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In order to develop permanent magnets with coercivities close to their physical limit, it is essential to obtain a better understanding of their magnetization reversal processes by directly observing their magnetic domains dynamically under an external magnetic field. Here, we review recent magneto-optical Kerr effect microscopy studies for correlating the magnetization reversal processes of Nd-Fe-B-based permanent magnets with different microstructural features. Angular--dependent coercivity measurements of conventional Nd-Fe-B sintered magnets with a ferromagnetic intergranular phase (IGP) suggest that the coercivity is governed by the pinning of magnetic domains at grain boundaries. Micromagnetic simulations based on the model created from experimentally observed microstructures predicted that the alternation of IGP to weak/non-ferromagnetic hinders the cascade-type domain wall propagation through an intergranular phase. Nevertheless, the angular-dependent coercivity of Ga-doped Nd-rich sintered magnets, that shows thick Nd-rich IGP partially isolating Nd2Fe14B grains, indicates that the reversal mechanism is still the pinning type. Hence, further exchange decoupling would lead to larger coercivity in Nd-Fe-B sintered magnets. We discuss how the magnetism of IGPs and tuning the anisotropy field of the IGP/Nd2Fe14B interfaces in ultra-fine-grained hot-deformed magnets alters the magnetization reversal process to reach the large coercivity of 2.5 T. [ABSTRACT FROM AUTHOR]

To develop low-cost, high-performance light rare earth element (LRE)-substituted Nd–Fe–B permanent magnets, we systematically studied the effect of substituting Nd with 20% light rare earth elements (LRE = Ce, Y, and La) on the hard magnetic properties of (Nd0.8LRE0.2)–Fe–B hot-deformed magnets. The (Nd0.8Ce0.2)2Fe14B magnet showed a coercivity of μ0Hc = 1.41 T, which is comparable to that of an LRE-free magnet. Y substitution was found to decrease μ0Hc and μ0Mr to 1.22 and 1.32 T, respectively, as was expected from the intrinsic properties of (Nd0.8Y0.2)2Fe14B. Unlike our expectation, the thermal stability of coercivity is not degraded by the Y substitution, demonstrating its potential for elevated temperature applications. With La substitution, coercivity is substantially degraded to 0.67 T because of the low intrinsic properties and the lack of the intergranular phase. The Y and Ce co-substituted (Nd0.8Ce0.1Y0.1)2Fe14B hot-deformed magnets showed better thermal stability of coercivity compared to the (Nd0.8Ce0.2)2Fe14B sample. [ABSTRACT FROM AUTHOR]

SmFe 12 -based compounds have been considered as promising candidates for next generation permanent magnet materials because of their excellent intrinsic hard magnetic properties with a minimum usage of rare earth elements. However, realizing high coercivity in anisotropic microstructure is a big challenge, which hinders their practical applications. In this work, a novel anisotropic granular microstructure of Sm(Fe 0.8 Co 0.2) 12 with a sufficiently large coercivity (µ 0 H c) of 1.2 T and with a high remanent magnetization of 1.50 T is demonstrated in thin films prepared by co-sputtering Sm(Fe,Co) 12 with boron. Detailed microstructure characterization using high resolution scanning transmission electron microscopy (STEM) and atom probe tomography (APT) indicate that the addition of B leads to the development of columnar-shaped Sm(Fe 0.8 Co 0.2) 12 grains with a size of ∼40 nm, which are surrounded by ∼3 nm-thick B-enriched amorphous intergranular phase. Domain wall pinning at the amorphous grain boundary phase is attributed to the high coercivity. This work provides a guiding principle for realizing high-coercivity anisotropic SmFe 12 -based permanent magnets, which can outperform Nd-Fe-B magnets and could be used as the next-generation high-performance permanent magnets in various applications. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Based on a modified hydrogenation disproportionation desorption recombination (HDDR) process we propose and realize a novel top-down processing route to synthesize anisotropic nano-composite magnet powders. Selection of alloy compositions with Nd content lower than the stoichiometry of Nd 2 Fe 14 B phase led to the formation of spherical shaped nano-sized α-Fe phase within the Nd 2 Fe 14 B matrix after HDDR process. Next anisotropic nanocomposite Nd-Fe-B/α-Fe powders with substantial coercivity were developed with optimized HDDR conditions and subsequent grain boundary engineering which remedied the initial lack of Nd-rich intergranular phase. Specifically, the infiltration of Nd 70 Cu 30 alloy increased the coercivity from 0.0 to 0.85 T. Note that low coercivity and the absence of texture in nanocomposite magnets have been the main challenges to realize the high (BH) max postulated for many years for anisotropic nanocomposite magnets. We employed micromagnetic simulations for optimum microstructure design of a nanocomposite Nd 2 Fe 14 B/α-Fe magnet that gives a large maximum energy product, (BH) max. The simulations are then correlated with macroscopic hysteresis properties and high-resolution electron microscopy as well as atom probe tomography. Another very remarkable result is the observation that the formation of α-Fe phase with a size up to 200 nm within the matrix of Nd 2 Fe 14 B grains can still result in a significant coercivity of 0.85 T. This is in contrast to common understanding of exchange-coupled systems and we explain this observation with sharp and defect free α -Fe/Nd 2 Fe 14 B interface, the latter a result of the disproportionation and recombination reactions. Image 1 [ABSTRACT FROM AUTHOR]

