Synthesis and Magnetic Properties of Nanocomposite Magnets Self-Organized from FeB-Nd-Nb Metallic Glasses
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Synthesis and Magnetic Properties of Nanocomposite Magnets Self-Organized from FeB-Nd-Nb Metallic Glasses R Tamura1,2 , S Kobayashi1, T Fukuzaki2 and M Kamiko3 1 Department of Materials Science & Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan 2 Polyscale Technology Research Center, Tokyo University of Science, Noda, Chiba 2788510, Japan 3 Institute of Industrial Science, University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan ABSTRCT Change of the magnetic property of Fe-B-Nd-Nb alloys was investigated with replacing Nb by a glass forming element Zr under constant quenching rate as well as heat treatment conditions. As a result, the coercivity significantly increases up to 1207 kA/m when the half of Nd is replaced by Zr, which is presumably due to grain refinement of the Nd2Fe14B phase. The self-organized nanograin magnets are attractive for future applications since their coercivity can be further improved by reducing the grain size via optimizing the Zr concentration, the quenching rate and the subsequent heat treatment condition. INTRODUCTION Recently, bulk metallic glasses were produced in Nb-added Fe-Nd-B alloys [1] and the best glass former with a critical diameter of 4 mm was obtained for the alloy composition of Fe65.28B24Nd6.72Nb4, allowing us to obtain nanograin magnets in a bulk form. The addition of Nb was also known to be effective in grain refinement of Fe-Nd-B magnets, and the reduction of the grain sizes is particularly important for the enhancement of the coercivity since the coercivity mechanism of the Nd2Fe14B is governed by nucleation of an inversed magnetic domain at grain boundaries [2]. According to Zhang et al.[1], the highest coercivity Hc is obtained for Fe64.32B22.08Nd9.6Nb4 where Hc reaches 1100 kA/m by the precipitation of the Nd2Fe14B phase from the metallic glass by a subsequent heat treatment. One drawback of the magnet exists in its low energy product, i.e., 33 kJ/m3, which is a consequence of its low remanence magnetization of 0.44 T. Several ways are known to improve the coercivity of a nanocomposite magnet; (1) refining the grain size on a fine scale, (2) increasing the volume fraction of the hard phase and (3) isolating the hard grains magnetically by the presence of a nonmagnetic phase at grain boundaries. On the other hand, the remanence magnetization is mainly determined by the saturation magnetization, or the Fe concentration, of a magnet. Therefore, in order to raise the remanence to a higher level, the Fe concentration of a magnet needs to be maintained at a high level. In the present work, we have aimed at producing Fe-Nd-B-Nb nanocomposite
magnets with high coercivity of typically > 1000 kA/m by reducing the grain size with the addition of grain refining elements such as Zr while maintaining the remanence at a substantially high level. Our final goal is to produce the extraordinary high coercivity magnets, i.e., > 1000 kA/m, in a bulk form. In the present work, we have studied Zr added Fe-B-Nd-Nb magnets self-organized from precursor met
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