Nanocomposite magnetis are composed of nanocrystalline soft and hard magnetic particles which are magnetically coupled with exchnage interaction. The first experimental observation of nanocomposite magnet was reported by Coehoon et al. in 1988 in annealed Fe₇₇Nd_4.5B_18.5 melt-spun ribbon. On crystallization of the amorphous ribbon, Fe₃B/Nd₂Fe₁₄B nanocomposite microstructure was obtained and its magnetization curve showed remanece higher than M_s/2 that is expected from randomly oriented isotropic polycrystalline magnets. The higher magnetization is the result of remanence enhancement due to exchange coupling between soft and hard magnetic grains. Figure 1 (a) show a typical example of Fe/Nd2Fe14B nanocomposite. Small particles are Fe and larger particles with uniform gray contrast are Nd₂Fe₁₄B grains. When the particle size is large, they are only magnetostatically coupled, so the magnetization curve show soft and hard magnetic componenents as shown in Fig. 1(c). When crystal grain size is refined to tens of nanometers, soft and hard magnetic grains are exchange coupled and the magnetization curve show only single phase magnetization behavior. In this case, the remanence Mr is much larger than 0.5Ms because of the remanence enhancement effect. Another feature is that the spring back effect as indicated in red curves in Fig. 1(c) appears as the rotation of the magnetization of soft magnetic phase is pinned by the magnetization of hard magnetic phase. Since relatively large magnetic flux density (B) can be obtained from the presence of the soft magnetic phase, nanocomposite magnets tend to exhibit relatively large (BH)_max in spite of its low rare earth content.

Fig. 1 (a) Typical Fe/Nd₂Fe₁₄B nanocomposite microstructure, (b) schematic illustration of magnetization, and (c) magnetization curves for decoupled and exchange coupled soft/hard composite.

Due to its isotropic nature, nanocomposite magnets are now regarded as economical medium performance magnetic materials for bonded magnet applications, although there were some expectations that nanocomposte magnets may break the record of the existing sintered magnets.

Recent trend is a development of anisotropic nanocomposite magent. Skomski predicted that a maxmum energy product exceeing 1000 kJ/m³ may be obtained from perfectly anistoropic SmFeN/FeCo nanocomposites. Our recent work on anisotropic Sm(Co,Cu)₅/FeCo multilayer demostrated that (BH)_max that is higher than the theoretical limit for SmCo5 single phase magnets could be achieve from a rigidly exchange coupled anisotropic soft/hard nanocomposite magnets.

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