In this paper, we report for the first time improved hard magnetic properties of Ce-substituted Nd-Fe-B based anisotropic permanent magnet despite the expected reduction of intrinsic hard magnetic properties of (Nd 1- x Ce x) 2 Fe 14 B compound for x > 0. With increasing Ce substitution, x , from 0 to 0.1, the coercivity (μ 0 H c) increased from 1.36 T to 1.44 T while the remanent magnetization (J r) remains to be 1.49 T when the magnets contain Co. Further increase in x decreases the remanent magnetization and coercivity. The ab initio calculations show that the magnetization would not be degraded if the Ce substitutes Nd at 4 g site in the Co-containing magnets, which is found to be the case by atom-resolved STEM-EDS mapping. The enhancement of coercivity of the sample with x = 0.1 originates from higher rare earth concentration in the grain boundary phase, resulting stronger pinning force against reversed domain wall motion. However, the improvement of hard magnetic properties with Ce substitution for Nd was not found in Co-free compounds. Image 1 [ABSTRACT FROM AUTHOR]

Abstract We have fabricated a hot-deformed magnet from the Fe 77.1 Nd 11.6 Pr 3.7 B 5.1 Cu 0.1 Co 1.0 Al 0.9 Ga 0.5 alloy composition that was developed for exchange-decoupled sintered magnet. The hot-deformed magnet showed a higher coercivity, μ 0 H c = 1.93 T, and better squareness, H k90 /H c = 0.96, compared to those of the sintered magnet, μ 0 H c = 1.48 T and H k90 /H c = 0.65. Microstructural analyses revealed the higher coercivity in the hot-deformed magnet originates from a larger contribution from the anisotropy term in the Kronmüller formula owing to fine grains, while the better squareness is primarily due to the exchange coupling of the Nd 2 Fe 14 B grains through ferromagnetic grain boundary phase, as suggested by atom probe tomography and magneto-optical Kerr effect microscopy. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

The microstructural evolution of hot-deformed Nd-Fe-B magnets in each stage of hot-deformation process was studied using transmission electron microscopy and three dimensional atom probe (3DAP). The anisotropic growth of initially isotropic grains in rapidly solidified alloy occurs by annealing without pressing. 3DAP analyses showed a higher concentration of rare-earth elements in the intergranular phase parallel to the flat surface of platelet shaped Nd2Fe14B grains compared to that in the intergranular phase at the side of platelets. [ABSTRACT FROM AUTHOR]

The low temperature grain boundary diffusion process using RE70Cu30 (RE=Pr, Nd) eutectic alloy powders was applied to sintered and hot-deformed Nd-Fe-B bulk magnets. Although only marginal coercivity increase was observed in sintered magnets, a substantial enhancement in coercivity was observed when the process was applied to hot-deformed anisotropic bulk magnets. Using Pr70Cu30 eutectic alloy as a diffusion source, the coercivity was enhanced from 1.65 T to 2.56 T. The hot-deformed sample expanded along c-axis direction only after the diffusion process as RE rich intergranular layers parallel to the broad surface of the Nd2Fe14B are thickened in the c-axis direction. [ABSTRACT FROM AUTHOR]

Magnetic isolation of hard magnetic grains with an intergranular phase is a key factor in obtaining high coercivity, which has been established in the microstructure of Nd-Fe-B magnets owing to the phase equilibrium of hard magnetic Nd 2 Fe 14 B with a low melting point phase. In this work, we investigated the phase equilibrium in the Sm-Fe-V system to establish an optimum composition range to develop high coercivity SmFe 12 -based permanent magnets. In addition to the solid 1:12/liquid Sm-rich phase equilibrium, an additional V-rich phase is found for the first time in the Sm-Fe-V phase diagram for V contents between 19 and 35 at.% at 1500 K. The equilibrium between the 1:12 phase and the two phases with low melting points is computationally modeled for Sm-Fe-V metastable phase diagram. This is experimentally confirmed by analyzing the microstructure of the Sm 8 Fe 72 V 20 thin film, where anisotropic columnar 1:12 grains are enveloped with predominantly V-rich phases along with some Sm-rich intergranular phases resulting in a coercivity (µ 0 H c) of 0.9 T. Although this large V content in Sm(Fe,V) 12 -based alloy is not beneficial for achieving a large magnetization, the finding of a new V-rich intergranular phase can benefit the development of anisotropic sintered magnets via two alloy powder methods; one alloy powder with large V-content for forming intergranular phase and the other with V-lean content to maintain a large magnetizations. To further improve the coercivity to its highest limit μ 0 H c ≈0.2–0.3 H a ≈2–3 T, decreasing the ferromagnetic elements in the intergranular phase is essential. [Display omitted] [ABSTRACT FROM AUTHOR]

0.2 at.%Nb doping to a Nd 12.83 Fe 76.7 Co 4.50 Ga 0.53 B 5.44 alloy suppress the surface crystallization in melt-spun ribbon, thereby suppressing the formation of non-oriented grains during the hot-deformation process to form bulk anisotropic magnet, which leads to the enhancement of remanence. High resolution transmission electron microscopy and three-dimension atom probe tomography revealed that the segregation of Nb at the grain boundaries hinders the grain growth during the hot-deformation process, thereby reducing the size and the aspect ratio of Nd 2 Fe 14 B grains and increasing coercivity. [ABSTRACT FROM AUTHOR]

