The formation of continuous thin layers of a Nd-rich amorphous phase surrounding Nd2Fe14B grains is the key microstructural feature of high-coercivity Nd–Fe–B magnets. Contrary to popular belief, a ...quantitative 3D atom probe investigation suggests that the thin grain boundary phase is ferromagnetic in both sintered and hydrogen disproportionation desorption recombination magnets. Model experiments using Nd–Fe–B thin films indicate that the intrinsic coercivity of μ0Hc=3T with improved temperature dependence is achievable by magnetically isolating Nd2Fe14B grains of ∼100nm.
Many new applications of Nd–Fe–B magnets have emerged in the past two decades. Recently, motor application has taken over VCM position which was the dominant application in Nd–Fe–B sintered magnet ...previously. Motor market is expected to expand in future because of strong energy saving requirements from the automobile and electric appliances markets.
Magnetic property improvement in residual magnetization (Br) and coercive force (
H
cJ) helped the rapid growth of the motor market. Magnetic properties are still rapidly improving and many technologies for powder alignment are also being invented. A new world record of magnetic properties with
B
r=1.555
T, (BH)
max=474
kJ/m
3
H
cJ=653
kA/m was established by NEOMAX Co. Ltd.
The dependence of the coercivity of hot-deformed anisotropic Nd–Fe–B magnets on grain size has been studied by processing the magnets at different temperatures. Higher coercivity was obtained in ...fine-grained magnets processed at lower temperature (700°C), in which intergranular phases formed uniformly along the grain boundaries. On the other hand, large Nd-rich triple-junction phases were frequently observed in the larger grain-sized magnets processed at higher temperature (900°C). Three-dimensional atom probe analyses showed that the Nd content in the intergranular phase decreased as the processing temperature increased. The origin of the coercivity difference in these hot-deformed magnets processed at different temperatures is discussed based on micromagnetic analysis of the observed microstructures complemented with finite-element micromagnetic simulations.
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.
Nd–Fe–B permanent magnets have been coated with 0.6wt.% dysprosium and annealed at various temperatures to study the impact of the temperature-dependent Dy diffusion processes on both the magnetic ...properties and the microstructure. When optimum annealing conditions are applied the Dy processed magnets with initial coercivity of ∼1100kAm−1 yield coercivity increases which can exceed 400kAm−1 without a significant reduction of the remanent magnetic polarization. The improved stability against opposing magnetic fields can be observed up to a depth of ∼3mm along the diffusion direction, restricting the application of the Dy diffusion process to either thin magnets or magnets with tailored coercivity gradients. While in the proximity of the Dy-coated surface, each grain has a Dy-enriched shell with a Dy content of ∼6at.%; the Dy concentration decreases exponentially to ∼1.8at.% after a diffusion depth of 400μm and to ∼1at.% after a diffusion depth of 1500μm, as was found with wavelength dispersive X-ray spectroscopy and scanning transmission electron microscopy–energy dispersive X-ray spectroscopy, respectively. In the vicinity of the Dy-coated surface, the mechanism of the Dy-shell formation is attributed to the melting/solidification of a heavy-rare-earth-rich intermediate phase during high-temperature annealing. This is based on the observation that a constant Dy concentration over the width of the shells was found. Also an epitaxial relation between the Dy-poor core and the Dy-rich shell was observed by electron backscattered diffraction, which is supported by results obtained with Kerr microscopy.
We have characterized the microstructures of as-sintered and optimally post-sinter annealed Nd-rich Ga-doped Nd–Fe–B magnets by scanning electron microscopy (SEM) and aberration-corrected scanning ...transmission electron microscopy (STEM). While the Nd2Fe14B grains in the as-sintered sample with a coercivity of 0.99T are in direct contact with each other, those in the optimally annealed sample with a coercivity of 1.8T are completely enveloped by typically 10-nm-thick Nd-rich phase that contains little Fe. This strongly suggests that the Nd2Fe14B grains in the optimally annealed Nd-rich Ga-doped Nd–Fe–B magnets are exchange decoupled in contrast to those in the commercial sintered magnets.
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The microstructure of hot-deformed Nd–Fe–B permanent magnets with different Nd contents was investigated in order to correlate them with the hard magnetic properties. A thick distinct Nd-rich grain ...boundary (GB) layer was observed in a high Nd content sample by scanning electron microscopy and transmission electron microscopy. Three-dimensional atom probe results showed a significant increase in the Nd content in the GB as the overall Nd content in the alloy increased. We found a clear correlation between the Nd concentration in the GB layer and the coercivity. The mechanism of the coercivity increase is discussed based on the microstructure characterization and micromagnetic simulation results.
We discuss the mechanism of the coercivity enhancement by the grain boundary diffusion process (GBDP) using Dy vapor based on detailed microstructural characterizations. Scanning electron microscopy ...and electron probe microanalysis showed that a (Nd,Dy)2Fe14B shell formed in the outer region of Nd2Fe14B grains while its thickness decreased from the surface to the center of a cube-shaped sample. Atom probe tomography showed that the Dy content at grain boundaries (GBs) was close to that in the (Nd,Dy)2Fe14B shell. High-temperature annealing (at 900°C) of a GB diffusion processed magnet led to the disappearance of the GB layers, which resulted in a substantial reduction in coercivity. This suggests that both the (Nd,Dy)2Fe14B shell and the Nd-rich GB phase layer are required microstructural features for the coercivity enhancement by the GBDP.
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 NdOx/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.
Micromagnetic simulations of magnetization reversals of Nd–Fe–B anisotropic magnets with exchange-coupled grains demonstrate that the local demagnetization factor decreases as the grain size is ...reduced. This explains the higher coercivity and the lower temperature dependence of coercivity in magnets with smaller grain sizes. When a fraction of Nd2Fe14B grains become a single domain state for average grain sizes <1μm, a higher magnetic field is needed to magnetize them, giving rise to a two-step initial magnetization curve.
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