The chemical formula of rare earth-iron-nitrogen permanent magnet materials is R_xFe_yN_z, and R stands for rare earth elements. Among them, anisotropic rare earth Fe-nitrogen permanent magnet materials include two types of materials with different chemical composition and crystal structure: (1) Nd(Fe,M)₁₂N_x or Pr(Fe,M)₁₂N_x, where M = Ti, V, Mo, etc., with ThMn₁₂ type tetragonal crystal structure, this kind of material is commonly called neodymium Fe-nitrogen; (2) Sm₂Fe₁₇N_x, with Th₂Zn₁₇ rhombohedral crystal structure, referred to as samarium iron nitrogen. The academic community collectively refers to these two types of materials as rare earth iron-nitrogen permanent magnet materials.

        The research literature on rare earth iron-nitrogen permanent magnet materials is mostly from China, Japan and European and American academic experts. These literatures mainly focus on the scientific discussion of material composition and process, and rarely discuss the progress of industrialization and market application. The author has long been engaged in industrialization development and marketing under the guidance of Academician Yang Yingchang, the main inventor of rare earth iron-nitrogen permanent magnet materials. Through more than ten years of efforts, Ningxia Junmagnetic New Material Technology Co., LTD. (Junmagnetic Technology Magvalley) has become one of the three global enterprises with original independent intellectual property rights and large-scale production capacity, and has conducted product application development with more than 100 domestic and foreign enterprises in the field of motor and consumer electronics. The progress of industrialization of rare earth iron-nitrogen permanent magnet materials is relatively comprehensive. In recent years, rare earth iron-nitrogen permanent magnet materials have become a hot direction of downstream market application development, especially since the concept of "less neodymium and no neodymium" motors was proposed, samarium iron-nitrogen permanent magnet materials have attracted much attention. Due to the lack of accurate and comprehensive information, the current industry and the related investment community, the understanding of the history of rare earth iron nitrogen permanent magnet materials research and development and industrial progress is not objective enough. In this paper, the development history of anisotropic rare earth ferric nitrogen (samarium-ferric nitrogen, neodymium ferric nitrogen) permanent magnet materials, especially the new progress in industrialization, will be introduced in combination with the published academic literature, patents, existing products on the market and ongoing development directions.

I. Overview

        Rare earth Fe-nitrogen permanent magnet material is a major original invention in the field of rare earth functional materials in China. From the proposal of physics idea to the core process and key equipment for large-scale production, it is an independent innovation.

        The first literature to propose the concept of anisotropic rare earth iron-nitrogen permanent magnet materials dates back to around 1990. The relevant literature was mainly written by Yang Yingchang research team of Peking University and the students of the team, and the relevant research results obtained invention patents in China, the United States, Japan and the European Union. Professor Zhang Hongjie, academician of the Chinese Academy of Sciences and president of the China Rare Earth Industry Association, pointed out at the 11th China Baotou Rare Earth Industry Forum in 2019: "In addition to Academician Yang Yingchang's samarium iron nitrogen permanent magnet material, the original technology of existing rare earth new materials is basically not mastered by China."

        The original contribution of Yang Yingchang's research team at Peking University is mainly reflected in two aspects:

        First, in 1990, the nitridation effect was discovered in rare earth iron alloys: by gas-solid reaction, nitrogen atoms were added to R(Fe,M)₁₂, M = Ti,V,Mo... Or... In Sm₂Fe₁₇, the corresponding nitride R(Fe,M)₁₂N_x or Sm₂Fe₁₇N_x is formed, and it is found that the magnetic properties of nitrides are comprehensively and greatly improved. For the first time in the world, the team used neutron diffraction technology to determine the crystal structure of samarium ferric nitrogen and neodymium ferric nitrogen, and found that nitrogen occupies specific interstitial crystal sites in these structures. Based on this, the effects of interstitial atoms on the crystal field and electronic structure are calculated. It is found that nitrogen atoms at specific crystal sites can sensitively adjust the crystal field structure of rare earth 4f electrons and the band structure of iron 3d electrons, resulting in an increase in the magnetic moment of iron atoms and a fundamental change in the crystal field structure of rare earth 4f electrons, which reveals that the nitride effect originates from the interstitial atomic effect of nitrogen. On the one hand, the interstitial atom effect changes the crystal field structure of rare earth ions, on the other hand, the Curie temperature of the alloy is significantly increased. This physical idea revealed by Yang Yingchang's research team has laid a theoretical foundation for the development of interstitial rare earth permanent magnet materials such as rare earth iron and nitrogen. The team's related research has twice won the National Natural Science Award, the Ho Liang Ho Li Foundation Science and Technology Progress Award, the International Rare Earth Permanent Magnet and Advanced Magnetic Materials Conference (REPM) Outstanding Achievement Award and other important honors and awards.

