Coercivity Of Nd Research Articles

Macroscopic phenomenon of exchange coupling and microscopic effect on magnetization reversal in sintered Nd–Fe–B magnets

Permanent magnets of Nd13.5Fe79.18M1.52B5.8 and Nd13.5Fe79.76M0.94B5.8 were prepared, respectively, via strip casting, jet milling and sintering followed by annealing. By adding the non-ferromagnetic elements M (Al, Cu, Ga and Zr) into the magnets, it could not only modify the microstructure, but also regulate the exchange coupling effect in the sintered magnets. From the macroscopic point of view, the recoil loops exhibit spring behavior in Nd13.5Fe79.76M0.94B5.8, indicating that the energy barrier can be overcome by the intergranular exchange coupling. From the microcosmic point of view, the exchange coupling can increase the domain wall size by suppressing the nucleation of reversed domains, and so the activation volume increases with thermal activation. In Nd13.5Fe79.76M0.94B5.8 the exchange coupling effect is stronger, and both the coercivity of 15.0 kOe and the remanence of 14.3 kGs are a little higher than those of Nd13.5Fe79.18M1.52B5.8 magnets in which the content of non-ferromagnetic elements is a little higher and the exchange coupling effect is weaker. Thus, the exchange coupling does not decrease the coercivity due to the exchange coupling suppressing the nucleation of reversed domains, though the microstructure is inhomogeneous in the sintered magnets of Nd13.5Fe79.76M0.94B5.8. Reducing the defect size and decreasing the defect concentration should be a practical way to improve the coercivity in Nd–Fe–B permanent magnets.

Obtain high coercivity of Nd–Ce–Fe–B magnet via grain boundary diffusion with Dy80Al10Cu10 alloy

Dy80Al10Cu10 and DyH3 were employed as diffusion sources to perform grain boundary diffusion (GBD) on 7 wt% Ce containing Nd-Fe-B magnets. The coated samples underwent a heat treatment at 1193.2 K for 6 h, followed by an annealing process at 913.2 K for 5 h. As a result, the coercivity increased from the original 1268.0 kA/m to 1681.1 kA/m and 1618.2 kA/m in Dy80Al10Cu10 and DyH3 diffused magnets, respectively. Notably, Dy80Al10Cu10 exhibited a more pronounced diffusion effect than DyH3. After GBD, continuous intergranular phases were observed around the matrix grains, which weakening the magnetic coupling effect between the ferromagnetic matrix grains, which in turn improved coercivity. Microstructure analysis revealed the presence of CeFe2 phase, which impeded the effectiveness of GBD due to its high melting point. However, when Dy80Al10Cu10 was employed as the diffusion source, the detrimental impact of CeFe2 was better inhibited compared to DyH3. The inclusion of Cu and Al elements in the Dy80Al10Cu10 alloy optimized the wettability and facilitated the formation of an antiferromagnetic RE6Fe13Cu phase at the triple junctions (TJs), which leads to the depletion of Fe and enrichment of RE in adjacent GBs. Furthermore, Cu aided in dissolving high melting point CeFe2 and promoting the formation of GBs with improved wettability, thereby contributing to an increase of coercivity.

Magnetic properties and Tb diffusion of sintered Nd–Fe–B magnets by suspension plasma spraying

For the first time, a TbF3 coating as a diffusion source was prepared on the surface of sintered Nd–Fe–B magnets by suspension plasma spraying. A sintered Nd–Fe–B magnet with a high coercivity and low remanence loss was obtained. The uniform and dense coating favored grain-boundary diffusion of heavy rare-earth elements, which improved the coercivity of Nd–Fe–B magnets. The effect of grain-boundary diffusion and TbF3 coating amount on the microstructure and magnetic properties of the magnet were studied. Optimal magnetic properties were obtained for a heat treatment of 900 °C for 10 h, when the TbF3 coating amount was 0.2132 mg/mm2. The coercivity reached 23.72 kOe with an increase of 60.16% compared with the original magnet, whereas the remanence did not decrease significantly. Microstructure analysis showed that a Tb-layer core–shell structure formed around the Nd2Fe14B grains after diffusion. The high magnetocrystalline anisotropy of the Tb core–shell structure formation improved the Nd–Fe–B magnet coercivity significantly. Therefore, the method of plasma spray grain-boundary diffusion is efficient and feasible.

