Hirokazu Kanekiyo

High‐coercivity iron‐rich rare‐earth permanent magnet material based on (Fe, Co)3B‐Nd‐M (M=Al, Si, Cu, Ga, Ag, Au)

High‐coercivity, high‐remanence permanent magnet materials have been obtained via a crystallization treatment of amorphous (Nd,Dy)xFe80.5−x−yCoyB18.5M1 alloys for 3≤x≤5 and 0≤y≤5, where M represent the additives Al, Si, Cu, Ga, Ag, and Au. It has been found that addition of Co and M results in a significant reduction of the crystal grain size and leads to significant improvements both in the intrinsic coercivity and in the energy product. The materials are superior to Nd2Fe14B‐based rapidly solidified materials in the remanence Br, the temperature coefficient of Br, and magnetizability. The typical values of temperature coefficients of Br and HcJ are, respectively, −0.05%/K and −0.35% K. The magnetizing force required to obtain substantial remanence is approximately 0.8 MA/m. Compaction‐molded resin‐bonded magnets produced from these materials have magnetic performance (Br, HcJ) in the range between 0.89 T, 0.29 MA/m and 0.80 T, 0.35 MA/m.

Microstructure and magnetic properties of high-remanence Nd/sub 5/Fe/sub 71.5/Co/sub 5/B/sub 18.5/M (M=Al, Si, Ga, Ag, Au) rapidly solidified and crystallized alloys for resin-bonded magnets

High-remanence, isotropic magnetic powder of a low rare earth content is developed for resin-bonded magnet applications. The materials are based on a metastable Fe(Fe, Co)/sub 3/B-Nd/sub 2/Fe/sub 14/B nanocrystalline mixture with the addition of a small amount of one of Al, Si, Cu, Ga, Ag, and Au. The effects of the additives on the microstructure are investigated by means of transmission electron microscopy. It is found that the additives considerably reduce grain sizes, which results in improvements in the hysteresis squareness of the material. The magnetic properties of a bonded magnet produced from Nd/sub 4.5/Fe/sub 73/Co/sub 3/Ga/sub 1/B/sub 18.5/ are described in detail. >

Electron holography of Nd-Fe-B nanocomposite magnets

Abstract Magnetization distribution in Nd–Fe–B nanocomposite magnets was investigated by electron holography, using a new pole piece apparatus dedicated to observations of nanocrystalline ferromagnetic materials. The exchange coupling between the magnetically soft and hard grains of 20–30 nm was experimentally verified by this microscopic study with improved resolution.

Direct formation of Fe3B/Nd2Fe14B nanocomposite permanent magnets in rapid solidification

Abstract Rapid solidification processing of Nd 4 Fe 77.5− x B 18.5 M x (M=Cr, Cu) alloys was studied in order to obtain better understandings of the “direct” process in which a Fe 3 B/Nd 2 Fe 14 B-type nanocomposite structure is formed during the rapid solidification of a melt. Effects of influential elements such as Cr and Cu on the critical cooling rate were also studied.

Nanostructure and magnetic properties of chromium-doped Fe3B-Nd2Fe14B exchange-coupled permanent magnets

Abstract Magnetic properties of Nd 4.5 Fe 77 − x Cr x B 18.5 and results from high-resolution scanning transmission microscopy (HRSTEM) analysis of Nd 4.5 Fe 77 B 18.5 and Cr-doped Nd 5.5 Fe 71 Cr 5 B 18.5 nanocomposite permanent magnets are reported. The magnets were prepared from amorphous alloys via a crystallization heat treatment. With increasing Cr content, the intrinsic coercivity ( H cJ ) increased and the remanence ( B r ) decreased considerably. The average grain size of crystallites was approximately 20 nm and found to be smaller in the Cr-doped sample than in the non-doped one. Two types of regions with the Fe/Nd atomic ratio of approximately 7 were detected by HRSTEM; some regions contain Cr and others do not. Presence of grains of the non-magnetic Nd 1.1 Fe 4 B 4 phase with negligible Cr contents were also revealed in both Nd 4.5 Fe 77 B 18.5 and Nd 5.5 Fe 71 Cr 5 B 18.5 samples. The experimental results suggest that, in Cr-doped Nd-Fe-B nanocomposite magnets, Cr is concentrated in Fe and/or Fe x B phases and depleted in the Nd 1.1 Fe 4 B 4 and some of Nd 2 Fe 14 B phases after crystallization. The increase in H cJ probably originates from depression of intergrain exchange couplings through Cr concentrated in the soft magnetic phases.