We revisited the microstructure of Sm 2 Co 17 -type sintered magnets to understand the underlying mechanism of substantial decrease of coercivity from 2.56 T for the optimally annealed sample to 0.14 T for the sample re-annealed at aging temperature of 850 °C for just 1 min and quenched (quenched sample). This low coercivity can be recovered by annealing of the magnet at 850 °C, slow cooling to 400 °C followed by quenching (slowly cooled sample) and with this is fully reversible between the two states. Magnetic domain observations of thermally demagnetized samples with c-axis out-of-plane showed an ultra-fine maze-like domain pattern comparable to the cell size in the slowly cooled sample. However, annealing at 850 °C, even for a short time, followed by quenching resulted in a substantial increase in the size of maze-like domain pattern. Microchemistry of the SmCo 5 cell boundary phase for two samples was analyzed using 3D atom probe. 8.6 at. % Cu and 7.7 at. % Fe was found in the cell boundary of the quenched sample while the SmCo 5 cell boundary phase of the slowly cooled sample contains 15.4 at. % Cu and 3.0 at. % Fe. Unlike the slowly cooled sample, a broader distribution of Cu than Sm was found in the cell boundary of the quenched sample. High resolution STEM-HAADF results showed SmCo 5 /Sm 2 Co 17 interface is sharp and defect free for the slowly cooled sample while it is rough and zigzag shaped for the quenched sample. Diffused Cu into the matrix phase from the cell boundary results in a gradual increase of magnetocrystalline anisotropy form matrix to the cell boundary phase. Large Fe concentration in the cell boundary phase reduces its magnetocrystalline anisotropy. Micromagnetic simulations showed that these microstructural features of the quenched sample cause a weak pinning strength of the cell boundary phase explaining its low coercivity. [ABSTRACT FROM AUTHOR]

Full-density recycled Nd-Fe-B sintered magnets were produced from collected waste Nd-Fe-B magnets with the coercivity, μ 0 H c , of 1.30 T and remanence, μ 0 M r , of 1.37 T. A small fraction of Nd 6 Dy 21 Co 19 Cu 2.5 Fe powder (Grain Boundary Modified (GBM™) powder) was blended to the powders obtained from the Nd-Fe-B waste magnets. The addition of 5.0 wt% GBM™ powder resulted in μ 0 H c = 2.36 T with μ 0 M r = 1.29 T in recycled magnets. The temperature coefficient of intrinsic coercivity, β , in the recycled magnet was measured to be −0.47%/ o C. Microstructure studies showed that the addition of the GBM™ powder increases the volume fraction of a metallic Nd-rich phase in the recycled magnets. 3D tomography revealed a good interconnection of Nd-rich phases through the grain boundaries of the recycled magnet. Microstructure studies suggested that obtained high coercivity in the recycled magnet with 5.0 wt% addition of GBM™ is due to the formation of distinct grain boundary phase combined with the formation of Dy-rich shell in the Nd 2 Fe 14 B grains with ∼0.8 at. % Dy. [ABSTRACT FROM AUTHOR]

The electrical resistivities of anisotropic Nd-Fe-B-based magnets were successfully increased in both parallel and perpendicular directions to the c -axis by hot-deforming Nd-Fe-B melt-spun flakes that were coated with DyF 3 using electrophoretic deposition. Consequently, the operating temperature under high-frequency AC magnetic field was reduced by 20 °C because of the reduction of eddy current loss. The resistive layer formed at the interface of original ribbons in the hot-deformed magnet was found to be NdF 3 rather than DyF 3. Transmission electron microscopy and atom probe tomography revealed that liquid Nd-rich intergranular phase reduces DyF 3 to form NdF 3 at the ribbon interfaces during hot-deformation at elevated temperature. Excess Dy diffuses into the original ribbon flakes through grain boundaries to form a Dy-rich shell in Nd 2 Fe 14 B grains, giving rise to a high coercivity of 1.8 T. This study demonstrates an alternative way to solve the thermal demagnetization problem of Nd-Fe-B-based permanent magnets during operation in electric motors via the resistivity increase of the magnet. [Display omitted] • DyF 3 -coated Nd-Fe-B ribbon was used to produce a high resistivity hot-deformed magnet. • High resistivity (ρ ∥ c = 530 μΩcm and ρ ⊥ c = 250 μΩcm) was obtained with 1.8 T coercivity. • High resistivity is attributed to the uniform distribution of fluoride layer. • Eddy current loss reduction was demonstrated in a high frequency (110.1 kHz) AC field. [ABSTRACT FROM AUTHOR]

Nd-M (M = Al, Cu, Ga, Zn, Mn) alloys with compositions close to eutectic points were investigated as diffusion sources for the grain boundary diffusion process to hot-deformed Nd-Fe-B magnets. Coercivity enhancement was observed for most of the alloys. Among them, the sample processed with Nd 90 Al 10 exhibited the highest coercivity of 2.5 T at room temperature. However, the sample processed with Nd 70 Cu 30 exhibited the highest coercivity of 0.7 T at 200 ° C. Microstructural observations using scanning transmission electron microscope (STEM) showed that nonferromagnetic Nd-rich intergranular phase envelops the Nd 2 Fe 14 B grains after the diffusion process. Abnormal grain growth and the dissolution of Al into the Nd 2 Fe 14 B grains were observed in the sample processed with Nd 90 Al 10 , which explains its inferior thermal stability of coercivity compared to the sample processed with Nd 70 Cu 30 . The coercivity enhancement and poor thermal stability of the coercivity of the Nd 90 Al 10 diffusion-processed sample are discussed based on microstructure studies by transmission electron microscopy. [ABSTRACT FROM AUTHOR]