Grain-size effect on coercivity of Nd–Fe–B nanomagnets: micromagnetics simulation based on a multi-grain model

The coercivity (H c) of nanocrystalline Nd–Fe–B magnet was simulated by multi-grain models in combination with hybrid Monte Carlo method. The simulated grain size dependence of H c with grain boundary (GB) model reproduces the available experiment nicely. The H c of direct contact (DC) model increase with the decreasing grain size (S G). This fact suggests that the H c of Nd–Fe–B magnets can be further increased by grain refinement even though the average S G down to several tens of nanometers. The difference of coercivity mechanism between GB-model and DC-model was understood by magnetization reversal analysis.

Comparison of (Pr, Nd)H grain boundary restructuring and diffusion on the magnetic properties of Nd–La–Ce–Fe–B sintered magnet

Exploiting high-performance 2:14:1-type permanent magnet at high La–Ce substitution level remains a bottleneck concern due to the severe magnetic dilution from inferior intrinsic magnetism and microstructural deterioration. In present work, to promote the La–Ce application, (Pr, Nd)Hx is introduced into LaCe-40 sintered magnet (La–Ce/TRE = 40 wt%, TRE: total rare earth) through two approaches, grain boundary restructuring (GBR) and grain boundary diffusion process (GBDP). Results show that (Pr, Nd)Hx GBR leads to a twofold coercivity by optimizing the distribution of GB phase and forming a Pr/Nd-rich magnetically hardening shell. However, the remanence is lowered drastically due to massive formation of REFe2 phase during high-temperature sintering. Comparably, (Pr, Nd)Hx GBDP magnet with an identical average composition to GBR one yields a nearly threefold coercivity without sacrificing the remanence, ascribing to the formation of more continuous RE-rich GB layer and thicker Pr/Nd-rich shell near the surface region. Above comparative study highlights the crucial role of surface magnetic isolation in enhancing the coercivity of Nd–La–Ce–Fe–B sintered magnet, and delights the prospect of abundant La–Ce for industrial application.

Mechanism of the enhanced coercivity for the dual-main-phase Ce\u2013Fe\u2013B magnet

The high coercivity of Nd–Fe–B magnets can also be obtained in the Ce–Fe–B magnets fabricated via the dual-main-phase (DMP) method in which the high abundance Ce was used to substitute Nd(Pr). The inhomogeneous distributions of the matrix grains in the DMP magnet play a key role in the enhanced magnetic performance. Compared with the single-phase magnet, more grain boundary phases encapsulating the matrix 2:14:1 grain are formed in the DMP magnet, which reduce the exchange coupling between adjacent magnetic grains. The switching field distribution and the interaction field distribution of the Ce–Fe–B magnets were determined by the first-order-reversal curves (FORC). The switching field peaks around 6 kOe, 11 kOe and 12 kOe in the FORC distribution indicate that three major reversal components coexist for the DMP magnet. The overlapp of the second and third switching field peaks reveals the presence of a pinning interaction within individual magnetic grains with a core–shell structure, which further improve the coercivity of the magnet.

Enhancement in hard magnetic properties of (Nd, Pr)–Fe–B melt-spun ribbons

The coercivity of RE2Fe14B-type permanent magnets is strongly influenced by the microstructural features such as grain boundary (GB) phases as well as grain sizes. We have combined micromagnetic simulations and experiments to elucidate the role of excess RE (Nd/Pr) in determining the resulting hard magnetic properties of Nd–Pr–Fe–B melt-spun ribbons. The intrinsic coercivity (Hc) at room temperature significantly enhanced from 9.7 kOe to 15.3 kOe with the increase in the Nd/Pr content. Furthermore, the effect of non-magnetic grain refining refractory carbide (TiC) on both the microstructure and magnetic hardening was studied. The addition of TiC showed a very high coercivity Hc of up to 19.0 kOe at room temperature. Micromagnetic simulation indicates that the coercivity enhancement is mainly due to the reduction of inter-grain magnetic interaction, which is due to the RE-rich nonmagnetic grain boundary (GB) phase and/or TiC distributed at the GB. This work provides useful information on the roles of non-magnetic grain boundary phases for improving the coercivity of Nd–Pr–Fe–B magnets. Combined with experimental and modeling results, we have discussed the mechanism responsible for the enhancements in coercivity and the suitability of the alloys for high-performance permanent magnet development.

