13 Search Results

Partial replacement of Fe by Co is an effective method to increase Curie temperature (T_C), which improves the thermal stability of magnetic properties in Nd₂Fe₁₄B-based permanent magnets. The correlation between Fe substitution and magnetic properties has been studied in Nd₂(Fe,Co)₁₄B via a first-principles calculation. The calculated Fe substitution energies indicate that the Co atoms avoid the 8j₂ site, which agrees with the experiments. The Co atoms are ferromagnetically coupled with Fe sublattice and show magnetic moments of about 1.2 to 1.7 μ_B at different crystallographic sites, less than that of Fe (2.1–2.7 μ_B), resulting in the decrease in total magnetizationmore » at ground state (0 K) with increasing Co content. The effective exchange interaction parameter, derived from the energy difference between varied magnetic structures, increases from 7.8 meV to 17.0 meV with increasing Co content from x = 0 to x = 14 in Nd₂Fe_14–xCo_xB. This change in the effective exchange interaction parameter is responsible for the enhancement of T_C in Nd₂(Fe,Co)₁₄B. The total magnetization at 300 K, derived from mean-field theory, shows a peak maximum value at x = 1 in Nd₂Fe_14–xCo_xB. The phenomenon results from the interplay between the reduction of the magnetic moment in the Fe(Co) sublattice and the enhancement of T_C with increasing Co content.« less

We investigate the effect of an applied magnetic field on the entire HDDR process using a customized reactor vessel and a warm-bore superconducting magnet. We analyzed the resulting properties produced at both a 0 applied field and a 2 Tesla applied field. We show that the application of a magnetic field throughout the HDDR process results in powders that exhibit a greater level of anisotropy compared to their ambient field counterparts.

Doping a small amount of Al can effectively enhance coercivity in Nd-Fe-B magnets. Here we investigated the partitioning behavior of Al and its effect on coercivity in Nd-Fe-B using first principles DFT (density functional theory) calculation and micromagnetic simulation. The calculated substitution energies of Fe by Al are negative at the crystallographic sites of 4c and 8j₂ while they are positive values at the other sites in Nd₂Fe₁₄B (2:14:1), implying a small solubility of Al in 2:14:1. Further, Al prefers to segregate at grain boundary (GB) and stabilize the Nd-Fe-Al phase with a Nd₆Fe₁₃Si-type tetragonal structure (6:13:1). The formation ofmore » the antiferromagnetic or weak ferrimagnetic 6:13:1-like phase depletes Fe and reduces the amount of ferromagnetic Nd–Fe type grain boundary phase (GBP), which weakens the inter-grain magnetic interaction. Micromagnetic simulations indicate that the 6:13:1-like GBP increases the pinning field of magnetic domain wall at GB and suppresses the nucleation of reversal magnetic domain on the grain surface of 2:14:1 during demagnetization process. The formation of Al-rich shell on 2:14:1 grain surface can further moderately increase the domain pinning field at GB and the nucleation field of 2:14:1 grain. Developing novel processing method to tailor Al segregation and promote formation of 6:13:1-like phase at GB can be a promising approach to improve coercivity in Nd-Fe-B magnet.« less

In this study, we report the effect of magnetic field annealing (MA) in relieving the stress induced during the milling of Nd-Fe-B sintered magnets. The effect of MA processing parameters and particle size on the magnetic properties were investigated. Based on the results, magnetic field annealing is more effective in improving the magnetic properties of larger size particles, compared to finer particles. Our computational study on the role of particle surface defects in bi-modal particle distribution agrees with results obtained experimentally. In conclusion, magnetic field annealed powder has been successfully used to develop 4.6 g.cm^-3 density bonded magnets with 75more » vol% of magnetic powder and polyamide 12 (nylon 12) polymer binder resulting in 11.3 MGOe energy product.« less