The coercivity of hot-deformed Nd–Fe–B magnet was substantially enhanced from 1.0 T to 2.6 T by the grain boundary diffusion process using Pr–Cu alloy. However, the temperature dependence of coercivity is larger compared to the sample diffusion processed with Nd–Cu. Microstructure studies showed a good isolation of platelet shaped Nd 2 Fe 14 B grains by Pr-rich intergranular phase, which explains pronounced coercivity at room temperature. Small portions of Nd 2 Fe 14 B become (Nd,Pr) 2 Fe 14 B phase, which has a higher anisotropy field compared to that of the Nd 2 Fe 14 B phase at room temperature, while it becomes lower than that of the Nd 2 Fe 14 B phase above ∼110 °C. Co is depleted from the (Nd,Pr) 2 Fe 14 B phase, which is considered to cause a slight decrease in Curie temperature. Micromagnetic simulations with the magnetically isolated grains including (Nd,Pr) 2 Fe 14 B regions showed that the degradation of thermal stability of coercivity in the Pr–Cu diffusion processed sample is due to the large temperature dependence of anisotropy field in the (Nd,Pr) 2 Fe 14 B regions. [ABSTRACT FROM AUTHOR]

Abstract: The grain boundary diffusion process using an Nd70Cu30 eutectic alloy has been applied to hot-deformed anisotropic Nd–Fe–B magnets, resulting in a substantial enhancement of coercivity, from 1.5 T to 2.3 T, at the expense of remanence. Scanning electron microscopy showed that the areal fraction of an Nd-rich intergranular phase increased from 10% to 37%. The intergranular phase of the hot-deformed magnet initially contained ∼55 at.% ferromagnetic element, while it diminished to an undetectable level after the process. Microscale eutectic solidification of Nd/NdCu as well as a fine lamellae structure of Nd70(Co,Cu)30/Nd were observed in the intergranular phase. Micromagnetic simulations indicated that the reduction of the magnetization in the intergranular phases leads to the enhancement of coercivity in agreement with the experimental observation. [Copyright &y& Elsevier]

Abstract: Anisotropic SmCo5 nanoflakes prepared by high-energy ball-milling with surfactants have great potential in applications for high-performance nanocomposite magnets. For such “nanocomposite” applications, the surface structure and chemistry of nanoflakes are crucial for achieving high coercivity. In this study, hot-pressed samples from anisotropic SmCo5 nanoflakes, ball-milled with different surfactants, oleic acid (OA) and oleylamine (OY), were investigated. Interface layers between the SmCo5 nanoflakes were found to consist of samarium oxides and a soft magnetic Co phase. These surface layers contribute to the degradation of hard magnetic performance, which is confirmed by scanning transmission electron microscopy-energy dispersive X-ray spectroscopy analysis of the cross-section of a single flake ball-milled with OA. Samples milled with OY show a much thinner interface layer in compacted samples, which means that the surface degradation during ball-milling with OY is much less than that with OA. The results show clearly that the choice of proper surfactant and the control of processing parameters are the key factors for improving the surface condition of the nanoflakes and the resulting hard magnetic properties. [Copyright &y& Elsevier]

Abstract: The compositions of grain boundaries (GBs) and other interfaces surrounding Nd2Fe14B grains in commercial Nd–Fe–B sintered magnets have been investigated by laser-assisted three-dimensional atom probe to understand the mechanism of the coercivity enhancement by post-sinter annealing. While only a slight segregation of Nd and Pr to the GBs was confirmed in the as-sintered sample, a thin Nd-rich amorphous phase layer was observed along the GBs with Cu segregation to the interfaces in the annealed sample. The segregation of Cu to NdO x /Nd2Fe14B interfaces was also found, suggesting that the Nd2Fe14B grains are enveloped by a Cu-enriched layer after the annealing. The concentration of Fe+Co in the thin GB layer was found to be as high as 65at.%, and a model amorphous film processed by sputtering with the same composition as the thin GB layer was found to be ferromagnetic. Ferromagnetic behavior of the thin GB layer suggested that Nd2Fe14B grains are magnetically coupled. The coercivity mechanism of the sintered magnets is discussed based on these new findings. [Copyright &y& Elsevier]

An anisotropic full density Nd–Fe–B nanocrystalline magnet was produced by spark plasma sintering of hydrogenation–decomposition–desorption–recombination processed powder and subsequent in situ hot deformation. The in situ hot deformation caused the Nd2Fe14B grains elongated normal to the applied pressure with an enhanced c-axis texture. Accordingly, the remanence (B r) and maximum energy product were improved from 1.07 to 1.35T and 192 to 224kJm−3, respectively, with a slightly reduced coercivity (H c) of ∼1000kAm−1. [Copyright &y& Elsevier]