        The second is to study the magnetic domain structure of nitride and its magnetization process, and successfully develop the industrialization of core technology and key equipment. Since 1990, rare earth nitrides have become a hot spot in the global development of new rare earth permanent magnet materials. However, many studies at home and abroad have shown that although nitride has excellent intrinsic magnetism, it is difficult to produce high-performance rare earth iron-nitrogen magnetic powder by using the existing process of manufacturing Ndfeb magnetic powder. In 1994, GM R & D Department of the United States adopted the rapid quenching process and Hitachi Company of Japan adopted the mechanical alloying technology to prepare it without success. Yang Yingchang's research team believes that the parametric coercivity, residual magnetic induction intensity and maximum magnetic energy product that mark the properties of permanent magnet materials are all structurally sensitive, related to the microstructure of the material, technically speaking, these depend on the manufacturing process of the material, and theoretically, depending on the magnetic domain structure of the material and its magnetization process. New materials should be developed according to their characteristics and suitable manufacturing methods. To this end, the team turned to the study of technical magnetization of nitrides, successfully observed the magnetic domain structure of rare earth iron nitrogen for the first time in the world, studied its magnetization mechanism, and explored a variety of different preparation techniques. Since 2011, with the support of the national "863" program, Peking University and Beijing Hengyuan Valley Technology Co., Ltd. have successfully carried out the development and construction of hundred-ton high-performance rare earth iron nitrogen permanent magnet powder production line, developed large-scale mass production technology and key equipment, and made further development and construction for market applications in Ningxia Junmagnetic New Material Technology Co., LTD.

        Looking back on the above research history, it can be seen that the research team of Peking University Yang Yingchang, as the main inventor of rare earth iron-nitrogen permanent magnet materials, has made internationally recognized, significant and pioneering contributions, so that China in the field of a new generation of rare earth permanent magnet materials has been free from the situation of being restricted by foreign patents for a long time. It has promoted China's transformation from a major country in rare earth resources to a powerful country in rare earth science and technology, and promoted the high-quality development of China's rare earth industry.

2. The industrialization development of rare earth iron-nitrogen permanent magnet materials

        Since entering the new century, because the price of rare earth samarium has been significantly lower than rare earth neodymium for a long time, the development of rare earth iron nitrogen permanent magnet materials mainly focuses on samarium iron nitrogen. In general, domestic and foreign academic tracking research is hot and cold, and continuous industrial technology development research is more rare, mainly because there is no consensus and confidence in the potential of this material in the past. Internationally, mainly the Magvalley technology development team led by Yang Yingchang of Peking University and Japanese enterprises represented by Sumitomo Metal Mining Co., Ltd. have carried out long-term uninterrupted research and development of industrialization core technologies, and finally developed successfully and obtained international patent protection. In 2022, Professor Coey, a fellow of the Royal Academy of Sciences, pointed out in his book Mordern Permanent Magnets that at present, anisotropic samarium iron-nitrogen permanent magnets are mainly produced by three production companies. Japan's Sumitomo Metal Mining Co., LTD., Nichia Chemical Industry Co., LTD., and Ningxia Junmagnetic New Material Technology Co., LTD. Among them, two Japanese enterprises use reduction diffusion process for production, and Ningxia Junmagnetic New Material Technology Co., Ltd. uses a new powder metallurgy method for production. Formed two process routes, three enterprises have independent intellectual property rights and achieve mass production situation. The core research and development teams of the three enterprises have generally carried out relevant research and development work since the 1990s.

        In 2021, the Japan Adhesive Magnet Association (JABM) released a report, introducing the product performance characteristics of the current anisotropic samarium-iron-nitrogen production enterprises with independent intellectual property rights and mass production capabilities in the world. The report was completed by Mr. Iriyama, and relevant literature was published in foreign industry authoritative journals, as shown in Table 1.

        Under the guidance of Yang Yingchang research team of Peking University, Ningxia Junmagnetic New Material Technology Co., Ltd. has also developed and built the world's only Nd Fe-nitrogen permanent magnet material mass production line, and uses the unique advantages of Nd Fe-nitrogen materials to carry out the application and development of ultra-thin permanent magnet sheets.

        In addition, Japan's Datong Electronics Co., Ltd. reported its development of injection molding and die molding isotropic samarium-iron-nitrogen magnets, which reported that they have magnetic properties that exceed the isotropic bonded magnets currently on the market, and have good corrosion resistance and thermal stability of nitrides.

