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A dual-main-phase magnet with excellent thermal stability was prepared by co-mixing two types of magnetic powders, which was the conventional N55-grade powder without Dy elements, while the Co and Dy contents reached 50 wt% and 6 wt% respectively in another Co-rich powder. Studies showed that the magnetic properties along with the thermal stabilities of the magnets exhibited significant differences with the changes in the mixing ratio of the two powders. Especially, when mixing the two powders in equal proportions, a great remanence temperature coefficient of −0.0725 %/°C at 20–100 °C was obtained with a rather high Curie temperature of 581 °C, possessing its magnetic properties with Br = 12.58 kGs, Hcj = 15.9 kOe, and (BH)max = 36.88 MGOe. Note that as the proportion of Co-rich powder increased to 2/3, although the Curie temperature of the prepared magnets further increased to 622 °C, both the remanence temperature coefficient and the magnetic properties deteriorated significantly. The phase composition and microstructure analyses revealed that when the proportion of added Co-rich component exceeded 1/2, the RE(Fe,Co)2B phase transformed into the RE(Fe,Co)4B phase, and a significant amount of RE(Fe,Co)1.8B0.2 phase precipitated at the grain boundaries, leading to a deterioration in magnetic performance. In addition, dynamic domain evolution analyses under variable temperature conditions demonstrated the positive effect of the appropriate introduction of Co/Dy on the anti-demagnetization in high temperatures. This work provides insights and an experimental foundation for the development of high-performance magnets with high thermal stability, used in high-temperature environments.

Sm-Fe-N material holds immense promise as new material for developing next generation permanent magnets. However, the achievement of high coercivity in this material remains a big challenge, and moreover new coercivity theories beyond the commonly considered nucleation-type mechanism needs exploration. This study describes both the yielding of anisotropic Sm-Fe-N powders with high coercivities and the discovery of new coercivity mechanism by a systematic study using a designed milling preparation method. As a result, a random-direction cleavage behavior with unexpected XRD peak shifts and formation of new micron-sized flower-like particles were found in the yielded Sm-Fe-N powders. The grain sizes of the powders are of nanoscale dimension and they refine gradually as the milling time increases. The measured coercivity is up to 15.0 kOe, which is a substantial increase of approximately 36.4% compared to the original commercial powders and among the highest reported values via the milling preparation method (commonly below 13 kOe). The yielded powders also demonstrate a new dual-type coercivity mechanism (combing both nucleation-type and pinning-type control effect), and the improved pining effect contributes to the high coercivities. This study opens the door of utilizing pinning effect for achieving high coercivities in anisotropic Sm-Fe-N material.

The multi-step magnetization behavior caused by a variety of microstructures is common in magnetic materials. The study of the effect of heterogeneous structure magnetization behavior on magnetic viscosity provides extensive guidance for the long-term stability of magnetic materials. The microstructure of Fe–Co–Ni–Al–Ti–Cu alloy was regulated by heat treatment process, and homogeneous structure (S810) and heterogeneous structure (S830) samples were prepared. In the S830 sample, there are three steps of magnetization reversal due to the presence of α1′ phases (split α1 phases). Part of the α1′ phases are reversed under a lower magnetic field, and the other part has strong interaction with the α1 phases forming a closed stable magnetic domain structure, which enhances the switch field of α1 phases. Both first-order reversal curves (FORC) analysis and electron holography have confirmed this phenomenon. The magnetic viscosity coefficient of S830 is higher, but its magnetic field dependence is better. The quantitative calculation of energy barriers shows that the energy barriers of α1′ phases are lower, which leads to the instability of S830 sample under low magnetic fields. The superposition of three magnetization reversal steps makes the variation of magnetic viscosity coefficient gentler.