Further enhancing the coercivity of Nd-Tb-Fe-B sintered magnets by Dy grain boundary diffusion

The coercivity of Nd–Fe–B sintered magnets was progressive enhanced by Tb alloying and Dy grain boundary diffusion (GBD). Every 1 wt% Tb doping led to about 0.37 T linear increase in the coercivity. For all magnets with different Tb content after Dy GBD, the coercivity increased by about 0.7 T with the formation of network enriched with Dy. As Dy diffusion flux was determined by Dy concentration gradient, the coercivity enhancement of Dy GBD was not affected by the Tb content of base magnets. Dy enriched at the TJs diffused along the grain boundary with the Nd-rich phase extending along the grain boundary during post diffusion annealing process. Similar with post sintering annealing, the coercivity increased by about 0.3 T after post diffusion annealing. Therefore, alloying and diffusing different heavy rare earth elements was an effective way to prepare Nd-Fe-B sintered magnets with higher magnetic properties.

Effect of annealing temperature on coercivity of Nd–Fe–B magnets with TbFeAl doping by process of hot-pressing**Project supported by the Major Project of “Science and Technology Innovation 2025” in Ningbo City, China (Grant Nos. 2018B10086 and 2018B10017).

The Nd–Fe–B magnets are pre-sintered and then processed with hot-pressing, and the resulting magnets are called the hot-pressed pretreated (HPP) magnets. The coercivity of the HPP magnets increases as the annealed temperature increases. When the annealing temperature is 900 °C, the coercivity of the magnet is only 17.6 kOe (1 Oe = 79.5775 A⋅m–1), but when the annealing temperature rises up to 1060 °C, the coercivity of the magnet reaches 23.53 kOe, which is remarkably increased by 33.7%. The microstructure analysis indicates that the grain surface of the HPP magnet becomes smoother as the annealed temperature increases. The microstructure factor α is changed according to the intrinsic coercivity model formula. The α of the magnet at 900 °C is only 0.578, but it is 0.825 at 1060 °C. Microstructural optimization is due mainly to the increase of coercivity of the HPP magnet.

On the magnetism of grain boundary phase and its contribution to the abnormal openness of recoil loops in hot-deformed magnets

The magnetism of grain boundary (GB) phase plays a critical role in the coercivity of Nd–Fe–B magnets. In this work, the aggregation of the GB phase was formed in the hot-deformed magnet due to the low-density high-velocity compaction (HVC) precursor. The ferromagnetism of GB aggregation was confirmed by the analysis of 57Fe Mössbauer spectrometry, and its correlation to the magnetic performance was investigated by micromagnetic simulation. Results show that the soft magnetic GB aggregation is responsible for the low coercivity of the hot-deformed magnet. Moreover, an abnormal large openness of the recoil loop under the reversal magnetic field HR was presented in this work, and the underlying mechanism is interpreted by introducing demagnetization energy. To compensate for the problem of low density, low-melting-point Nd70Cu30 powders were added into the HVC magnet, which makes the GB aggregation paramagnetic and enhances the coercivity.

Application of a Dy3Co0.6Cu0.4Hx Addition for Controlling the Microstructure and Magnetic Properties of Sintered Nd-Fe-B Magnets.