Crystallization process of a rapidly quenched Fe-B-Nd nanocomposite magnet

Abstract Superior magnetic properties of Nd–Fe–B nanocomposite magnets rely on their nanoscaled structure composed of hard-magnetic Nd 2 Fe 14 B and soft-magnetic Fe 3 B phases, which results from a glassy state upon subsequent annealing. It has been known from X-ray diffraction (XRD) analysis that for this system, an addition of Cr is critical to control the crystallization route to produce the desired mixture of Nd 2 Fe 14 B and Fe 3 B phases. We have investigated the partitioning of Cr in a Nd 5 Fe 74.5 B 18 Cr 2.5 alloy throughout its crystallization process using atom probe field ion microscopy (APFIM). Cr is found to be enriched in Fe 3 B, up to 4.7 at.%, and the Cr concentration in Nd 2 Fe 14 B is determined to be 0.70 at.%. Together with the results of XRD, we have discussed the effect of Cr, based on the idea that Cr alters the phase decomposition route by stabilizing the Fe 3 B phase.

Effect of Cr doping on crystallization behavior of Fe3B/Nd2Fe14B nanocomposite permanent magnets

Abstract Crystallization behavior and magnetic properties of melt-spun NdxFe82–xB18 and NdxFe79–xCr3B18 (x = 3.5–5.5 at%) alloys have been investigated by means of X-ray diffraction, transmission electron microscopy and magnetization measurements. For both the systems, a similar step proceeds until Fe3B and Nd2Fe23B3 crystallize from the amorphous state. For the Nd5Fe77B18 alloy the metastable compound Nd2Fe23B3 then decomposes along with Fe3B into Fe and nonmagnetic Nd1.1Fe4B4 without any occurrence of hard-magnetic Nd2Fe14B. For the Nd5Fe74Cr3B18 alloy, the decomposition of Nd2Fe23B3 into Nd2Fe14B and Fe3B occurs, resulting in a nanocomposite structure with a large fraction of Nd2Fe14B and a uniform grain size distribution. It is concluded that Cr contributes to increasing coercivity of Fe3B/Nd2Fe14B nanocomposite magnets by inducing the desired scheme of decomposition of the Nd2Fe23B3 compound.

Influence of heating rate on the microstructure and magnetic properties of Fe3B/Nd2Fe14B nanocomposite magnets

The influence of heating rate on the microstructure and magnetic properties of the Fe3B/Nd2Fe14B nanocomposite permanent magnet materials produced by the crystallization of the Nd4.5Fe73B18.5Co2Cr2 amorphous alloy has been studied.

Mechanism of grain size refinement of Fe3B/Nd2Fe14B nanocomposite permanent magnet by Cu addition

Minor addition of Cu to Nd4.5Fe77B18.5 melt-spun alloy is effective on reducing the grain size of the Fe3B/Nd2Fe14B nanocomposite permanent magnet produced via the crystallization route from the amorphous phase, thereby improving hard magnetic properties. Three-dimensional atom probe and transmission electron microscopy observations have shown that Cu clusters with a number density of ∼1024 m−3 is formed prior to the nucleation event of the Fe3B primary crystal. In the nucleation and growth stage of the Fe3B primary crystals, Cu clusters are in direct contact with the primary particles, suggesting that Cu clusters serve as heterogeneous nucleation sites for the Fe3B primary particles, thereby increasing the number density of the particles.

Solidification and crystallization behaviors of Fe3B/Nd2Fe14B-based nanocomposite permanent-magnet alloys and influence of micro-alloyed Cu, Nb and Zr

Abstract Rapid solidification process of Nd 2 Fe 14 B-based nanocrystalline permanent-magnet alloys and crystallization processes of the relevant amorphous alloys are investigated. Accumulated experimental results are discussed in terms of temperature–time–transformation diagrams for Nd 4 Fe 77.5 B 18.5 . Solidification and crystallization kinetics is strongly influenced by minor amounts of additives, particularly Cr, Cu, Nb, and Zr, knowledge of which has been applied to design alloy composition. Development of a high coercivity Fe 3 B/Nd 2 Fe 14 B-type nanocomposite magnet based on Nd–Dy–Fe–B–Cr–Co–Cu–Nb/Zr is described as an example.