        Recently, due to the performance advantages of samarium iron nitrogen permanent magnet materials began to appear rapidly in the market, there are also domestic enterprises claimed to have carried out industrial development, but from a technical point of view, no one has proposed a technical route different from the aforementioned existing two processes, has not yet seen the new patented technology, from the market, there is no independently developed products.

        The industrial development of rare earth iron-nitrogen permanent magnet materials mainly refers to the large-scale mass production of the above-mentioned high-performance rare earth iron-nitrogen magnetic powders. The preparation of high-performance rare earth ferric nitrogen magnets depends on the premise of mass production of high-performance magnetic powder, and is also affected by the following two aspects: one is the progress of anisotropic bonded magnet molding technology and equipment, and the other is the improvement of the magnetic field orientation and magnetization level of anisotropic magnetic materials. Japanese companies have been leading in both areas. In recent years, with the joint efforts and the expansion of international cooperation between Magvalley Technology and downstream enterprises, as a manufacturer of high-performance rare earth iron nitrogen magnetic powder, Magvalley Technology has been able to provide international leading level of technical support and process guidance for downstream application development in these two aspects, which has largely solved the two bottleneck problems of production efficiency and magnetic field orientation. Also participate in end-user motor design.

3.The performance advantages of rare earth iron-nitrogen permanent magnet materials and the technical level that existing products have reached

        As a new generation of rare earth permanent magnet materials, rare earth iron-nitrogen permanent magnet materials have shown many advantages and characteristics from the technical level that existing products can achieve.

        First, from the point of view of resources, such materials do not use heavy rare earths at all, especially samarium iron nitrogen, do not use neodymium, but use samarium. On the one hand, the raw material cost pressure of heavy rare earths can be avoided, and on the other hand, the price fluctuation impact of neodymium can be avoided.

        Second, from the point of view of magnetic properties, the maximum magnetic energy product of anisotropic samarium iron nitrogen magnetic powder on the market is more than twice that of the mainstream rare earth bonded magnetic powder isotropic NdFeb, and the Curie temperature is higher than the latter by more than 150 ° C, and has excellent oxidation resistance and corrosion resistance.

        Third, the rare earth iron-nitrogen magnetic powder has good particle size characteristics, and its D50 is about 2μm, which is far lower than the particle size of Ndfeb magnetic powder. This makes the rare-earth iron-nitrogen magnets exhibit better mechanical properties, greater freedom of forming, and can be hybridized with other magnetic particles to further improve the density of the bonded magnets. This particle size characteristic can also reduce the wear of the production mold and increase the life of the mold by more than 50%, thus reducing the production cost.

        Based on the above performance characteristics, it can be seen that the application and development of rare earth iron-nitrogen permanent magnet materials has reshaped the concept of "bonded magnetic", reconstructed the situation of rare earth permanent magnet materials, and further provided a broader idea and greater freedom for the design and development of downstream motors. This change, first of all, the performance range of "bonded magnetic" has been greatly expanded, in the past, the highest performance of injection bonded magnetic is usually about 10MGOe, the highest performance of molded bonded magnetic is usually about 12MGOe, and now due to the emergence of high performance samarium iron nitrogen magnetic powder, The highest performance of the injection bonded magnetic has reached 18MGOe, and the highest performance of the molded bonded magnetic has reached more than 25MGOe. Although this performance is still lower than the sintered NdFeB, but because it is a bonded magnet, it has a greater degree of freedom in magnetic circuit design, so that it can be applied to some scenarios that can only be applied to sintered NdFeB before, and shows the advantages of high resistivity and high mechanical properties, especially for the development of high-frequency high speed motor miniaturization and lightweight needs. In addition, samarium iron nitrogen magnetic powder can also be made into hybrid magnets with inexpensive ferrite magnetic powder, providing a more cost-effective bonding magnet in the field of maximum magnetic energy product of 3 to 7MGOe.

        Due to the corrosion resistance of rare earth iron and nitrogen, this new type of bonded magnet is usually no longer required to be coated with rust prevention as the previous rare earth bonded magnet, which is both environmentally friendly and reduces the cost. It can be said that due to the application of rare earth iron and nitrogen, "bonded magnetic" is no longer a low magnetic material, no longer a material only applied to the field of micro motors, no longer a material that cannot compete with rare earth sintering magnetic, but a new permanent magnet material with a wide range of magnetic properties, strong oxidation resistance and corrosion resistance, good mechanical properties and high resistivity.