We have investigated synthesis, magnetic properties and microstructure of Sm 5 Fe 17 -based hard magnetic phase with a Sm 20 Fe 70 Ti 10 composition. Ultrahigh coercivities, μ 0 H c =7.18 T at room temperature and μ 0 H c =8.86 T at 10 K, have been achieved. The room-temperature coercivity, determined from high-field pulse measurements, reaches 35% of the anisotropy field μ 0 H a =20.7±0.8 T. Further, it is demonstrated that a coercivity of 2.18 T is maintained even at 500 K. The Curie temperature T C of Sm 20 Fe 70 Ti 10 is 577 K and the calculated exchange stiffness parameter A is 7.72 pJ/m. Detailed transmission electron microscopy investigations show a two-phase microstructure consisting of the Sm 5 Fe 17 -based hard magnetic matrix phase with grain size below 200 nm and finer, below 100 nm, Fe 2 Ti grains. Majority of the Fe 2 Ti phase is located at the grain boundaries with some finer inclusions found also inside the 5:17 grains. Despite the high fraction of the Fe 2 Ti grains, nearly single-phase demagnetization loops are observed. In order to enhance M s , the effect of Ti content on phase constitution, magnetic properties and microstructure was studied in detail. M s increases and H c decreases for the Ti-lean compositions. [Display omitted] [ABSTRACT FROM AUTHOR]

High phase-purity Sm(Fe 0.8 Co 0.2) 11 Ti and Sm 0.8 Zr 0.2 (Fe 0.8 Co 0.2) 11 Ti powders with the tetragonal ThMn 12 crystal structure have been synthesized by reduction-diffusion method. The average particle size is 1.47 ± 0.82 μm for the Zr-free and 2.29 ± 1.22 μm for the Zr-doped samples. The larger particles are multi-grain, without a chemically distinct grain boundary phase as revealed by TEM investigations. The intrinsic magnetic properties are μ 0 M s = 1.42 ± 0.04 T, μ 0 H a = 9.8 ± 0.4 T for Zr-free and μ 0 M s = 1.37 ± 0.04 T, μ 0 H a = 9.7 ± 0.4 T for Zr-doped samples. Temperature-dependent XRD studies show that the ThMn 12 -type phase begins to decompose around 500 °C which presents difficulties for further development into fully dense bulk magnets. Successful introduction of grain boundary phase with good wettability has been realized by infiltrating low-melting eutectic alloys, resulting in fully dense bulk samples. However, coercivity remains low, μ 0 H c = 0.34 T. The underlying microstructural reasons for not achieving large H c are discussed. Image 1 • Producing Sm(Fe 0.8 Co 0.2) 11 Ti and Sm 0.8 Zr 0.2 (Fe 0.8 Co 0.2) 11 Ti fine powders using reduction diffusion process. • Realizing bulk magnet by use of low melting point alloys as the grain boundaries. • The microstructure origins for the low coercivity of bulk magnet were investigated. [ABSTRACT FROM AUTHOR]

The research interest in SmFe 12 -based permanent magnets has recently been revived in search for new permanent magnet materials that are not dependent on critical rare earth elements such as Nd and Dy. Although the superiority of the intrinsic hard magnetic properties of the Sm(Fe 0.8 Co 0.2) 12 compound as compared to those of Nd 2 Fe 14 B has been experimentally confirmed, its instability as a bulk phase and inability to achieve high coercivity hinder the realization of SmFe 12 -based high-performance permanent magnets. In this Viewpoint, by critically reviewing recent experimental and theoretical studies on SmFe 12 -based compounds, we discuss the prospects for the development of SmFe 12 -based high-performance permanent magnets that can outperform Nd-Fe-B-based magnets. [ABSTRACT FROM AUTHOR]

