy field gradually decreases from 188 kOe to 49 kOe. Compared with all the existing Fe-based permanent magnets, (Sm,Ce)2Fe17N3 compounds exhibit the superior intrinsic magnetism upon the same Ce substitution level. Magnetically hard anisotropic (Sm1−xCex)2Fe17N3 (x = 0.2–0.6) powders were prepared by surfactant-assisted ball milling. (Sm0.8Ce0.2)2Fe17N3 fine powder shown magnetic properties of μ0Mr = 1.24 T, HcJ = 9.33 kOe and (BH)max = 28.82 MGOe, which are comparable with Ce-free Sm2Fe17N3. For high Ce-containing (Sm0.4Ce0.6)2Fe17N3 powder, the coercivity can be enhanced significantly from 2.4 kOe to 5.9 kOe by Zn-treatment. These findings suggest the great prospects of Ce-rich (Sm,Ce)2Fe17N3 compounds as a promising alternative material system for high performance magnets.

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.

Microstructure evolution of Pr-doped SmCo4B-based ribbons with improved magnetization

Searching for high-performance permanent magnets is always challenging for modern technological applications. The novel SmCo4B phase has a higher anisotropy field than SmCo5, and this paper disclosed the effect of Pr addition on the phase transformation, microstructure evolution, and magnetism of the multi-element doped Sm1−xPrxCo3.04Fe0.9Cu0.06B ribbons. The results show that Pr addition decreases the amorphous forming ability of the as-spun ribbons. After annealing, Pr addition promotes the growth of strip-like Sm-Co-B grains, and 8 at% Pr addition increases the saturation magnetization and remanence of the Sm-Co-B ribbons by 96.2% and 48.0%, respectively. Pr addition weakens the anisotropy field of the Sm-Co-B phase, resulting in a gradual decrease in coercivity. We revealed the transformation mechanism of strip-like grains during melt-spinning and annealing, found the relationship between phase content, dislocation density, strain, and grain size, and proposed the corresponding microstructure evolution models.

High-throughput search for potential permanent magnet materials

High-performance permanent magnets, which have a wide range of applications in the information age, are characterized by significant magnetic anisotropy mainly affected by spin-orbit coupling (SOC). The authors perform a highly efficient search for permanent magnet materials in the inorganic crystal structure database by focusing on materials containing $3d$ transition elements with specific Wyckoff positions, where certain partially occupied orbital multiplets can significantly enhance the effect of SOC. According to common standards of permanent magnets, the authors propose five new permanent magnet candidates. They believe that these potential permanent magnet materials deserve further experimental study.