        From the application point of view, the unique advantages of rare earth iron-nitrogen permanent magnet materials in the field of micro motors are obvious. The latest practice shows that, driven by the concept of "less neodymium" and "no neodymium", some internationally renowned automobile companies have begun to use this new bonding magnet to design motors that previously only used rare earth sintered magnets, and the obvious example is the new energy vehicle drive motor. Another notable example is that based on the high magnetic properties and submicron particle size characteristics of rare earth iron nitrogen magnetic powder, since the use of thermoplastic materials to develop rare earth iron nitrogen magnetic elastomers, it has rapidly set off application development enthusiasm in the field of consumer electronics, among which "halogen-free flame retardant magnetic elastomers" have been applied in mobile phones, tablet computer charging data lines, smart phone watchbands and other fields.

        At present, the rare earth iron-nitrogen permanent magnet material products sold on the market are mainly magnetic powder, particles for injection magnets and various types of bonded magnets. Among them, anisotropic samarium-iron-nitrogen magnetic powder has high magnetic properties, fine particle size, uniform distribution and strong oxidation resistance, and is suitable for the preparation of new high-performance rare earth bonded magnets of various forms by injection, molding, extrusion, calendering, 3D printing and other processes, as shown in Table 2.

        Samarium-iron-nitrogen and other rare earth iron-nitrogen permanent magnet materials through the hybrid with ferrite magnetic powder and other rare earth magnetic powder, using traditional PA12, PPS and epoxy resin made of bonded magnet products, the maximum magnetic energy product can reach 25MGOe. The performance range of 3~25MGOe can not only fully cover the performance range of traditional bonded magnets on the market, but also further break through the performance limits of bonded magnets. Flexible magnets and magnetic elastomer materials prepared with rare earth Fe-nitrogen permanent magnet materials also make full use of the characteristics of such magnetic powder materials with high magnetic properties, fine particle size and strong oxidation resistance. While realizing high magnetic properties, flexible magnets can also have high surface finish and excellent flexibility, so that magnetic elastomer materials have good tensile properties and flexibility. Provides an excellent use experience for products in wearable application scenarios. This combination of high magnetic properties and good mechanical properties, but also the traditional ferrite and NdFeb magnetic materials do not have, see Figure 1, Figure 2, Figure 3, Table 3.

        Taking samarium ferric nitrogen injection magnet as an example, using the MGC12 series samarium ferric nitrogen particles produced by Ningxia Junmagnetic New Material Technology Co., LTD., the Halbach sinusoidal quadrupole magnetized ring magnet produced can reach the surface magnetic flux density of 2900Gs at the size of φ11.3 × φ5 × 11.6mm; The halogen-free flame-retardant magnetic data line extruded particle material made of anisotropic samarium ferric nitrogen magnetic material can achieve a surface magnetic flux density of 650Gs at a thickness of 0.6mm and ensure good flexibility.

4.The prospect of rare earth iron-nitrogen permanent magnet materials

        Although the concept of rare earth iron-nitrogen permanent magnet materials was proposed in the 1990s, it is still in the initial stage in terms of its industrial development and application. First, its theoretical magnetic potential is far from being played out, the anisotropic field of samarium iron nitrogen is 21T, NdfeB is 9T, theoretically the maximum magnetic energy product of samarium iron nitrogen is more than 60MGOe, and the current highest performance of its bonded magnetic powder is only about 40MGOe, and rare earth iron nitrogen sintered magnets are still in the development process. Second, the industrial application and development of rare earth iron nitrogen materials is still in the early stage. Rare earth Fe-nitrogen permanent magnet materials do not contain heavy rare earths and are not affected by price fluctuations of neodymium. Based on the accumulation of application and development experience in recent years, they are increasingly becoming new materials used in industry practice. Through practical efforts, manufacturers of the industrial chain such as rare earth iron-nitrogen permanent magnet devices, motors, and consumer electronics products have broken the previous stereotype and fear of anisotropic permanent magnet materials, and are further expanding the application range of rare earth iron-nitrogen permanent magnet materials through the improvement and improvement of device preparation technology and magnetic access/magnetization technology. With the further improvement of anisotropic samarium iron-nitrogen magnetic powder preparation technology and the research and development of low-temperature sintering samarium iron-nitrogen, rare earth iron-nitrogen permanent magnet materials will increasingly become an important member of the magnetic material family, providing important material support for the upgrading of the downstream electronic information industry and the pursuit of a better life for human beings.

Author: Wang Xin-An¹, Liao Siyu¹, Cheng Ben-Pei^1,2

1. Ningxia Junmagnetic New Material Technology Co., LTD(Junmagnetic Technology Magvalley) 

2. Peking University

Note: This article was published in the 6th issue of "Rare Earth Information" in 2024, if you need to reprint, please indicate the source.