We have studied the thermal stability of coercivity in anisotropic Nd-Fe-B magnet from 300 K to 500 K by combining the micromagnetic simulation with the numerical estimation of coercivity reduction caused by thermal activation. The numerically achieved coercivities agree well with the measured value of a conventional sintered magnet. The concave shape of H c (T) which is different from linear shape of H A (T) in anisotropic Nd-Fe-B magnets, as well as the break of linearity in the fitting of H c / M s vs. H A / M s are found both originated from the temperature dependence of magnetization of the ferromagnetic grain boundary phase. This curving of H c (T) can be eliminated via transformation of grain boundary magnetism into non-ferromagnetic. We have demonstrated this hypothesis experimentally in the hot-deformed magnet heavily infiltrated with Nd–Cu alloy. The parameters, α and N eff , achieved in the fitting of Kronmüller equation are found to be largely deviated from those achieved in micromagnetic method, which is due to the ignorance of temperature dependent grain boundary magnetization and the thermal activation effect in the fitting. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We studied the angular dependence and thermal stability of coercivity of Nd-rich Ga-doped Nd–Fe–B sintered magnets. Experimentally measured angular dependence of coercivity for the as-sintered magnet agrees well with the micromagnetic simulation results for exchange-coupled polycrystalline model that assumes the magnetization reversal is dominated by the domain wall depinning at the grain boundaries. On the other hand, the measured angular dependence of coercivity of the post-sinter annealed sample with an enhanced coercivity deviates from fully exchange-decoupled model in spite of previous experimental results on inter-grain exchange decoupling. Based on micromagnetic simulations and Kronmüller equation, it is suggested that the entire elimination of magnetic exchange coupling between Nd 2 Fe 14 B grains can further improve the coercivity and its thermal stability of the 5 µm-grain-sized Dy-free magnet. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Bulk sintered SmFe 11 Ti-based magnets with high energy density have not been realized yet despite three decades of research efforts. In order to move a step forward in this direction, in this work we demonstrate a process for producing fine, anisotropic, single-crystalline SmFe 11 Ti-based particles with high roundness by using jet-milling. Particle size of 2.7 ± 0.6 μm is realized in the case of Sm(Fe 0.8 Co 0.2) 10.5 Cu 0.5 Ti composition. XRD results show that the ThMn 12 phase is preserved during milling, whereas peaks from the intergranular phases vanish indicating partial removal and amorphization as confirmed by TEM studies. Increased roundness can be achieved by optimizing feeding rate and grinding gas pressure, which is beneficial for local demagnetization field reduction according to micromagnetic simulations. Grain boundary infiltration experiments using low-melting alloys enable liquid-phase sintering at a temperature as low as 500 °C. The resultant bulk samples are anisotropic and show full density, although coercivity remains only 0.32 T. The possible reasons are linked to defects in the microstructure, where formation of slip bands lead to Sm vacancies and can lower the local magnetocrystalline anisotropy and the coercivity. It was found that the volume fraction of the defects can be reduced by low temperature annealing of the jet-milled powders. Image 1 • Producing fine and spherical shaped single-crystalline SmFe 11 Ti-based powders with ThMn 12 -structure using jet-milling. • Formation of slip bands with Sm vacancies in the powders after jet-milling. • Reduction of the volume fraction of the defects by low temperature annealing of jet-milled powders. [ABSTRACT FROM AUTHOR]

We achieved a record-high coercivity of 1.40 T in a SmFe 12 -based ThMn 12 -type anisotropic sintered magnet. This magnet was prepared by sintering jet-milled powder made from strip-cast alloys with a nominal composition of Sm 8 Fe 73.5 Ti 8 V 8 Al 2 Cu 0.5 , using the conventional process for anisotropic sintered magnet production. Detailed microstructural investigations conducted using scanning electron microscopy and transmission electron microscopy revealed that the formation of Fe-lean intergranular phases (with < 20 at.% of Fe) surrounding the fine SmFe 12 phase grains resulted in the high coercivity. Nanobeam electron diffraction patterns showed that these Fe-lean intergranular phases (IGPs) exhibiting excellent wettability in this magnet are mainly composed of two phases: a hexagonal close-packed Mg-type Sm-based phase and a primitive cubic CsCl-type SmCu-based phase, which co-exist in the same intergranular region. Such Fe-lean IGPs minimize the intergranular exchange coupling in the SmFe 12 main phase, which contributes to the high coercivity, as revealed by magneto-optical Kerr effect microscopy. Overall, this study provides detailed microstructure-coercivity correlations and insights for the future development of high-performance SmFe 12 -based sintered magnets. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, we developed a series of anisotropic hot-deformed Nd-Fe-B magnets with ultrafine grain sizes varying from 125 nm to 234 nm in the lateral direction. Although grain refinement is known to increase the coercivity of magnets, all samples exhibited identical record high coercivity of 2 T. Microstructural analysis showed similar intergranular phase (IGP) composition and crystallographic texture, but an increased number of coarse and isotropic equiaxed grains in the flake boundary regions of the magnets with larger grain size. Although this could explain significant differences in the initial magnetization curves and magnetic domain patterns, the experimental data alone were not enough to identify the main factors limiting the coercivity. To elucidate these, a micromagnetic simulation was performed using a realistic 1:1 scale model. We showed that grain refinement below 200 nm has a limited potential for coercivity enhancement of hot-deformed Nd-Fe-B magnets unless the exchange interaction is weakened by reducing the magnetization of the IGP. Negative contributions to the coercivity from texture deterioration, coarse grains, and magnetostatic interaction were quantified for different values of IGP magnetization. A clear direction on how to further improve the coercivity of hot-deformed magnets was provided. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The magnetic properties and microstructures of two Nd–Fe–B thick films with different Nd contents have been studied. The films were deposited in the amorphous state and were crystallized by post-deposition annealing. Both films show a strong 〈001〉 fibre texture out-of-plane. The film with the higher Nd content has a large room temperature coercivity of 2.7T, while the one with the lower Nd content has a room temperature coercivity of only 0.7T. The difference in coercivity may be explained by the fact that the film with the higher Nd content exhibits a continuous Nd-rich grain boundary phase, giving better isolation of the Nd2Fe14B grains with respect to magnetic exchange interactions. The extrusion of Nd-rich liquid to the top surface of the film with high Nd content during post-deposition annealing led to the formation of ripples in the Ta capping layer, indicating that the films are under compressive stress. This stress-induced flow of the Nd-rich material up through the film explains the excellent distribution of the Nd-rich grain boundary phase. Atom probe tomography has revealed the presence of Cu in the Nd-rich grain boundary phase, explaining the formation of the liquid phase at the relatively low temperature of 550°C due to the eutectic reaction of Nd and Cu. [Copyright &y& Elsevier]

