C. J. Yang

Low temperature magnetic properties and microstructure of rapidly solidified Nd-Fe(-Co) and Sm-Co(-Fe) Laves compounds

Laves phases of NdFe2, Nd(Fe0.5Co0.5)2, SmCo2, and Sm(Fe0.5Co0.5)2 stoichiometries were prepared using a rapid solidification technology. Low temperature magnetic properties show ferromagnetic behavior for Nd(Fe0.5Co0.5)2, SmCo2, and Sm(Fe0.5Co0.5)2. Laves compounds, while a sort of antiferromagnetism has been suggested for the supposed NdFe2 compound. This rapidly solidified NdFe2 alloy is believed to consist of an unstable rhombohedral NdFe7 phase plus fine particles of a Nd‐rich phase. Some evidence of a phase transition from the mixture of unstable NdFe7 compound plus Nd‐rich to Nd2Fe17 plus Fe‐Nd‐O phase are obtained after annealing the NdFe2 alloy. The ternary Laves compound, Sm(Fe0.5Co0.5)2, exhibits a high coercivity of 4 kOe at room temperature with a Curie temperature of 400 °C, while the Nd(Fe0.5Co0.5)2 compound shows a magnetic moment of 3.0–3.5 μB.

X‐ray refinement and magnetic properties of nitrogenated melt‐spun Sm2Fe17 compound

Sm2Fe17N2.9 compound powders were prepared from parent alloys of Sm2Fe17 which were made by both the arc‐melting and rapid solidification processes. The cast Sm2Fe17 contained mixed phases of SmFe3 and α‐Fe, and correspondingly formed less Sm2Fe17Nx with a rather large amount of residual α‐Fe after nitrogenation. The melt‐spun Sm2Fe17 compound, however, was single phase and exhibited a negligible amount of residual α‐Fe after nitrogenation. The residual amount of free iron was found to increase as a function of milling time and impede the development of promising permanent magnetic properties. The melt‐spun Sm2Fe17Nx compound powders exhibited a coercivity value iHc of 5 kOe, which is double that of the as‐cast Sm2Fe17Nx powders and a high remanence, Br=60 emu/g. The average magnetic moment of Fe atoms in the crystal is estimated to be 2.29 μB and the Fe atom in the 6c site shows the highest magnetic moment of 2.65 μB. The expansion in the c‐axial direction of the nitrogenated crystal was found to be main...

Crystal structure and magnetic properties of a nitrogenated melt-spun Sm2Fe17 compound

Abstract Sm2Fe17N2.9 compound powders were prepared from parent alloys of Sm2Fe17 which were made by both arc melting and rapid solidification processes. The cast Sm2Fe17 contained mixed phases of SmFe3 and α-Fe, and correspondingly formed less Sm2Fe17Nx with a rather large amount of residual α-Fe after nitrogenation. The melt-spun Sm2Fe17 compound, however, was single phase and exhibited a negligible amount of residual α + Fe after nitrogenation. The residual amount of free iron was found to increase as a function of milling time and to impede the development of promising permanent magnetic properties. The melt-spun Sm2Fe17Nx compound powders exhibited a coercivity value iHc of 5 kOe, which is double that of the as-cast Sm2Fe17Nx powders and a high remanence Br = 60 e.m.u. g−1. The average magnetic moment of Fe atoms in the crystal is estimated to be 2.29 μB and the Fe atom in the 6c site exhibits the highest magnetic moment of 2.65 μB. The expansion in the c axial direction of the nitrogenated crystal was found to be mainly due to extension of Sm(6c)Fe(c) and Fe(18f)Fe(18f) distances.