The (NdLaCeY)-Fe-B magnets with 45–50 wt% high-abundance rare earth substitution were systematically investigated by co-mixing high boron content matrix phase and Pr–Fe alloy powders (0, 1, 3, 5, 7, and 9 wt%) during the preparation process. The coercivity of magnets increased successively with the addition of Pr70Fe30 (wt.%) and reached 9.32 kOe at the addition amount of 9 wt% from the initial 7.33 kOe without addition, while the remanence had a very small decrease of only 0.13 kGs. The analyses showed that the B-rich phase in the high-boron powders reacted with Pr70Fe30 to form the main phase shell, forming a core-shell structure with a Pr-rich shell and a Y-rich core. Meanwhile, the grain boundary was significantly widened with the introduction of Pr70Fe30. Then, just the improved anisotropy of the main phase grain surface, accompanied with the enhanced magnetic de-coupling effect between the main phase grains, resulted in a great coercivity increment. Notably, on the remanence, the beneficial effect of increased main-phase proportion by regenerated grain shell had a competitive relationship with the deteriorating effect of the magnetic dilution by the non-magnetic material introduction, thus under the combined effects, the remanence firstly increased and then decreased with the addition of Pr–Fe.

The magnetically hardened shells constructed by the spontaneous core–shell structure due to the enrichment of Y in the core region of (Y/RE)-Fe-B facilitate the enhancement of coercivity. In this work, we systematically compared the metallurgical behaviors of Y in R2Fe14B (RE = Nd, Tb) to investigate the modulation effect of Y on the distribution of REs in the matrix phase. During the strip casting, Y was depleted in the RE-rich phase in Y1/3Nd2/3-Fe-B while enriched in Y1/3Tb2/3-Fe-B. Under the sintering and annealing, Y was simultaneously enriched in the core of matrix phase grain, resulting in the spontaneously formation of Y-rich core and Nd-rich shell structure as well as the Y-rich core and Tb-rich shell structure. Further statistics on the concentration distribution of Y in the matrix phase indicated that the degree of aggregation for Y in Y1/3Nd2/3-Fe-B is higher than that of Y1/3Tb2/3-Fe-B, due to the different formation energy of Re2Fe14B phases. Notably, the spontaneous Y-rich core and Tb-rich shell structure demonstrates the modulation effect of Y on the distribution of Tb, inspiring an innovative scheme for construction of magnetically hardened shells.

High-performance Nd-Fe-B-based rare-earth permanent magnets play a crucial role in the application of traction motors equipped in new energy automobiles. In particular, the anisotropic hot-deformed (HD) Nd-Fe-B magnets prepared by the hot-press and hot-deformation process show great potential in achieving high coercivity due to their fine grain sizes of 200–400 nm, which are smaller by more than an order of magnitude compared to the traditional sintered Nd-Fe-B magnets. However, the current available coercivity of HD magnets is not as high as expected according to an empirical correlation between coercivity and grain size, only occupying about 25% of its full potential of the anisotropy field of the Nd2Fe14B phase. For the sake of achieving high-coercivity HD magnets, two major routes have been developed, namely the grain boundary diffusion process (GBDP) and the dual alloy diffusion process (DADP). In this review, the fundamentals and development of the HD Nd-Fe-B magnets are comprehensively summarized and discussed based on worldwide scientific research. The advances in the GBDP and DADP are investigated and summarized based on the latest progress and results. Additionally, the mechanisms of coercivity enhancement are discussed based on the numerous results of micromagnetic simulations to understand the structure–property relationships of the HD Nd-Fe-B magnets. Lastly, the magnetization reversal behaviors, based on the observation of magneto-optic Kerr effect microscopy, are analyzed to pinpoint the weak regions in the microstructure of the HD Nd-Fe-B magnets.

The magnetic properties and microstructure of Nd-Ce-Y-Fe-B magnets with Nd-Co boundary additions (0, 1, 3, and 5 wt%) have been systematically investigated. The coercivity, remanence, and Curie temperature of the magnets showed to be all simultaneously improved with appropriate Nd-Co addition, especially, a significant coercivity increase of 15% accompanied with the remanence optimization was obtained at 3 wt% Nd-Co addition. The analyses revealed that the coercivity enhancement was attributed to the widened grain boundary width and the reduced magnetic exchange coupling between grains by the Nd-Co intergranular introduction. By designing the partial substitution, e.g. Co for Fe and Nd for Ce in matrix phase grains, the remanence improvement was achieved with the increased net magnetic moment, compensating the magnetic dilution brought by the extra alloy additions. Meanwhile, the Curie temperature of the magnet was improved with the weakened negative interaction. The combined intrinsic and microstructural modification in this study provides new insights into the preparation of high-abundance magnets with high coercivity, high remanence, and excellent temperature stability.