This work has demonstrated that a high coercivity of 2.5 T can be achieved in a 5.6-mm-thick Nd-Fe-B anisotropic magnet by applying the two-step diffusion process using Nd 50 Dy 30 Cu 20 and Nd 80 Cu 20 eutectic alloys to a Dy-free Nd-Fe-B hot-deformed magnet. This is in contrast to the fact that about 5 wt.% Dy was needed to reach this coercivity level in sintered magnets. The overall Dy concentration in the sample was determined to be only 0.45 wt.% Dy, which is about 10 % of that used in the conventional Dy-alloyed sintered magnets with a comparable coercivity. Due to the low Dy concentration, a relatively high remanence of 1.32 T is retained even after the two-step diffusion process with an excellent temperature coefficient of coercivity of –0.41 %/°C. [Display omitted] [ABSTRACT FROM AUTHOR]

We explored the possibility of Pr-Fe-B and (Pr 0.75 Ce 0.25)-Fe-B based hot-deformed magnets as magnetic field source for cryogenic magnetic refrigeration. Thanks to the elimination of spin reorientation effect in the rare earth (RE)-Fe-B system, Pr-Fe-B based hot-deformed magnet shows a large remanence of µ 0 M r = 1.75 T with a coercivity of 7.4 T at 20 K. To our surprise, (Pr 0.75 Ce 0.25)-Fe-B based hot-deformed magnet shows even higher remanence of 1.76 T with a coercivity of 4.8 T at 20 K. The measured maximum energy product of the anisotropic (Pr 0.75 Ce 0.25)-Fe-B hot-deformed magnets is 612 kJ/m3 at 20 K, demonstrating their potential as a low-cost permanent magnet for cryogenic applications. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We introduce a novel method to develop high coercivity with improved squareness in bulk hot-deformed Nd–Fe–B magnets with a thickness of 5.6 mm. A low-melting-point Tb 20 Dy 10 Nd 40 Cu 30 eutectic diffusion source was infiltrated into the thick magnets through grain boundaries as the 1st step, followed by the 2nd step eutectic diffusion using Nd 80 Cu 20. The 1st step led to the formation of rare earth (RE)-rich intergranular phase as well as heavy rare earth (HRE)-rich shell surrounding platelet-shaped Nd 2 Fe 14 B grains. A large concentration gradient toward the center of the thick magnet caused a large switching field distribution, which was the main limitation of this method after the first step and in the conventional infiltration process. The second step Nd-Cu diffusion makes the HRE-shell more uniform throughout the entire volume, which improved the squareness factor from 0.83 to 0.91 with a slight coercivity increase from 2.38 T to 2.43 T. Magneto-optical Kerr effect microscopy showed unlike the conventional infiltration processed magnet, strong pinning of the magnetic domain wall motion at the side grain boundaries occur even in the near center of the two step diffusion processed magnet. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We demonstrate a 20% Ce-substituted Nd-Fe-B hot-deformed magnet with a large coercivity (µ 0 H c) of 1.69 T and a remanent magnetization of 1.32 T, which are comparable to those for Nd-Fe-B sintered magnets. The temperature coefficient of coercivity (β) of the Ce-substituted magnet was -0.593%/°C in the range of 27–127 °C, which is superior to that of a Nd-Fe-B sintered magnet (β = -0.669%/°C) with a comparable coercivity at room temperature. Furthermore, the coercivity and its thermal stability can be increased to µ 0 H c =1.83 T and β = -0.51%/°C by applying the Nd 80 Cu 20 eutectic grain boundary diffusion process to an optimized (Nd 0.75 Ce 0.25)-Fe-B hot-deformed sample. A coercivity of 0.74 T was achieved at 150 °C in the Nd-Cu diffusion processed hot-deformed magnets, which is much higher than that of sintered magnets. Microstructure investigations and micromagnetic simulations indicate that the modification of grain boundary phase and formation of a Nd-rich shell near the grain boundaries of the developed (Nd,Ce)-Fe-B hot-deformed magnet is the cause of the improved coercivity at room temperature. Thus, in this work, we realize a microstructure-optimized low-cost (Nd 0.8 Ce 0.2)-Fe-B hot-deformed magnet that can outperform Nd-Fe-B sintered magnets. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The influence of the aspect ratio of grains on the thermal stability of coercivity of Nd-Fe-B hot-deformed magnets was systematically investigated by experimental and numerical approaches. With increasing amount of Nb doping from 0.2 at.% to 0.6 at.% in the hot-deformed magnets, the aspect ratio of grains, defined as the ratio of the width to the height of a Nd 2 Fe 14 B grain (D c / D ab), decreased while the average grain size was preserved. No change of temperature coefficient of coercivity (β) was observed in the as hot-deformed samples with different grain aspect ratios. However, the reduction of aspect ratio (increase of the height and reduction of the width of platelet shaped Nd 2 Fe 14 B grains) improves β for the sample infiltrated with a Nd-Cu eutectic alloy; i.e. , from β = −0.42 %/°C to −0.40 %/°C for the aspect ratio reduction from 4.7 to 3.1. Micromagnetic simulations indicated that the grain aspect ratio is not a dominant factor to the thermal stability of coercivity for exchange-coupled anisotropic polycrystals. While smaller aspect ratio reduces demagnetizing field caused by magnetically isolated grains, resulting in the improvement of the thermal stability of the coercivity for Nd-Cu infiltrated hot-deformed magnets. Image, graphical abstract [ABSTRACT FROM AUTHOR]