The focus of new technologies on the formation of inhomogeneous distributions of heavy rare-earth metals (REMs) in hard magnetic Nd–Fe–B materials is of scientific importance to increase their functional properties, along with preserving existing sources of heavy REMs. This paper focused on the coercivity enhancement of Nd2Fe14B-based magnets by optimizing the microstructure, which includes the processes of grain boundary structuring via the application of a Dy3Co0.6Cu0.4Hx alloy added to the initial Nd–Fe–B-based powder mixtures in the course of their mechanical activation. We have studied the role of alloying elements in the formation of phase composition, microstructure, the fine structure of grains, and the hysteretic properties of hard magnetic Nd(R)2Fe14B-based materials. It was shown that the Dy introduction via the two-component blending process (the hydrogenated Dy3Co0.6Cu0.4 compound is added to a powder mixture) resulted in the formation of the core-shell structure of 2–14–1 phase grains. The efficient improvement of the coercivity of Nd(RE)–Fe–B magnets, with a slight sacrifice of remanence, was demonstrated.

Intrinsic Coercivity of Very Hard Nd–Fe–B Magnets Enlarged by Electromagnet Measurements

The intrinsic coercivity of Nd–Fe–B permanent magnets measured by the electromagnet method is substantially enlarged if it is close to or greater than the saturation magnetization of the electromagnet pole caps. This enlargement is caused by a dominant demagnetizing field against overall magnetic reversal.

First-principles study on substitution effects in Nd2(Fe, X)14B

We study the magnetic properties of the substitution effects in Nd2(Fe, X)14B (X = Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge) by first-principles calculations in order to understand which added elements have the ability to improve the coercivity of Nd–Fe–B magnets. By analyzing the formation energies of these systems and the magnetic anisotropy K1 of Nd atoms, we find that adding Mg, Ca, Sc, Cu, Zn, and Ga into Nd–Fe–B magnets is a useful approach to improve K1, a factor which is strongly related to the coercivity of Nd–Fe–B magnets. This anisotropy improvement is due to the fact that the substitution weakens the orbital coupling between 3d electrons and Nd 5d electrons, allowing the 5d electrons to extend in a uniaxial direction.

Effect of Heat Treatment Time on Dy–Cu Alloy Diffusion Process in Dy-Containing Commercial Nd–Fe–B Sintered Magnets

In this paper, the grain boundary diffusion process (GBDP) using a Dy70Cu30 (at.%) alloy as the diffusion source was performed in a commercial sintered Nd–Fe–B magnet, and the effect of heat treatment time on the microstructure and magnetic properties of the magnet was investigated in detail. For the processed magnets heat-treated at 860 °C, as heat treatment time increased, the coercivity and the depth of (Nd,Dy)2Fe14B core–shell structure increased first and then decreased. However, when the heat treatment time was more than 2 h, the diffusion path of Dy from the Dy-rich shell phase into the Nd2Fe14B grains was revealed, and a nearly homogeneous (Nd,Dy)2Fe14B phase was formed, which brought on the decrease in both the depth of visible core–shell structure and the coercivity of Nd–Fe–B magnet.

Magnetic properties and magnetization reversal mechanism of Nd-Fe-B nanoparticles synthesized by a sol-gel method

Nd–Fe–B oxide powders with various pH were prepared using chloride and nitrate precursors including NdCl3·6H2O, FeCl3·6H2O, H3BO3, Nd2O3, Fe(NO3)3·9H2O, HNO3, citric acid (CA), ethylene glycol (EG) by Pechini type sol–gel method. The pH of chloride and nitrate base sols were 0 and 2.2, respectively. Mixed oxide powders were obtained by calcination and annealing the gels. These oxides by using a reduction-diffusion process under high vacuum and employing CaH2 as reducing agent at 800°C were hated to prepare Nd2Fe14B nanoparticles. The role of pH on phase, morphologies, microstructure, and magnetic properties of the powders were investigated. The results show that with a decrease in pH, the average particle size and coercivity of Nd–Fe–B oxide powders were decreased and increased, respectively. Nd2Fe14B nanoparticles were formed successfully after reduction process. The average particle size of reduction treated products were 30 and 65nm for powders which made of chloride and nitrate base metal salts, respectively. Final powders which made of chloride and nitrate base metal salts had a saturation magnetization of 127.7emu/g and 122.8emu/g while the coercivity of samples were 3.32kOe and 1.82kOe, respectively. The experimental results in the angular dependence of coercivity indicated that the normalized coercivity of the permanent magnets Hc(θ)/Hc(0) obeys the 1/cosθ law and intermediate between the 1/cosθ law and Stoner-Wohlfarth formula for different Nd2Fe14B magnets which made of nitrate and chloride base metal salts, respectively. Also, the results show that different Nd2Fe14B magnets which made of nitrate and chloride base metal salts had the maximum energy product of 5 and 16 MGOe, respectively. The Henkel plot showed that magnetic phases in synthesized NdFeB magnets which made of chloride and nitrate base metal salts were coupled by exchange and dipolar interactions, respectively. Different average particle size, morphology and microstructure were the reasons for variation of magnetic properties.