Using the heavy rare earth Dy element to improve coercivity is the most common solution for hindering the reduction in magnetic properties in the Nd–Fe–B magnet, and the effective utilization of Dy has become the focus of research in industrial society. In this work, we investigated the influence of DyHx addition and diffusion methods on the microstructure, magnetic performance, and thermal stability of the Nd–Y–Fe–B magnet with a Y-rich core structure. The coercivity of the DyHx addition magnet increases from 9.45 kOe to 15.51 kOe when adding 1.6 wt.% DyHx, while the DyHx diffusion magnet increases to 15.15 kOe. According to the analysis of the microstructure and elemental distribution, both Dy-rich shells were basically formed due to the diffusion process of Dy atoms. The Dy-rich shell in the DyHx addition magnet was similar with the original core–shell structure in the Nd–Y–Fe–B magnet. However, the distinct dual-shell structure consisting of a thinner Dy-rich shell and a Y-lean shell was constructed in the DyHx diffused magnet, contributing to the superior coercivity increment and Dy utilization efficiency. Furthermore, the remanence of the DyHx diffused magnet is up to 12.90 kG, which is better than that of the DyHx addition magnet (12.59 kG), due to fewer Dy atoms entering the 2:14:1 matrix grain to cause the antiferromagnetic coupling with Fe atoms. Additionally, the thermal stability of the DyHx diffusion magnet is also better than that of the DyHx addition magnet, owing to the elevated coercivity at room temperature, which expands the application range of the Nd–Y–Fe–B magnet to a certain extent.

The grain boundary diffusion process (GBDP) of heavy rare earth Tb is an effective method to improve the coercivity of Nd-Fe-B magnets, and the matrix grain size has a crucial effect on the diffusion efficiency and depth of the Tb element. In this work, magnets with different grain sizes have been fabricated using powder metallurgy to investigate the effect of grain size on Tb diffusion efficiency and the microstructure of Nd-Fe-B-type magnets. After the Tb diffusion process, the coercivity increment of the magnet with 4.9 μm large grain is 8.60 kOe, which is much higher than that of the magnet with 3.0 μm small grain (~5.90 kOe), which clearly demonstrates that the coercivity increment decreases as the grain size decreases. Microstructure analysis suggested that grain refinement significantly increases the total surface area, resulting in narrowing and discontinuity of the grain boundary phase (GBP). Therefore, as the channel for diffusion, the narrowing and discontinuity of the GBP are unfavorable for diffusion, resulting in a decrease in diffusion efficiency.

In this study, the effects of Y2O3 addition on the magnetic properties, microstructure and magnetization reversal behavior of Sm(Co0.79Fe0.09Cu0.09Zr0.03)7.68 magnet were investigated. By addition of Y2O3, the coercivity was increased from 21.34 kOe to 27.42 kOe at 300 K and from 5.14 kOe to 6.27 kOe at 823 K. A magnet with a maximum magnetic energy product of 9.86 MGOe at 823 K was obtained. With the interdiffusion of Y and Sm after appropriate addition, the Cu content within the cell boundary phase close to the oxide was detected to be nearly twice as high as that away from the oxide. We report for the first time that a collection of lamellar phases were formed on both sides of the inserted oxide, providing a strong pinning field against magnetic domain wall motion based on in-situ Lorentz TEM observation. Furthermore, the ordering process of the original magnet was delayed after Y2O3 addition, resulting in the refinement of cellular structure, which can also enhance the domain wall pinning ability of cellular structures based on micromagnetic simulation. However, excessive addition of Y2O3 led to large Cu-rich phase and Zr-rich impurity phase precipitated at the edge of the oxide, resulting in the destruction of cellular structures and a significant reduction in coercivity. This study provides a new technical approach to regulate the microstructure of Sm2Co17 type magnets. Addition of Y2O3 is expected to play a significant role in improvement of high temperature magnetic properties.