We investigated the influence of Ti addition on the coercivity and microstructure of Ga-doped R-Fe-B (R = Nd + Pr) based sintered magnets. Detailed microstructure observations of the 0.5 at.%Ti-doped sample with the highest coercivity of 2.38 T revealed a near-complete grain isolation through the formation of thick (0.5–1.0 μm) grain boundary (GB) phases such as the R 6 (Fe,Ga) 14 and the double hexagonal close-packed (DHCP) Nd. This suggests that these GB phases are non-ferromagnetic and decouple intergrain exchange. The formation of the R 6 (Fe,Ga) 14 phase is found to be triggered by the formation of hexagonal TiB 2 at high temperatures, which leads to a B-lean composition of GB phases that fall into a narrow compositional region, in which the R 6 (Fe,Ga) 14 forms during post-sinter annealing at intermediate temperatures. Despite the high coercivity, the interface between the R 6 (Fe,Ga) 14 and R 2 Fe 14 B phases in the post-sinter annealed 0.5 at.%Ti sample exhibits zig-zag morphology which is expected to deteriorate the coercivity with local demagnetizing field. Thus, further efforts to improve interfacial roughness should lead to further improve coercivity. Image 1 • Origins of high coercivity in a Ti-containing Nd-Fe-B sintered magnet are clarified. • Near-complete grain isolation by thick GB phases enhances coercivity. • The role of R 6 (Fe,Ga) 14 phase formed after annealing on the coercivity is studied. • Formation of TiB 2 phase provides conditions for formation of R 6 (Fe,Ga) 14 phase. [ABSTRACT FROM AUTHOR]

We investigated the microstructure of the Dy-free high coercivity sintered magnet that was developed by optimizing the chemical composition of Nd-Fe-B sintered magnet with 0.1 at.%Ga doping in order to understand the reasons for the high coercivity of μ 0 H c  = 2.0 T and the improved squareness of 0.95 compared to a recently developed 0.5 at. %Ga doped magnet. The as-sintered 0.1Ga sample shows a relatively high coercivity of 1.5 T owing to the finer grain size of ∼3.3 μm and the higher Al concentration up to 1.3 at.% compared to post-sinter annealed 0.5Ga magnet that has a grain size of ∼5.5 μm and Al concentration of 0.9 at. %. The post-sinter annealing leads to a substantial coercivity increase from 1.5 to 2.0 T due to the formation of a thick and crystalline intergranular grain boundary (GB) phase containing 30-60 at.% of Nd. The trace Ga addition of 0.1 at.% improved the wettability of the liquids and facilitated the formation of thick GB phase during the post-sinter annealing. The good squareness is mainly attributed to the ferromagnetic nature of the intergranular GB phase as well as the strong crystal alignment to the easy axis. Finite element micromagnetic simulations of the demagnetization processes of the models incorporating experimentally determined GB thickness and chemistry well explain the simultaneous achievement of the high coercivity and good squareness observed in this magnet in contrast to the very poor squareness observed in the post-sinter annealed 0.5Ga sample. [ABSTRACT FROM AUTHOR]

Anisotropic SmFe 12 -based sintered magnets require an optimum microstructure and alloy composition to have a large coercivity μ 0 H c and remanent magnetization μ 0 M r. In this study, we investigated the microstructure transformation and extrinsic performance change in the anisotropic Sm(Fe,Ti,V) 12 -based sintered magnets via reducing the Ti content and partially substituting Fe with Co. Reduction of Ti content in anisotropic sintered Sm 8 Fe 73.5+ x Ti 8- x V 8 Ga 0.5 Al 2 magnet eliminated the secondary Fe 2 (Ti,V) phase, resulting in a remanence of μ 0 M r = 0.8 T and energy density of (BH) max = 113 kJ/m3 (14.2 MGOe) in x = 3. A relatively high coercivity μ 0 H c of 0.6 T is maintained owing to the formation of thin intergranular phase (IGP). Substitution of Co for Fe in the Sm 8 (Fe 0.95 Co 0.05) 73.5 Ti 8 V 8 Ga 0.5 Al 2 sintered magnet resulted in a low remanence and low coercivity μ 0 H c. The electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) observations indicate the formation of twins inside 1:12 grains which deteriorate the texture and remanent magnetization. Magneto-optical Kerr effect (MOKE) microscopy and micromagnetic simulations revealed that although the magnetization reversal starts at the twinned grains, Sm-rich intergranular phase hinders propagation of reversed domains to the neighboring grains, preventing the deterioration of coercivity. However, the formation of ferromagnetic SmFe 2 -based phase of the MgCu 2 type in Co-containing magnet is responsible for a further decrease of coercivity to μ 0 H c = 0.55 T. This study provides an insight on the optimum microstructure to realize high-performance anisotropic bulk SmFe 12 -based sintered magnets, i.e., ones without twins and ferromagnetic SmFe 2 -based phase of the MgCu 2 type, but with refined grain size, and thin non/weak ferromagnetic IGP isolating the SmFe 12 grains. For SmFe 12 -type phase, reducing the nonmagnetic elements and increasing its saturation magnetization remain crucial to realize the potential to compete with the Nd 2 Fe 14 B phase. [Display omitted] [ABSTRACT FROM AUTHOR]