Coercivity enhancement mechanism in Dy-substituted Nd–Fe–B nanoparticles synthesized by sol–gel base method followed by a reduction-diffusion process

In current work, Nd15−xDyxFe77.5B7.5 (at%) nanoparticles with different Dy-content (x=0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) were synthesized by sol–gel method followed by a reduction-diffusion process. The effects of Dy on the magnetic properties and the relations between the microstructure and the coercivity of Dy-substituted Nd–Fe–B nanoparticles have been studied. The coercivity of Nd–Fe–B nanoparticles with the addition of Dy first increase, reaches a maximum, and then starts to decrease. The coercivity of Dy-substituted Nd–Fe–B nanoparticle synthesized by sol–gel method increased from 938.9 to 1663.9kA/m while the remanence decreased slightly from 1.16 to 1.06T. The results show that with an increase in Dy content the variation of maximum energy product ((BH)max), lowest-order uniaxial magnetocrystalline anisotropy constant (Ku1), and Curie temperature (Tc) had a trend as same as the coercivity. The Henkel plot showed that the existence of exchange coupling interaction between grains, and the exchange coupling interactions increased with increasing x from 0.0 to 2.0 and then decrease with further increasing x≥2.5. The optimum magnetic properties of Nd–Fe–B nanoparticles with (BH)max =40.38 MGOe, Hc=1663.9kA/m, Br=1.08T were obtained by substituted 2.0at% Dy. The effects of increasing temperature on magnetic properties of Dy-substituted Nd–Fe–B nanoparticle magnets with 2.0at% Dy was investigated. The reduced spin-reorientation temperature was obtained for Dy-substituted Nd–Fe–B nanoparticles with 2.0at% Dy. Below 100K a spin-reorientation transition was takes place. The temperature coefficient of coercivity (β) was −0.36, −0.46, −0.41, −0.34, −0.29, −0.24, −0.25%/°C at different temperature 50, 100, 150, 200, 250, 300, 350°C, respectively. Mössbauer spectroscopy was applied to study the composition and properties of Dy-substituted Nd–Fe–B magnet. Microstructure analysis showed a homogeneous distribution of Dy in produced samples. The possible reason for observed magnetic behavior is improving the intrinsic material parameter and optimizing the microstructure by a uniform enhancement of magnetocrystalline anisotropy by formation the nanocrystalline compound (Nd,Dy)2Fe14B.

Controlling of Saturation of Magnetization of Nd–Fe–B Nanoparticles Fabricated by Chemical Method

Nd–Fe–B oxide powders with various size distributions and structures were prepared using nitrite-based metal salts including Nd2O3, Fe(NO3)3⋅9H2O, HNO3, H3BO3, citric acid (CA), and ethylene glycol (EG) by a Pechini-type sol–gel method. Mixed oxide powders were obtained by calcination and annealing the gels. These oxides by using a reduction–diffusion process under vacuum and employing CaH2 as a reducing agent at 800 ∘C were heated to prepare Nd2Fe14B nanoparticles. The role of different EG/metal salt molar ratios on phase, morphologies, microstructure, and magnetic properties of the powders were investigated by employing XRD, FE-SEM, TEM, and VSM techniques, respectively. The results show that with an increase in EG/metal salt molar ratio, the average particle size of Nd–Fe–B oxide powders increased while the coercivity of Nd–Fe–B oxide powders decreased. The presence of excess EG/CA/metal salt molar ratio, from 2:2:1 to 50:2:1, played a role of a space-filling template or capping agent and helped initial oxide particles embedded in the gel matrix to grow as sheet-like particles. Nd2Fe14B nanoparticles can successfully be synthesized by increasing the EG/CA/metal salt molar ratio from 2:2:1 to 50:2:1. The reduced samples made of nitrite-based metal salts with a EG/CA/metal salt molar ratio of 50:2:1 had a saturation magnetization of 123 emu/g and a coercivity of 315 Oe. By controlling the ratio of precursors, the hard-phase Nd2Fe14B could be successfully synthesized.