Critical rare-earth free La ₂ Fe ₁₄ B (2:14:1) has the potential to be a gap permanent magnet. However, La ₂ Fe ₁₄ B decomposes into La, α-Fe, and LaFe ₄ B ₄ phases below 1067 K. The phase stability and coercivity have been studied in La ₂ Fe ₁₄ B magnet using first principles DFT (density functional theory) calculation and micromagnetic simulation. For a perfect La ₂ Fe ₁₄ B cube (edge length of 256 nm) without any structural defects and soft magnetic secondary phases, the coercivity (8.5 kOe) is reduced to ∼40% of its magnetocrystalline anisotropy field ( H _Amore » = 20 kOe). Further, the coercivity sharply reduces to 3.2 kOe upon forming a thin layer (2 nm) of α-Fe on the surface of the La ₂ Fe ₁₄ B cube particle. The DFT calculations indicate that a partial replacement of La by other rare-earth (RE) elements can enhance the structural stability of 2:14:1. The gains in cohesive energy are 0.75, 0.10, and 0.33 eV per formula unit in (La _0.5 RE _0.5 ) ₂ Fe ₁₄ B with RE = Ce, Pr, and Nd, respectively. Stabilizing the 2:14:1 structure and mitigating the formation of soft magnetic structural defects or impurity phases such as α-Fe is necessary to develop La ₂ Fe ₁₄ B based magnet, which can be moderately achieved via partial substitution of La by other rare earth elements such as Ce, Pr, and Nd.« less

ighly dense and magnetically anisotropic rare earth bonded magnets have been fabricated via packing bimodal magnetic particles using a batch extrusion process followed by compression molding technology. The bimodal feedstock was a 96 wt% magnet powder mixture, with 40% being anisotropic Sm-Fe-N (3 μm) and 60% being anisotropic Nd-Fe-B (100 μm) as fine and coarse particles, respectively; these were blended with a 4 wt% polyphenylene sulfide (PPS) polymer binder to fabricate the bonded magnets. The hybrid bonded magnet with an 81 vol% magnet loading yielded a density of 6.15 g cm^-3 and a maximum energy product (BH)m of 20.0 MGOemore » at 300 K. Scanning electron microscopy (SEM) indicated that the fine-sized Sm-Fe-N particles filled the gap between the large Nd-Fe-B particles. Rietveld analysis of the X-ray diffraction data showed that the relative contents of the Nd₂Fe₁₄B and Sm₂Fe₁₇N₃ phases were 61% and 39%, respectively, in the hybrid bonded magnet. The PPS binder coated most of the magnetic particles homogeneously. Compared with the magnetic properties of the initial Nd-Fe-B and Sm-Fe-N powders, the reduction in the remanence, from the demagnetization curve, is ascribed to the dilution effect of the binder, the non-perfect alignment, and the internal magnetic stray field.« less

MnBi is a candidate material for high-temperature magnets because of its increasing coercivity with increasing temperatures up to 255 °C. However, most efforts in fabricating bulk MnBi magnets have run into the problem of preserving the coercivity (H_cj) of its feedstock powders. About 70% of powder’s H_cj would be lost during the densification process. Our micromagnetic modeling shows that the coercivity mechanism of the MnBi bulk magnet is controlled by nucleation of the reversal magnetization domains, and the large H_cj loss that occurred during the powder consolidation process can be attributed to the inter-grain magnetic coupling. To attain a highmore » H_cj, the grains in the MnBi bulk magnet must be separated with a non-magnetic grain boundary phase (GBP). To validate this GBP hypothesis, we engineered MnBi bulk magnets with two different types of GBP. The first type of GBP was created in-situ by precipitating excessive Bi from the grains; the second type was created ex-situ by coating silicates on the feedstock powders before the consolidation. While both GBP work, the ex-situ approach resulted in a better H_cj due to a more uniform GBP distribution. We report the H_cj loss was reduced from 70% to 15%, and the (BH)max of a warm sintered bulk magnet reached 8.9 MGOe.« less

Relatively resource-rich but property inferior-Ce-Fe-B magnet can be improved by partial replacement of Ce by Nd and/or Pr. In addition to the amount of Nd/Pr, their distribution profile in microstructure plays an important role. From our first principles density functional theory (DFT) calculation, the substitution energy of Ce by Pr/Nd is negative in Ce₂Fe₁₄B (2:14:1) while that for laves phase, CeFe₂ is positive, implying that Nd/Pr stabilize 2:14:1 and suppress the formation of the CeFe₂ phase. Further, micromagnetic simulation indicates that homogenized distribution of Nd/Pr improves squareness of demagnetization curve, while core (Ce-rich)-shell (Nd/Pr-rich) 2:14:1 grain structure enhances coercivity. Magneticmore » properties of Ce-Fe-B can be optimized by manipulating distribution profile of chemical element in microstructure based on their subtle difference in thermodynamic property, which is an effective pathway to design optimized chemical composition and processing route for high-performance magnet.« less

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. Here, 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 (H_c) 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 highmore » coercivity H_c 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.« less