Magnetic phase transition in rapidly solidified Nd-Fe alloy

Abstract The magnetic phase transition taking place in melt-spun NdFe 2 alloy during annealing has been discussed. The high coercive ( i H c = 2–3.5 kOe) NdFe 7 phase at a Curie temperature 230–235°C is found to be formed in the melt-spun NdFe 2 alloy. The NdFe 7 phase transforms into Nd 5 Fe 17 and Nd 2 Fe 17 together with the Nd-rich phase under a vacuum atmosphere. Under a controlled oxygen atmosphere, the NdFe 7 and (Nd-rich) phases transform directly into Nd 2 O 3 and α-Fe. The Nd 5 Fe 17 phase is observed to be magnetically soft ( i H c = 0.18 kOe) and to exhibit a higher Curie temperature ( T c = 240–245°C) than that of the NdFe 7 phase. The melt-spun NdFe 2 alloy shows a magnetic anomaly indicating the spin reorientation of NdFe 7 crystallizes at 325–350 K.

Magnetic properties of sintered Alnico 5 magnet via rapid solidification technology

A process for making Alnico magnets (grade 5–8) via rapid solidification technology has been developed. The process includes extractive melt spinning of alloy powders, press forming, heat treatment under a magnetic field, and aging. The Alnico 5 magnets made by this process showed superior magnetic properties to those of cast magnet of the corresponding composition. The superior magnetic properties of the sintered magnets with Br=13.2 kG, Hc=680 Oe, and (BH)max=6.02 MGOe are due to the spinodal decomposition which is completely free of γ phase.A process for making Alnico magnets (grade 5–8) via rapid solidification technology has been developed. The process includes extractive melt spinning of alloy powders, press forming, heat treatment under a magnetic field, and aging. The Alnico 5 magnets made by this process showed superior magnetic properties to those of cast magnet of the corresponding composition. The superior magnetic properties of the sintered magnets with Br=13.2 kG, Hc=680 Oe, and (BH)max=6.02 MGOe are due to the spinodal decomposition which is completely free of γ phase.

The effect of Laves phase on the magnetic properties of Fe‐Co‐Nd‐B alloys

The formation of a Laves phase, Nd(Fe,Co)2, in Nd(Fe0.92−xCoxB0.08)5.5 magnets prepared by powder metallurgical processing of melt‐spun powders has been confirmed. The Laves phase forms after crystallization in melt‐spun alloys when x is larger than 0.17, and is likely to occupy 15 vol % of the sintered magnet when the Co content x is 0.22. ‘‘Two‐phase’’ magnets of Fe77Nd16B7+(5–20 wt. %) NdFe2 were prepared by rapid solidification and powder metallurgy. As the volume fraction of rapidly solidified Laves phase NdFe2 (actually a mixture of NdFe7 compound plus Nd‐rich phase) increased, the magnetic properties, particularly for coercivity, were found to decrease.

Crystal structure and low‐temperature magnetic properties of melt‐spun Sm2Co7B3 compounds

Low‐temperature magnetic properties and crystal structures of melt‐spun Sm2Co7B3 compound were characterized. The magnetic measurements in the temperature range 77–450 K indicated that a spin reorientation took place at about 150–160 K. A huge anisotropy was observed (Ha=135 kOe at 300 K, 725 kOe at 77 K) for Sm2Co7B3, although the magnetic moment is rather low. The crystal structure of the Sm2Co7B3 compound was analyzed in detail by Rietveld analysis of powder diffraction for the first time, and revealed that B(4h) atoms are not placed in the Sm(2e) layer but in between the Sm(2e) and Co(6i1) layers.

The effect of the Laves phase on the magnetic properties of Fe-Co-Nd-B alloys

Abstract The formation of the Laves phase in Nd(Fe 0.5 Co 0.5 ) 2 stoichiometry has been confirmed in Nd(Fe 0.92 − x Co x B 0.08 ) 5.5 magnets prepared from melt-spun powders as well as from powder metallurgy. The Laves phase starts to form after crystallizing the melt-spun alloys when x is larger than 0.17. The Laves phase of Nd(Fe 0.5 Co 0.5 ) 2 is likely to occupy 15 vol% of the sintered magnet when the Co content x is 0.22. Two-phased magnets of Fe 77 Nd 16 B 7 + (5–20 wt%) NdFe 2 were artificially prepared by rapid solidification and powder metallurgy. As the volume fraction of rapidly solidified Laves phase NdFe 2 , actually a mixture of NdFe 7 + Nd-rich compounds, increased the magnetic properties were found to dccrease, particularly for coercivity.