A Phenomenological Analysis on the Anisotropic Shrinkage of Nd-Fe-B Compacts during Sintering

Magnetic compaction is a well-known and effective method to improve the coercivity of Nd–Fe–B sintered magnets. During magnetic compaction, the application of an external magnetic field causes the anisotropic alignment of Nd2Fe14B grains as a ferromagnetic phase. After the magnetic compaction, the sintering process is carried out to densify the Nd–Fe–B compacts. In this paper, to identify the relationship between the anisotropic alignment of Nd2Fe14B grains and the densification behavior during the sintering process, we phenomenologically analyzed the shrinkage of magnetically compacted Nd–Fe–B sintered magnets according to the directions of magnetic field that were perpendicular direction and parallel direction to the magnetic aligned direction. In addition, the shrinkage and the grain size were measured at various sintering temperatures. It was concluded that the difference in the shrinkage and grain growth rates between the directions that are the perpendicular and the parallel could be evaluated based on their anisotropic characteristics and the crystal structure of the Nd2Fe14B phase. Shrinkage analysis indicated that the perpendicular direction to the magnetic aligned direction yields a larger shrinkage compared with the parallel direction to the magnetic aligned direction. In addition, grain size analysis revealed that the grain size was $\sim 10$ % larger in the perpendicular plane as compared with the grain size in the parallel plane.

Preparation and Magnetic Properties of Nd–Co-Substituted M-Type Strontium Ferrite by Microwave-Assisted Synthesis Method

M-strontium hexaferrites (Sr1−xNdxFeyCo1−y O19) with the substitution of Co2+ and Nd3+ were prepared by the microwave-assisted solid-phase reaction process. M-strontium hexaferrite with magnetoplumbite structure can be formed at 950 °C for 90 min by applying microwave-assisted calcination. For Co2+-substituted M-strontium ferrites, the saturation magnetization (σs) and remanent magnetization (σr) tend to decrease with the increasing calcination temperature, accompanying by the improvement of coercivity for the enhancement of morphology anisotropy. For Nd–Co-co-substituted strontium ferrites, the appearance of α-Fe2O3 phase and the conversion of Fe3+(5 μB) into Fe2+ (4 μB) decrease the net magnetic moment, resulting in the dilution of the σs and σr. The coercivity of Nd–Co-co-substituted strontium ferrite increases slightly for the improvement of magnetocrystalline anisotropy, which could be attributed to the enhancement of anisotropy field for the substitution of Fe3+ ions by Co2+ ions and the formation of grains with plate-like shape.

Role of hydrogen in Nd–Fe–B sintered magnets with DyHx addition

In order to improve the coercivity of Nd–Fe–B sintered magnets, DyH2 and DyH3 fine powders were prepared and used as additive for preparing Nd–Fe–B sintered magnets. The effects of DyHx powders addition on the microstructures and the magnetic properties of the magnets have been investigated. It was found that hydrogen will react with oxygen of NdOx rich intergranular phases to form Nd rich phases by dysprosium hydride addition. The Nd-rich grain boundary phases are more homogenous and continuous because the volume fraction of Nd-rich grain boundary phases increases with respect to the Nd oxide phases. After desorption, fine dysprosium powders become more active and wrap matrix phases well so that the diffusion of dysprosium to the surface layer of matrix phases is convenient, so dysprosium decreases in grain boundary phases and aggregates in surface layer of matrix phases. Then, intrinsic coercivity of NdFeB sintered magnets is improved from 14.96kOe to 20.5kOe and 20.31kOe by 2.0wt.% DyH3 and 2.0wt.% DyH2 addition, respectively. This study has shown that DyHx addition can reduce the content of oxygen in grain boundary phases. This can be an effective method for massive production.