Synthesis of nanoalloys containing three or more elements with controllable size and morphology and desired phase is very challenging. Rare earth (RE)-transition metal (TM) nanoalloys have great potential applications, however, their controllable synthesis is even a daunting task. Here we find that the direct gas-phase ordering of Sm-Co-based nanoalloys (around 5–15 nm) into 1:5 phase without size and shape change can be achieved by in-flight annealing through reducing their crystallization-ordering temperature (COT) with additive Cu or Ni. The homogenous element-distribution and desired phase in these trimetallic nanoalloys are evidenced by high-resolution-transmission-electron-microscopy analysis despite a preferred surface segregation from thermodynamic calculation and analysis. Our results demonstrate a promising way to synthesize trimetallic nanoalloys especially RE-TM with independently controlled size, composition and phase as well as their building-blocks with novel functionalities. The reduction in COT of Sm-Co-Cu nanoalloys is further confirmed by post-annealing the deposited nanocluster film. A significant coercivity-enhancement is achieved at a lower annealing-temperature. Compared to FePt, the higher anisotropy and much lower COT of Sm-Co-based nanoalloy make it emerge as a promising next-generation recording media with density beyond FePt. Our study also opens a door for fabricating trimetallic or even multicomponent nanoalloys with controllable size, composition and phase. Keywords: Nanoalloys, Gas phase ordering, Crystallization temperature, Cluster beam deposition, High magnetic anisotropy

A novel practice of surface nanocrystallization (SNC) to protect sintered Nd-Fe-B magnet from corrosion was achieved by a controlled hydrogenation-disproportionation-desorption-recombination (HDDR). The SNC magnet showed much better corrosion resistance compared with the originally unprocessed. The nanocrystallized surface of 50 μm thick layer with dispersed Nd-rich intergranular phases played crucial role in blocking intergranular corrosion. For SNC magnet, corrosion test showed a highly oxidized and chemically homogeneous film had formed preventing corrosion deep inwards. Hardness testing revealed the surface was strengthened showing a totally different cracking pattern. To verify its applicable ability, the influences of hydrogen reaction with bulk magnet on microstructures and magnetic properties in different depth were studied. For a large-sized magnet, the detrimental effects were almost negligible. This technique could be a possible new method of surface processing utilized in large bulk Nd-Fe-B magnet for good corrosion resistance and surface strength. Keywords: Sintered Nd-Fe-B magnet, Corrosion resistance, Surface nanocrystallization

The chemical bath deposition (CBD) method is used to diffuse the heavy rare earth element in order to obtain the high coercivity magnets with low heavy rare earth element. The jet mill powders are soaked in the alcohol suspension of Dy(CH3CHOHCH3)3 (Dy-ipa) so that Nd2Fe14B powder particles are surrounded by Dy-ipa homogeneously. By adding 1.0 wt. % Dy, the coercivity of magnet is increased from 14.47 kOe to 17.55 kOe with slight reduction of remanence after grain boundary diffusion (GBD) in the sintering and annealing processes. The temperature coefficient of coercivity optimizes from -0.629 %/°C to -0.605 %/°C as well as that of remanence improves from -0.108 %/°C to -0.100 %/°C. The CBD method is helpful for thermal stability and alignment either. The relation between the microstructure and the coercivity has been studied systematically.

The dynamic magnetic domain structure in magnetization and demagnetization process of hot-deformed and NdCu-diffused Nd2Fe14B magnets were in-situ observed by Lorentz transmission electron microscopy (LTEM). The demagnetization process of hot-deformed sample is dominated by domain-wall pinning, while that of NdCu-diffused sample is mainly the magnetization reversal of single grains or grain aggregations. This firstly observed result gives an explicit evidence to understand the coercivity mechanism of magnetically segregated magnet. The effect of magnetic field of TEM on decrease in domain wall energy was theoretically analyzed, which helps to understand the in-situ observation process of magnetic materials.

Magnetization reversal mechanism is found to vary with cellular structures by a comparative study of the magnetization processes of three (Sm, Dy, Gd) (Co, Fe, Cu, Zr)z magnets with different cellular structures. Analysis of domain walls, initial magnetization curves and recoil loops indicates that the morphology of cellular structure has a significant effect on the magnetization process, besides the obvious connection to the difference of domain energy density between cell boundary phase (CBP) and main phase. The magnetization of Sample 2 (with a moderate cell size and uniformly continuous CBPs) behaves as a strong coherence domain-wall pinning effect to the domain wall and lead to a highest coercivity in the magnet. The magnetization of Sample 1 (with thin and discontinuous CBPs) shows an inconsistent pinning effect to the domain wall while that of Sample 3 (with thick and aggregate CBPs) exhibits a two-phase separation magnetization. Both the two cases lead to lower coercivities. A simplified model is given as well to describe the relationships among cellular structure and magnetization behavior.